• Factor V Leiden (FVL) is the most common inherited thrombophilia, present in ~5% of Caucasians and responsible for 20–40% of venous thromboembolism (VTE) cases in this population.

  • It causes activated protein C resistance (APCR), impairing the inactivation of Factor Va and promoting thrombosis.

  • Heterozygous carriers have a 3–8x increased VTE risk; homozygotes have a 50–100x higher risk.

Why Not the Others?

• A) Protein S deficiency – Prevalence: 0.03–0.13%; less common than FVL.

• B) Antithrombin deficiency – Rare (0.02–0.2%); severe but uncommon.

• D) Prothrombin G20210A mutation – Second most common (2–3% of Caucasians), but less prevalent than FVL.

• Factor VIII inhibitors (neutralizing antibodies) develop in ~30% of severe hemophilia A patients (FVIII <1%) due to:

  • Replacement therapy with exogenous FVIII, which the immune system recognizes as “foreign.”

  • The risk is highest in high-risk genotypes (e.g., large F8 gene deletions, nonsense mutations).

• Inhibitors render FVIII infusions ineffective, increasing bleeding risk.

Why Not the Others?

• A) Viral infection – May trigger immune responses but is not a primary cause of inhibitors.

• B) Vaccination – No proven link to inhibitor development.

• D) Iron deficiency – Unrelated to inhibitor formation.

• Activated protein C resistance (APCR) is most commonly caused by the Factor V Leiden (FVL) mutation (R506Q substitution in the F5 gene).

  • This mutation makes Factor Va resistant to degradation by activated protein C (APC), leading to prolonged thrombin generation and increased thrombotic risk.

  • FVL accounts for >90% of hereditary APCR cases and is the most common inherited thrombophilia in Caucasians.

Why Not the Others?

• B) Factor VIII – Mutations cause hemophilia A (bleeding), not APCR.

• C) Protein S – Deficiency reduces APC cofactor activity but does not cause resistance to APC.

• D) Antithrombin – Inhibits thrombin/Factor Xa; mutations cause thrombophilia but not APCR.

• The dRVVT ratio is calculated as:

  Ratio=dRVVT Screen TimedRVVT Confirm Time=6249≈1.26Ratio=dRVVT Confirm TimedRVVT Screen Time​=4962​≈1.26

• Interpretation:

  • Ratio ≥1.2 suggests lupus anticoagulant (LA) presence (per ISTH guidelines).

  • LA interferes with phospholipid-dependent clotting, prolonging the screen (low phospholipid) but not the confirm (high phospholipid).

Why Not the Others?

• A) 1.18 – Below the 1.2 threshold (negative for LA).

• C) 1.35 / D) 1.47 – Overestimates the ratio.

• PT and aPTT test the plasma coagulation pathways (extrinsic, intrinsic, and common).

• If both are normal, the bleeding disorder is likely not due to a clotting factor deficiency (e.g., not vitamin K deficiency, hemophilia, or DIC).

• Platelet function defects (e.g., von Willebrand disease, aspirin use, or thrombocytopenia) can cause bleeding without affecting PT/aPTT because platelets are involved in primary hemostasis (clot formation at the vessel level).

Why not the others?

• A) Vitamin K deficiency → Prolongs PT (affects Factors II, VII, IX, X).

• C) Factor IX deficiency → Prolongs aPTT (Hemophilia B).

• D) DIC (Disseminated Intravascular Coagulation) → Typically prolongs both PT and aPTT due to factor consumption.

• Factor VII is the only coagulation factor that, when deficient, prolongs PT (extrinsic pathway) while aPTT remains normal (intrinsic pathway unaffected).

• This is because Factor VII is unique to the extrinsic pathway, while other factors (VIII, IX, XI) affect the intrinsic pathway (prolonging aPTT) .

Why Not the Others?

• A) Factor VIII & B) Factor IX: Deficiencies prolong aPTT (Hemophilia A/B) but leave PT normal .

• D) Factor XI: Deficiency also prolongs aPTT (intrinsic pathway) with normal PT .

• D-dimer is a specific marker for cross-linked fibrin degradation, which occurs in DIC (due to secondary fibrinolysis) but is absent in primary fibrinolysis (where fibrinogen is directly cleaved without clot formation).

  • DIC: Elevated D-dimer (from fibrin clot breakdown) + low platelets/fibrinogen (consumption).

  • Primary fibrinolysis: Normal/low D-dimer + isolated hypofibrinogenemia (no platelet consumption) .

Why Not the Others?

• A) Fibrinogen: Low in both DIC and primary fibrinolysis (not discriminatory) .

• B) Platelet count: Low in DIC (consumption) but normal in primary fibrinolysis .

• D) aPTT: Prolonged in DIC (factor consumption) but may be normal in primary fibrinolysis unless fibrinogen is critically low .

• Enhanced response to low-dose ristocetin suggests increased platelet agglutination due to abnormal von Willebrand factor (VWF) with heightened affinity for platelet GPIb (seen in Type 2B or 2M VWD).

• Normal response to high-dose ristocetin rules out severe deficiencies (Type 3 VWD) or global dysfunction (Type 1 VWD).

• Type 2B VWD is most likely, where mutant VWF binds platelets spontaneously, causing thrombocytopenia and paradoxical aggregation at low ristocetin doses.

Why not others?

• A) Type 1 VWD shows reduced aggregation at all ristocetin doses (quantitative VWF deficiency).

• C) Type 3 VWD has no aggregation (absent VWF).

• D) Normal platelets show no enhanced response to low-dose ristocetin.

• Factor XII (Hageman factor) deficiency causes a prolonged aPTT due to impaired contact activation of the intrinsic pathway.

  • However, it is not associated with bleeding because FXII is not involved in in vivo hemostasis.

  • Paradoxically, FXII deficiency may increase thrombotic risk (due to defective fibrinolysis).

Why Not the Others?

• A) Normal clotting tests – Incorrect; aPTT is always prolonged.

• B) Severe bleeding – Seen in FVIII/FIX deficiency (hemophilia), not FXII.

• D) Low platelets – Unrelated (FXII deficiency does not affect platelets).

The thrombin time (TT) is most sensitive to heparin, which directly inhibits thrombin and prevents fibrinogen → fibrin conversion. While other factors can prolong TT, heparin is the most common and primary cause in clinical practice.

Key Reasons:

1. Heparin (C) → Directly binds and inactivates thrombin, markedly prolonging TT.

2. Low fibrinogen (B) → Can prolong TT but is less common than heparin interference.

3. Factor V deficiency (A) & Protein C defect (D) → Do not affect TT (they influence PT/aPTT).

• aPTT is the standard test for monitoring unfractionated heparin (UFH) therapy because heparin potentiates antithrombin, which primarily inhibits Factor Xa and thrombin (IIa) in the intrinsic pathway.

• The aPTT reflects heparin’s anticoagulant effect, with a therapeutic range typically 1.5–2.5x the control value.

Why Not the Others?

• A) PT (Prothrombin Time) → Measures the extrinsic pathway; insensitive to heparin.

• C) Fibrinogen → Assesses fibrinogen levels but does not reflect heparin activity.

• D) Thrombin Time (TT) → Too sensitive to heparin (often prolonged even at low doses) but not used for routine monitoring.

To calculate the normalized dRVVT ratio, follow these steps:

Step 1: Calculate the Screen Ratio

Screen Ratio=dRVVT Screen (Patient)dRVVT Screen (PNP)=7535=2.14Screen Ratio=dRVVT Screen (PNP)dRVVT Screen (Patient)​=3575​=2.14

Step 2: Calculate the Confirm Ratio

Confirm Ratio=dRVVT Confirm (Patient)dRVVT Confirm (PNP)=3836=1.06Confirm Ratio=dRVVT Confirm (PNP)dRVVT Confirm (Patient)​=3638​=1.06

Step 3: Compute the Normalized Ratio

Normalized Ratio=Screen RatioConfirm Ratio=2.141.06≈2.03Normalized Ratio=Confirm RatioScreen Ratio​=1.062.14​≈2.03

Interpretation:

• Normalized ratio ≥1.2 suggests lupus anticoagulant (LA).

• Here, 2.03 strongly indicates LA (consistent with antiphospholipid syndrome).

Why Not the Others?

• A) 1.89 – Underestimates the ratio.

• C) 2.25 / D) 2.48 – Overestimates the ratio.

• Lupus anticoagulant (LA) is diagnosed using a stepwise approach:

  1. Screening test (e.g., dRVVT or aPTT) – Prolonged clotting time suggests possible LA.

  2. Mixing study – If the prolonged clotting time does not correct with normal plasma, it suggests an inhibitor (like LA) rather than a factor deficiency.

  3. Confirmatory test – Hexagonal phase phospholipid assay (or excess phospholipid addition) confirms LA by shortening the clotting time, proving phospholipid dependence.

Why not the others?

• A) Thrombin time – Assesses fibrinogen conversion to fibrin; not relevant for LA.

• C) Fibrinogen assay – Measures fibrinogen levels; unrelated to LA.

• D) Mixing study – Helps distinguish an inhibitor (like LA) from factor deficiency but does not confirm LA alone.

• Glanzmann thrombasthenia is characterized by absent platelet aggregation to agonists like ADP, collagen, and epinephrine due to a deficiency or dysfunction of the glycoprotein IIb/IIIa (GPIIb/IIIa) complex, which is required for fibrinogen binding and platelet-platelet bridging .

• Ristocetin-induced aggregation remains normal because it depends on the GPIb/IX/V complex (intact in Glanzmann), which mediates von Willebrand factor (VWF)-dependent platelet agglutination .

Why Not the Others?

• A) Bernard-Soulier syndrome: Shows absent ristocetin aggregation (GPIb/IX/V defect) but normal response to ADP/collagen .

• C) von Willebrand disease: Typically exhibits reduced ristocetin aggregation (due to VWF dysfunction) but normal ADP/collagen responses unless severe.

• D) Factor VII deficiency: Affects PT only (extrinsic pathway); no impact on platelet aggregation

• Factor VIII is the most labile coagulation factor in stored plasma, losing activity quickly due to its instability at room temperature and sensitivity to pH changes.

• Degradation begins rapidly (even in refrigerated conditions), making it critical to freeze plasma promptly for accurate testing.

• Other factors (II, VII, X) are more stable in stored plasma.

Why Not the Others?

• A) Factor II (Prothrombin) – Stable for days in plasma.

• B) Factor VII – Relatively stable compared to FVIII.

• D) Factor X – Also stable in stored plasma.

• Thrombin time (TT) is the most sensitive test for detecting heparin in a blood sample because it directly measures the time for thrombin to convert fibrinogen to fibrin.

  • Heparin potently inhibits thrombin (Factor IIa), leading to significant prolongation of TT even at low concentrations.

  • TT is so sensitive that it will be prolonged with therapeutic or even subtherapeutic heparin levels.

• aPTT (not listed) is used for monitoring heparin but is less sensitive than TT.

Why Not the Others?

• A) PT (Prothrombin Time) – Measures the extrinsic pathway; insensitive to heparin (only high doses affect it).

• B) Fibrinogen – Measures fibrinogen levels; unaffected by heparin unless the assay is thrombin-based (then falsely low).

• D) D-dimer – Reflects fibrinolysis, not heparin activity.

• Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) both affect the intrinsic coagulation pathway, which is measured by the aPTT.

• The aPTT will be prolonged in both conditions, while the PT, platelet count, and thrombin time remain normal (they are not dependent on Factors VIII or IX).

Why Not the Others?

• A) PT (Prothrombin Time): Evaluates the extrinsic pathway (Factor VII) and common pathway (Factors X, V, II, fibrinogen)—unaffected in hemophilia.

• C) Platelet count: Measures platelet number (normal in hemophilia).

• D) Thrombin time: Tests fibrinogen conversion to fibrin (normal in hemophilia).

• Vitamin K deficiency leads to impaired synthesis of vitamin K-dependent factors (II, VII, IX, X, protein C/S), causing prolonged PT and aPTT.

• Correction with vitamin K confirms the diagnosis, as supplementation restores factor carboxylation and normalizes clotting times .

Why Not the Others?

• A) Liver failure: Prolonged PT/aPTT does not correct with vitamin K (hepatocyte dysfunction impairs factor synthesis) .

• C) DIC: Consumptive coagulopathy causes low fibrinogen, high D-dimer, and no vitamin K response .

• D) Hemophilia A: Prolongs aPTT only (Factor VIII deficiency); unaffected by vitamin K

• The bleeding time test evaluates platelet adhesion and primary hemostasis by measuring the time for bleeding to stop after a standardized skin incision.

• It reflects the ability of platelets to adhere to damaged endothelium (via von Willebrand factor) and form a plug .

Why Not the Others?

• A) PT (Prothrombin Time): Assesses the extrinsic coagulation pathway (unrelated to platelet function).

• B) Platelet count: Measures platelet quantity, not adhesion or function.

• D) Thrombin time: Evaluates fibrinogen conversion to fibrin (plasma coagulation).

• Heparin contamination from an IV line is a common preanalytical error causing falsely prolonged aPTT and thrombin time (TT).

• First step: Redraw the sample properly (e.g., discard the first 5 mL of blood, use a non-heparinized line, or perform a fresh venipuncture).

• Rationale:

  • Heparin binds antithrombin, inhibiting thrombin (↑TT) and intrinsic pathway factors (↑aPTT).

  • Repeating the test with an uncontaminated sample avoids unnecessary follow-up tests (e.g., mixing studies, factor assays).

Why Not the Others?

• B) Mixing study – Premature if heparin contamination is suspected.

• C) Neutralize heparin (e.g., with protamine) – Only for confirmed heparin effect (not first-line for suspected contamination).

• D) Factor assays – Unnecessary until heparin is ruled out.

• DIC (Disseminated Intravascular Coagulation) involves widespread clotting followed by fibrinolysis, leading to:

  • ↑ D-dimer (most reliable marker) → Reflects breakdown of cross-linked fibrin (fibrinolysis).

  • ↓ Platelets (A) (thrombocytopenia from consumption).

  • ↓ Fibrinogen (B) (consumed by clotting).

  • Factor VIII (D) is variable (can be low due to consumption or elevated as an acute-phase reactant).

Key Point:

While multiple labs are abnormal in DIC, D-dimer is the most consistently elevated due to fibrin degradation.

• Factor V is the most labile (unstable) coagulation factor in stored plasma, losing activity rapidly within 24–48 hours at room temperature or even under refrigerated conditions.

• This instability is why fresh frozen plasma (FFP) must be used for Factor V replacement, as standard stored plasma loses Factor V activity quickly.

Why Not the Others?

• A) Factor II (Prothrombin): Stable for 5 days in refrigerated plasma.

• C) Factor IX: Stable in frozen plasma for longer periods.

• D) Factor XIII: Extremely stable, even in stored serum.

• Lupus anticoagulant (LA) is typically associated with thrombosis, but bleeding can occur if:

  • Anti-prothrombin antibodies bind and accelerate prothrombin clearance, causing hypoprothrombinemia (low Factor II).

  • This rare condition is termed lupus anticoagulant-hypoprothrombinemia syndrome (LAHPS).

• Lab findings:

  • Prolonged PT/aPTT (due to low prothrombin).

  • Positive LA tests (e.g., dRVVT, aPTT).

Why Not the Others?

• A) Factor VIII inhibition – Causes acquired hemophilia A (unrelated to LA).

• B) Low fibrinogen – Seen in DIC/liver disease, not LA.

• D) Platelet dysfunction – Occurs in antiplatelet antibodies (e.g., ITP), not LA.

• For low molecular weight heparin (LMWH), the anti-Xa therapeutic range is typically 0.5–1.2 U/mL (varies by indication and institutional guidelines).

  • Prophylactic dosing: Targets 0.2–0.5 U/mL.

  • Therapeutic dosing: Targets 0.5–1.2 U/mL (for VTE/treatment).

• An anti-Xa level of 0.9 U/mL falls within the therapeutic range for most treatment protocols.

Why Not the Others?

• A) Subtherapeutic – Would be <0.5 U/mL for treatment dosing.

• C) Supratherapeutic – Would be >1.2 U/mL (requires dose reduction).

• D) Risk of bleeding – Not directly predicted by anti-Xa levels; clinical assessment is needed.

• The 5M urea clot solubility test detects Factor XIII deficiency.

  • Factor XIII cross-links fibrin to stabilize clots.

  • In its absence, clots dissolve in 5M urea or 1% monochloroacetic acid within 2–24 hours (normal clots remain stable).

Why Not the Others?

• A) Factor V: Prolongs PT/aPTT; no role in clot stability.

• B) Factor VIII: Affects intrinsic pathway (prolongs aPTT); unrelated to fibrin cross-linking.

• D) Factor VII: Prolongs PT only; no impact on clot solubility.

The cell-based model of coagulation describes clotting as occurring in three overlapping phases:

1. Initiation – Begins when tissue factor (TF) on TF-bearing cells (e.g., subendothelial fibroblasts, activated monocytes) binds Factor VIIa, forming the TF-FVIIa complex and generating small amounts of thrombin (FIIa).

2. Amplification – Thrombin activates platelets and cofactors (FV, FVIII, FXI) on the platelet surface.

3. Propagation – Large-scale thrombin burst occurs on activated platelet membranes (not endothelial cells or collagen directly).

Why Not Others?

• A) Activated platelet membranes – Critical for propagation, not initiation.

• B) Endothelial cells – Normally anticoagulant; only procoagulant if injured.

• C) Subendothelial collagen – Exposed after vascular injury but does not initiate clotting alone (requires TF).

✅ Key Point: TF-bearing cells trigger the cascade, while platelets amplify/propagate it.

• Factor XII (Hageman factor) deficiency is the most likely cause of a persistently prolonged aPTT that corrects with mixing in a patient without bleeding symptoms.

  • Factor XII is part of the contact activation pathway (intrinsic coagulation) but is not essential for in vivo clotting.

  • Deficiency causes laboratory abnormalities only (no clinical bleeding).

• Correction with mixing confirms a factor deficiency (not an inhibitor).

Why Not the Others?

• A) Factor VIII (Hemophilia A) – Causes bleeding (prolonged aPTT, corrects with mixing).

• B) Factor IX (Hemophilia B) – Causes bleeding (prolonged aPTT, corrects with mixing).

• C) Factor XI – Mild bleeding risk (e.g., post-surgery/trauma).

• Lupus anticoagulant (LA) is an antiphospholipid antibody that prolongs aPTT by interfering with phospholipid-dependent clotting tests in vitro.

• Paradoxically, it is associated with thrombosis (not bleeding) due to hypercoagulability in vivo .

Why Not the Others?

• A) Anti-factor IX: Rare; causes bleeding (prolonged aPTT with low Factor IX levels).

• B) Anti-thrombin: Neutralizes thrombin, leading to bleeding (prolongs TT/aPTT).

• D) Anti-platelet antibody: Causes thrombocytopenia (e.g., ITP) but does not affect aPTT.

• Thromboxane A₂ (TXA₂) is the key amplifier of platelet aggregation, synthesized from arachidonic acid via cyclooxygenase-1 (COX-1).

  • It promotes:

    • Platelet activation (via GPIIb/IIIa receptor exposure).

    • Vasoconstriction (to reduce blood loss).

    • Secondary aggregation (by recruiting nearby platelets) .

Why Not the Others?

• A) Cyclooxygenase (COX-1): Enzyme that converts arachidonic acid to prostaglandin H₂ (precursor for TXA₂), but not the final product.

• B) Arachidonic acid: Substrate for COX-1; itself does not amplify aggregation.

• C) Prostacyclin (PGI₂): Inhibits platelet aggregation (counteracts TXA₂) .

• A prolonged aPTT that does not correct with a 50:50 mixing study (patient plasma + normal plasma) indicates the presence of an inhibitor, not a factor deficiency.

• Lupus anticoagulant (B) is an acquired antiphospholipid antibody that interferes with phospholipid-dependent clotting tests (e.g., aPTT) but paradoxically causes thrombosis, not bleeding.

Why Not the Others?

• A) Factor VIII deficiency & C) Factor IX deficiency: Both are factor deficiencies that correct with mixing (missing factors are replaced by normal plasma).

• D) Normal variant: A prolonged aPTT without clinical bleeding or correction would be unusual; inhibitors like lupus anticoagulant are more likely.

• Glanzmann thrombasthenia (GT) is characterized by absent platelet aggregation to agonists like ADP and collagen due to a deficiency or dysfunction of the glycoprotein IIb/IIIa (GPIIb/IIIa) complex, which is required for fibrinogen binding and platelet-platelet bridging .

• Ristocetin-induced aggregation remains normal because it depends on the GPIb/IX/V complex (intact in GT), which mediates von Willebrand factor (VWF)-dependent platelet agglutination .

Why Not the Others?

• A) von Willebrand disease (VWD): Typically shows reduced ristocetin aggregation (due to VWF dysfunction) but normal ADP/collagen responses unless severe .

• B) Storage pool disorder: Causes impaired secondary wave aggregation with ADP/epinephrine (due to granule secretion defects) but normal ristocetin response .

• D) Aspirin effect: Prolongs bleeding time and reduces ADP/collagen aggregation (via thromboxane A2 inhibition) but does not abolish aggregation entirely

• The dilute Russell viper venom test (dRVVT) is a specialized assay used to detect lupus anticoagulants (LAs).

• Its screen reagent contains:

  1. Russell viper venom (RVV): Directly activates Factor X, bypassing the need for Factors VII, VIII, or IX.

  2. Low phospholipid concentration: Intentionally reduced to enhance sensitivity to phospholipid-dependent antibodies (e.g., LA).

• If LA is present, the dRVVT screen time will be prolonged due to antibody interference with phospholipid-dependent clotting.

Why Not the Others?

• A) High phospholipids – Would mask LA effects (used in confirmatory reagents to neutralize LA).

• B) No phospholipid – Clotting would fail (phospholipids are essential for FX activation).

• D) Hexagonal phospholipid – Used in neutralization assays (e.g., Staclot LA) to confirm LA, not in dRVVT.

• ATP release assay directly measures platelet granule secretion (e.g., dense granule ADP/ATP release), which is impaired in storage pool disorders (e.g., δ-storage pool deficiency).

• It quantifies the biochemical output of platelet activation, unlike aggregation tests that only assess clumping.

Why Not the Others?

• A) Bleeding time: Nonspecific for secretion defects (prolonged in many platelet disorders).

• B) Platelet aggregation with thrombin: Detects GPIIb/IIIa function (e.g., Glanzmann thrombasthenia) but not granule release.

• D) aPTT: Evaluates plasma coagulation, not platelet function.

• Factor XIII deficiency is not detected by routine coagulation tests (PT/aPTT/TT), which are normal because Factor XIII acts after fibrin clot formation.

• The 5M urea solubility test is the classic screening test:

  • Normal clots (cross-linked by Factor XIII) remain stable in urea.

  • Factor XIII-deficient clots dissolve within 2–24 hours due to lack of cross-linking .

Why Not the Others?

• A) PT & B) aPTT: Measure clotting time up to fibrin formation (unaffected by Factor XIII).

• C) Thrombin time: Tests fibrinogen → fibrin conversion (also normal in Factor XIII deficiency).

• An uncorrected aPTT mixing study (immediate 50:50 mix with normal plasma fails to normalize aPTT) indicates the presence of a circulating inhibitor, most commonly:

  • Factor VIII inhibitor (in hemophilia A patients receiving replacement therapy).

  • Autoantibodies (in acquired hemophilia A).

• In a bleeding child, this suggests congenital hemophilia A with an alloantibody inhibitor (developed against infused FVIII).

Why Not the Others?

• A) von Willebrand disease – Prolongs bleeding time but corrects aPTT with mixing (no inhibitor).

• B) Hemophilia B (Factor IX deficiency) – Corrects with mixing (unless an inhibitor is present, which is rare).

• D) Vitamin K deficiency – Prolongs PT (not isolated aPTT) and corrects with mixing.

• Type 3 von Willebrand disease (VWD) is characterized by:

  • Severe deficiency of von Willebrand factor (vWF):

    • vWF antigen <10% (here, 3%).

    • Ristocetin cofactor activity <10% (here, <1%).

  • Low Factor VIII (1–10%): Due to lack of vWF (which stabilizes FVIII).

• Type 3 VWD is autosomal recessive, causing severe mucosal bleeding (similar to hemophilia A).

Why Not the Others?

• A) Type 1 – Partial quantitative deficiency (vWF antigen/activity 5–30%), FVIII usually >10%.

• B) Type 2 – Qualitative defects (e.g., 2A, 2B, 2M, 2N); FVIII varies but vWF activity:antigen ratio is abnormal.

• C) Type 2B – Gain-of-function mutation causing spontaneous platelet binding (low vWF but thrombocytopenia)

• Ristocetin-induced platelet aggregation requires the binding of von Willebrand factor (VWF) to platelet glycoprotein Ib (GPIb).

• Bernard-Soulier syndrome (B) is caused by a deficiency or dysfunction of GPIb, leading to absent ristocetin aggregation (since platelets cannot bind VWF).

Why Not the Others?

• A) Glanzmann thrombasthenia: Shows normal ristocetin aggregation (GPIb is intact) but absent aggregation to ADP/collagen (due to GPIIb/IIIa defect).

• C) von Willebrand disease (VWD): May show reduced ristocetin aggregation (due to low VWF activity) but not complete absence unless severe.

• D) Storage pool disease: Normal ristocetin aggregation (defect is in granule secretion, not GPIb).

• PT and aPTT are both prolonged → Suggests a vitamin K-dependent factor deficiency (Factors II, VII, IX, X) or warfarin effect.

• Normal thrombin time → Rules out heparin (which would prolong TT).

• Reduced protein C (54%) and protein S (48%) → Expected with warfarin, as they are vitamin K-dependent anticoagulants.

• No bleeding/clotting history → Makes thrombophilia (A) or hemophilia (B) less likely.

Why Not the Others?

• A) Thrombophilia → Typically presents with thrombosis, not prolonged PT/aPTT.

• B) Hemophilia → Prolongs aPTT only (PT normal).

• C) Heparin use → Prolongs aPTT and thrombin time (TT would be elevated).

• The template method (a standardized version of the Ivy technique) typically reports a reference range of 2–9 minutes for bleeding time .

• This method uses a device to create uniform incisions (e.g., 1 mm deep × 5–10 mm long) for consistency, unlike older methods like the Duke technique (earlobe puncture, normal: 2–5 min) .

Key Points:

1. Template Method:

   • Normal range: 2–7 minutes (some sources extend to 9 minutes) .

   • Values >10–15 minutes suggest platelet dysfunction or thrombocytopenia .

2. Why Not Other Options?

   • A) 1–2 min: Too short; seen only in hypercoagulable states (not standard).

   • B) 2–4 min: Matches the Duke method (earlobe), not the template .

   • D) 6–12 min: Exceeds the upper limit of normal (abnormal range).

• Heparin’s anticoagulant effect depends entirely on its binding to antithrombin (AT).

• The heparin-AT complex undergoes a conformational change that accelerates AT’s inhibition of key clotting factors:

  • 1000x faster inhibition of Factor Xa

  • 1000x faster inhibition of thrombin (Factor IIa)

• This mechanism is why AT deficiency causes heparin resistance.

Why Not the Others?

• A) Protein C – An anticoagulant, but heparin does not bind it.

• B) Protein S – A cofactor for Protein C, unrelated to heparin.

• C) Factor Xa – Heparin enhances AT’s inhibition of Xa but does not bind Xa directly.

• Factor VIII is primarily produced by vascular endothelial cells (and to a lesser extent by hepatocytes), unlike other clotting factors (e.g., II, IX, fibrinogen), which are liver-synthesized .

• This explains why liver disease often spares Factor VIII levels (may even elevate them due to inflammation), while other factors decrease .

Why Not the Others?

• B) Factor IX & D) Factor II (Prothrombin): Synthesized by the liver (vitamin K-dependent).

• C) Fibrinogen: Liver-derived; low levels are seen in advanced liver disease/DIC .

• Unfractionated heparin (UFH) primarily inhibits Factor Xa and thrombin (IIa) via antithrombin, prolonging the intrinsic pathway.

• The aPTT is the standard test for monitoring UFH because it reflects heparin’s anticoagulant effect in the therapeutic range (typically targeting 1.5–2.5x the control value).

• Anti-Xa assays (not listed) are an alternative but are less commonly used for routine UFH monitoring.

Why Not the Others?

• A) PT (Prothrombin Time) – Measures the extrinsic pathway (unaffected by heparin).

• B) INR (International Normalized Ratio) – Used for warfarin, not heparin.

• D) Platelet count – Important for detecting heparin-induced thrombocytopenia (HIT) but does not assess anticoagulation efficacy.

• Prolonged PT (with normal or mildly prolonged aPTT) and bleeding after antibiotics suggest vitamin K deficiency, because:

  • Broad-spectrum antibiotics (e.g., cephalosporins) can:

    1. Kill gut bacteria that synthesize vitamin K₂.

    2. Disrupt dietary vitamin K absorption (fat-soluble vitamin).

  • Vitamin K is required for γ-carboxylation of Factors II, VII, IX, X (PT measures VII, the most sensitive).

• Classic triad: Prolonged PT, bleeding, and no prior liver disease.

Why Not the Others?

• A) Liver failure – Would also cause low fibrinogen, elevated bilirubin, and prolonged aPTT (all factors affected).

• B) Aspirin overdose – Causes platelet dysfunction (normal PT, prolonged bleeding time).

• D) Factor VIII inhibitor – Prolongs aPTT (not PT) and causes bleeding.

• Polycythemia vera (hematocrit >60%) causes falsely prolonged PT/aPTT due to:

  • High red cell mass reduces plasma volume in the sample, leading to excess citrate anticoagulant relative to plasma.

  • Citrate chelates calcium, impairing clotting factor activation in the assay.

• No bleeding occurs because in vivo coagulation is normal (the artifact is lab-specific).

Why Not the Others?

• A) Liver disease – Causes true factor deficiencies (would likely have bleeding).

• C) Factor X deficiency – Rare; would cause bleeding.

• D) DIC – Consumes platelets/factors (thrombocytopenia, elevated D-dimer).

• Warfarin inhibits vitamin K epoxide reductase (VKOR), reducing the synthesis of vitamin K-dependent clotting factors (II, VII, IX, X) and natural anticoagulants (Protein C and Protein S) .

• Among these, Protein C is the most clinically significant because:

  • It has a shorter half-life (~6–8 hours) compared to clotting factors like prothrombin (Factor II, half-life ~60 hours). This means Protein C levels drop rapidly after warfarin initiation, creating a transient hypercoagulable state in the first few days of therapy .

  • This imbalance can lead to warfarin-induced skin necrosis, a rare but severe complication in patients with underlying Protein C deficiency .

Why Not the Others?

• A) Antithrombin – Not vitamin K-dependent; unaffected by warfarin.

• B) Protein S – Also vitamin K-dependent, but its depletion is less critical than Protein C in the early phase of warfarin therapy.

• C) Factor V Leiden – A genetic mutation (not an anticoagulant) causing activated Protein C resistance; warfarin does not directly affect it.

• The patient’s presentation (abnormal bleeding, normal platelet count, prolonged bleeding time, and reduced ristocetin cofactor activity) is classic for von Willebrand disease (VWD).

  • Ristocetin cofactor (VWF:RCo) measures the functional ability of von Willebrand factor (VWF) to bind platelets, which is impaired in VWD .

  • Prolonged bleeding time reflects defective platelet adhesion due to VWF dysfunction .

  • Normal platelet count rules out thrombocytopenia (e.g., Bernard-Soulier syndrome) .

Why Not the Others?

• A) Bernard-Soulier syndrome: Causes thrombocytopenia (low platelet count) and absent ristocetin-induced platelet aggregation (due to GPIb deficiency), but ristocetin cofactor (plasma VWF activity) is normal .

• C) Glanzmann thrombasthenia: Presents with normal ristocetin cofactor but defective platelet aggregation to ADP/collagen (GPIIb/IIIa defect).

• D) Factor VII deficiency: Prolongs PT (extrinsic pathway) but does not affect bleeding time or ristocetin cofactor

• Rapid clot development on TEG (shortened R-time, or reaction time) indicates hypercoagulability, meaning the blood is clotting too quickly.

• In a post-cardiac surgery patient, this suggests insufficient anticoagulation (e.g., inadequate heparin reversal or prothrombotic state).

• This increases the risk of postoperative thrombosis (e.g., graft occlusion, stroke).

Why Not the Others?

• A) Over-anticoagulation would show prolonged R-time (slower clot initiation).

• B) Adequate anticoagulation would show normal R-time and clot kinetics.

• D) Platelet disorder would primarily affect MA (maximum amplitude, clot strength), not the speed of clot initiation.

• Severe hemophilia A is defined by Factor VIII activity levels <1% of normal.

  • Patients experience spontaneous bleeding (joints, muscles, CNS) and prolonged hemorrhage after trauma/surgery.

  • Requires prophylactic Factor VIII replacement to prevent crippling arthropathy.

Why Not the Others?

• B) 1–5% – Moderate hemophilia A (bleeding with minor trauma).

• C) 6–30% – Mild hemophilia A (bleeding only after major trauma/surgery).

• D) >30% – Normal range (asymptomatic carriers may have 30–50%).

• Type I protein S deficiency is characterized by low levels of both free protein S antigen and activity, with normal C4b-binding protein.

  • This indicates a quantitative deficiency (reduced synthesis or increased clearance of protein S).

• Type II (C) would show normal antigen levels but reduced activity (functional defect).

• Type III (D) would show low free protein S but normal total protein S (due to increased C4b-binding protein, which is not the case here).

Why Not the Others?

• A) No deficiency – Incorrect, since both activity and antigen are low.

• C) Type II – Functional defect (normal antigen, low activity).

• D) Type III – Only free protein S is low (total protein S is normal), but this requires high C4b-binding protein, which is normal in this case.

• Aspirin irreversibly inhibits cyclooxygenase-1 (COX-1), blocking thromboxane A2 (TXA2) production, which is required for platelet activation and aggregation.

• Arachidonic acid is the substrate for COX-1, so aspirin ingestion leads to markedly reduced platelet aggregation in response to arachidonic acid .

Why Not the Others?

• A) Normal bleeding time: Aspirin prolongs bleeding time due to impaired platelet function (though this test is rarely used now) .

• B) Increased ristocetin aggregation: Ristocetin-induced aggregation depends on VWF-GPIb interaction and is unaffected by aspirin .

• D) Decreased D-dimer: D-dimer reflects fibrinolysis (e.g., DIC, VTE); aspirin has no effect on fibrin breakdown

• Heparin exerts its anticoagulant effect by potentiating antithrombin, which primarily inhibits Factor Xa and thrombin (Factor IIa) in the intrinsic pathway.

• The aPTT is the standard test to monitor unfractionated heparin (UFH) therapy, with a target range typically 1.5–2.5x the control value .

Why Not the Others?

• A) PT (Prothrombin Time): Measures the extrinsic pathway; insensitive to heparin.

• C) INR (International Normalized Ratio): Used for warfarin monitoring, not heparin.

• D) Bleeding time: Reflects platelet function; unaffected by heparin.

• Platelet aggregation tests evaluate how well platelets clump together in response to agonists (e.g., ADP, collagen, ristocetin).

• Bernard-Soulier syndrome (C) is a platelet function disorder caused by a GP1b/IX/V receptor defect, impairing ristocetin-induced aggregation (but normal response to other agonists).

Why Not the Others?

• A) Thalassemia → Diagnosed by hemoglobin electrophoresis, not platelet function.

• B) Hemophilia → A coagulation factor deficiency (VIII/IX); platelet aggregation is normal.

• D) DIC → Diagnosed by D-dimer, PT/aPTT, platelet count, and fibrinogen—not aggregation studies.

• Type 2B von Willebrand disease (VWD) is caused by gain-of-function mutations in the vWF A1 domain, leading to:

  • Enhanced binding of vWF to platelets (even at low ristocetin concentrations).

  • Thrombocytopenia (due to spontaneous platelet aggregation and clearance).

• Diagnostic test:

  • Low-dose ristocetin (0.5 mg/mL) induces abnormally high platelet aggregation (unlike other VWD types).

  • Lab findings:

    • ↓ vWF antigen, ↓ ristocetin cofactor activity, ↓ large vWF multimers (consumed by platelets).

Why Not the Others?

• A) Type 1 – Normal aggregation (quantitative deficiency only).

• B) Type 2A – Reduced aggregation (loss-of-function mutation → defective platelet binding).

• D) Type 2N – Normal aggregation (defect in FVIII binding, mimics hemophilia A).

• Glanzmann thrombasthenia is caused by a deficiency or dysfunction of platelet glycoprotein IIb/IIIa (GPIIb/IIIa), the receptor for fibrinogen.

• This defect prevents platelet aggregation in response to ADP, collagen, and epinephrine, while ristocetin-induced aggregation remains normal (as it depends on GPIb, not GPIIb/IIIa).

Why Not the Others?

• A) von Willebrand disease: Shows normal ADP aggregation but reduced ristocetin aggregation (due to VWF dysfunction).

• C) Bernard-Soulier syndrome: Has normal ADP aggregation but absent ristocetin aggregation (GPIb/IX/V defect).

• D) Factor VIII deficiency: Does not affect platelet function (prolongs aPTT only).

• Primary hemostasis involves platelet plug formation at the site of vascular injury and is evaluated by tests that assess platelet number and function:

  • Bleeding time (A): Measures platelet adhesion/aggregation (though rarely used now).

  • Platelet count (B): Quantifies platelet number.

  • Platelet aggregation studies (C): Assess platelet response to agonists (e.g., ADP, collagen).

• PT (D) evaluates the extrinsic coagulation pathway (Factor VII) and is part of secondary hemostasis (fibrin clot formation), not primary hemostasis .

Why Not the Others?

• A, B, C: All directly assess platelet-related aspects of primary hemostasis.

• D) PT: Reflects plasma coagulation factors, unrelated to platelet function .

• Lupus anticoagulant (LA) is detected by phospholipid-dependent tests (e.g., aPTT, dRVVT) that use low phospholipid concentrations to enhance sensitivity.

  • LA antibodies bind phospholipids, prolonging clotting times in low-phospholipid reagents.

  • High-phospholipid reagents (confirmatory step) neutralize LA, shortening the clotting time.

• The aPTT LA-sensitive reagent is designed with just enough phospholipid to detect LA interference.

Why Not the Others?

• A) No phospholipid – Clotting would fail (phospholipids are essential for coagulation).

• B) High phospholipid – Masks LA effects (used in confirmatory tests).

• D) Heparin neutralizer – Irrelevant (LA testing requires heparin-free samples).

The patient’s lab results (prolonged PT/aPTT, low fibrinogen, thrombocytopenia, and elevated D-dimer) are classic for DIC, a consumptive coagulopathy where widespread clotting depletes platelets and coagulation factors, followed by secondary fibrinolysis (high D-dimer).

Why Not the Others?

• A) Liver disease: May cause prolonged PT/aPTT and low fibrinogen, but typically platelets are normal/mildly reduced, and D-dimer is not elevated unless concurrent DIC.

• B) Hemophilia: Prolongs aPTT only (intrinsic pathway defect) with normal PT, fibrinogen, platelets, and D-dimer.

• C) von Willebrand disease: Usually presents with normal PT/aPTT (unless severe Type 3) and normal fibrinogen/D-dimer; primary issue is platelet dysfunction

• Factor XIII is a transglutaminase that cross-links fibrin polymers by forming covalent bonds between lysine and glutamine residues, stabilizing the clot and making it resistant to fibrinolysis .

• Deficiency leads to fragile clots and delayed bleeding (e.g., umbilical stump hemorrhage) despite normal PT/aPTT .

Why Not the Others?

• A) Factor VIII: Cofactor for Factor IX in intrinsic pathway (no role in fibrin cross-linking).

• C) Factor V: Cofactor for Factor X in prothrombinase complex (common pathway).

• D) Factor X: Activates prothrombin (Factor II) but does not modify fibrin structure.

• PT (Prothrombin Time) is the standard test to monitor warfarin therapy because warfarin inhibits vitamin K-dependent factors (II, VII, IX, X), with Factor VII (short half-life) affecting PT first.

• Results are reported as INR (International Normalized Ratio) for standardization (therapeutic range typically 2.0–3.0 for most conditions).

Why Not the Others?

• A) Heparin: Monitored by aPTT (or anti-Xa assay for LMWH).

• C) Aspirin: No effect on PT; assessed by platelet function tests (e.g., PFA-100).

• D) Streptokinase (thrombolytic): Monitored clinically or with fibrinogen/D-dimer, not PT.

• Unfractionated heparin (UFH) works by activating antithrombin (AT) to inhibit Factor Xa and thrombin (IIa).

• The aPTT is the standard test for monitoring UFH, but in some cases (e.g., subtherapeutic aPTT with suspected heparin resistance, antithrombin deficiency, or lupus anticoagulant interference), the anti-Xa assay is more reliable.

• Anti-Xa assay directly measures heparin’s effect on Factor Xa inhibition and is not affected by conditions that alter aPTT (e.g., lupus anticoagulant, factor deficiencies).

Why not the others?

• A) Anti-IIa assay – Measures thrombin inhibition but is less commonly used than anti-Xa for heparin monitoring.

• B) Platelet count – Used to check for heparin-induced thrombocytopenia (HIT), not for heparin efficacy.

• D) PT (Prothrombin Time) – Monitors warfarin, not heparin.

The PT (Prothrombin Time) test evaluates the extrinsic coagulation pathway, which is initiated by tissue factor (Factor III) and involves Factor VII.

• aPTT (A) assesses the intrinsic pathway.

• TT (C) measures the conversion of fibrinogen to fibrin (common pathway).

• Fibrinogen assay (D) quantifies fibrinogen levels but does not specifically test the extrinsic pathway.

• Acute DIC (Disseminated Intravascular Coagulation) is characterized by consumption of platelets and clotting factors, leading to:

  • Thrombocytopenia (low platelets due to microthrombi formation).

  • Prolonged PT/aPTT (factor depletion).

  • Elevated D-dimer (fibrinolysis of microclots).

  • Low fibrinogen (consumed by widespread clotting).

Why Not the Others?

• A) Increased platelet count – Occurs in reactive thrombocytosis (e.g., inflammation), not DIC.

• B) Shortened PT – Suggests hypercoagulability (e.g., acute phase reaction), not DIC.

• C) Decreased D-dimer – Rules out DIC (D-dimer is always elevated in DIC due to fibrinolysis).

• In COVID-19, a hypercoagulable state develops due to endothelial injury, inflammation, and thrombin generation, leading to venous/arterial thrombosis (e.g., PE, DVT, stroke).

• D-dimer (a fibrin degradation product) is the most reliable predictor of mortality because:

  • It reflects ongoing thrombosis and fibrinolysis.

  • Elevated levels correlate with disease severity (higher in ICU patients/non-survivors).

  • Studies show D-dimer >2-4x upper limit of normal predicts poor outcomes (ARDS, death).

Why Not Others?

• A) PT (Prothrombin Time) – May be prolonged in DIC/sepsis but less specific for COVID-19 thrombosis.

• B) aPTT – Often normal or mildly prolonged; not a strong prognostic marker.

• D) Bleeding time – Irrelevant in COVID-19 (not used in coagulopathy assessment).

• The secondary wave in ADP- or epinephrine-induced platelet aggregation reflects dense granule secretion (e.g., ADP, serotonin, calcium), which amplifies platelet activation and recruitment.

  • Primary wave: Initial reversible aggregation (GPIIb/IIIa activation by weak agonists).

  • Secondary wave: Irreversible aggregation (driven by secreted ADP/TXA₂ feedback loops) .

Why Not the Others?

• A) Fibrin formation: Occurs during coagulation (not platelet aggregation).

• B) Platelet shape change: Happens before aggregation (pseudopodia formation).

• D) Clot retraction: Mediated by actin-myosin contraction (post-aggregation) .

• The International Normalized Ratio (INR) was developed to standardize PT (Prothrombin Time) results across different laboratories and thromboplastin reagents.

• It adjusts for variations in thromboplastin sensitivity using the formula:
  INR = (Patient PT / Mean Normal PT)^ISI
  where ISI (International Sensitivity Index) is specific to the reagent/instrument combination .

Why Not the Others?

• A) aPTT: Reported in seconds (no INR equivalent; sensitivity varies by reagent).

• B) Thrombin time: Measures fibrinogen conversion; no standardization needed.

• D) Bleeding time: A platelet function test; results are in minutes.

• Factor assays are performed to identify specific clotting factor deficiencies when screening tests (PT/aPTT) are abnormal.

  • Prolonged PT + aPTT: Suggests a common pathway factor deficiency (e.g., Factor X, V, II, or fibrinogen) or combined deficiencies.

  • Factor assays quantify individual factor levels (e.g., Factor VIII, IX, X) to pinpoint the defect.

Why Not the Others?

• B) PT prolonged, aPTT normal: Likely Factor VII deficiency (extrinsic pathway only)—still may need factor assay but less complex.

• C) Bleeding time prolonged: Indicates platelet dysfunction (e.g., von Willebrand disease)—test VWF activity/ristocetin cofactor instead.

• D) Low fibrinogen: Diagnosed by fibrinogen assay, not factor assays for other factors.

• A shortened aPTT indicates accelerated intrinsic pathway clotting, often due to:

  1. Elevated acute phase proteins (e.g., Factor VIII, fibrinogen) during inflammation, infection, or stress.

  2. Hypercoagulable states (e.g., early DIC, cancer).

• Factor VIII (an acute phase reactant) is the most common cause, as it accelerates Factor X activation.

Why Not the Others?

• A) Factor XII deficiency – Prolongs aPTT (no bleeding risk).

• B) Anticoagulant therapy (e.g., heparin) – Prolongs aPTT.

• D) Vitamin K deficiency – Affects extrinsic pathway (prolongs PT), not aPTT.

• Factor XIII deficiency causes delayed bleeding (e.g., 24–72 hours post-trauma/surgery) due to impaired fibrin clot stabilization, despite normal screening tests (PT, aPTT, platelets, fibrinogen).

  • FXIII crosslinks fibrin, preventing premature clot breakdown.

  • Diagnosis:

    • Urea solubility test (clot dissolves in 5M urea if FXIII is deficient).

    • FXIII activity assay (gold standard).

Why Not the Others?

• A) von Willebrand disease – Typically causes mucosal bleeding and may prolong bleeding time (but aPTT can be normal in mild cases).

• C) DIC – Would show abnormal PT/aPTT, low platelets/fibrinogen.

• D) Hemophilia A – Prolongs aPTT.

• Factor VIII is a critical cofactor in the intrinsic coagulation pathway, accelerating the activation of Factor X (with Factor IXa).

• Elevated Factor VIII levels increase the speed of clot formation, leading to a shortened aPTT (activated partial thromboplastin time).

• This effect is independent of bleeding/thrombotic risk—high Factor VIII alone does not confirm hypercoagulability but may contribute to thrombotic tendency.

Why Not the Others?

• A) Prolong aPTT – Only occurs with Factor VIII deficiency (e.g., hemophilia A) or inhibitors.

• B) No change – Incorrect; Factor VIII directly impacts the intrinsic pathway measured by aPTT.

• D) Invalid result – Unrelated; invalid results arise from technical errors (e.g., hemolysis, heparin contamination).

• The clot retraction test assesses the ability of platelets to contract (via actin/myosin filaments), pulling fibrin strands together and squeezing out serum from the clot.

• It evaluates platelet function, particularly the integrity of contractile proteins (e.g., thrombosthenin) and GPIIb/IIIa (fibrinogen receptor).

Why Not the Others?

• A) Platelet count: Measured by CBC; clot retraction is independent of platelet number (unless severe thrombocytopenia).

• C) Fibrinogen levels: Tested via fibrinogen assay; clot retraction requires normal fibrinogen but doesn’t measure it directly.

• D) Coagulation factors: Evaluated by PT/aPTT; clot retraction depends on platelets, not soluble factors.

Thromboelastography (TEG) assesses whole blood coagulation dynamics, including:

• Clot initiation (time to start clotting, influenced by coagulation factors),

• Clot strength (platelet and fibrinogen contribution),

• Fibrinolysis (clot breakdown).

Unlike isolated tests (e.g., platelet count or PT/INR), TEG provides a comprehensive evaluation of the entire clotting process.

Why not others?

• A) TEG does not directly measure platelet count or fibrinogen levels (though it reflects their functional impact).

• C) TEG detects clot formation abnormalities but does not specifically identify individual factor deficiencies.

• D) Platelet adhesion is only one aspect; TEG evaluates broader platelet function within clot strength.

• The PFA-100 is the most clinically useful screening test for aspirin-induced platelet dysfunction because it:

  • Measures closure time in response to collagen/epinephrine or collagen/ADP.

  • Is sensitive to aspirin’s COX-1 inhibition (prolongs epinephrine closure time).

Why Not the Others?

• A) PT & B) aPTT: Unaffected by aspirin (evaluate plasma coagulation, not platelet function).

• D) Platelet count: Normal in aspirin therapy (aspirin affects function, not number).

• The thrombin time (TT) test measures how long it takes for fibrinogen to be converted into fibrin when thrombin is added.

• It evaluates the final step of the coagulation cascade and helps detect abnormalities in fibrinogen or inhibitors like heparin or fibrin degradation products (FDPs).

Why Not the Others?

• B) Intrinsic factor activity → Assessed by aPTT, not TT.

• C) Platelet aggregation → Tested via platelet function assays (e.g., PFA-100).

• D) D-dimer → Measures fibrin breakdown (used for DIC or VTE diagnosis), not fibrin formation.

• Activated Protein C Resistance (APCR) is most commonly caused by the Factor V Leiden (FVL) mutation (a G1691A missense mutation in the F5 gene), which makes Factor Va resistant to inactivation by Activated Protein C (APC) .

• Molecular testing for Factor V Leiden is the gold standard to confirm hereditary APCR, as it directly detects the R506Q substitution in Factor V .

• Functional APCR assays (e.g., aPTT-based tests) are screening tools, but DNA-based testing (e.g., PCR) is required for definitive diagnosis .

Why Not the Others?

• A) MTHFR mutation: Associated with hyperhomocysteinemia, not APCR.

• B) PAI-1 polymorphism: Linked to fibrinolysis disorders, not APCR.

• D) Prothrombin G20210A: A thrombophilic mutation unrelated to APCR

• Immediate correction of aPTT after mixing suggests the initial prolongation was due to a factor deficiency (since adding normal plasma provides missing factors).

• Prolongation after incubation (typically 1–2 hours) indicates a time-dependent inhibitor, such as:

  • Factor VIII inhibitor (acquired hemophilia A)

  • Lupus anticoagulant (antiphospholipid antibody)

• These inhibitors take time to neutralize the added factors in the normal plasma.

Why Not the Others?

• A) Factor deficiency – Would correct immediately and remain corrected after incubation.

• C) Low fibrinogen – Affects thrombin time (TT) or clot firmness, not aPTT in this pattern.

• D) Antithrombin deficiency – Does not cause aPTT prolongation in mixing studies

• Heparin resistance occurs when standard heparin doses fail to achieve therapeutic anticoagulation (e.g., inadequate aPTT prolongation).

• The most common cause is antithrombin (AT) deficiency, because:

  • Heparin binds to AT to potentiate its inhibition of thrombin (IIa) and Factor Xa.

  • Low AT levels (congenital or acquired) reduce heparin’s effectiveness, requiring higher doses or AT concentrate.

Why Not the Others?

• A) Protein C – Deficiency causes thrombophilia but does not impair heparin’s mechanism.

• B) Protein S – Similar to Protein C; unrelated to heparin’s action.

• D) Factor VII – Extrinsic pathway factor; deficiency prolongs PT but does not affect heparin.

• The Clauss method is the gold standard for measuring fibrinogen levels.

• Thrombin is added directly to diluted patient plasma, converting fibrinogen into fibrin.

• The time taken for clot formation is measured and compared to a standard curve to determine fibrinogen concentration.

Why Not the Others?

• A) Calcium – Used in PT/aPTT tests but not in the Clauss method.

• B) Kaolin – An activator in aPTT testing, not fibrinogen assays.

• D) Tissue thromboplastin – Used in PT testing (extrinsic pathway activation).

• D-dimer is a specific marker for the breakdown of cross-linked fibrin (not fibrinogen), indicating fibrinolysis due to clot formation.

• It is generated when plasmin cleaves cross-linked fibrin (from stabilized clots), releasing D-dimer fragments.

Why Not the Others?

• A) Fibrinogen degradation: Produces FDPs (fibrin degradation products), but D-dimer is not elevated because fibrinogen lacks cross-linking.

• B) Platelet lysis: Unrelated to D-dimer; measured by platelet activation markers (e.g., PF4, β-thromboglobulin).

• D) Von Willebrand factor activity: Assessed by VWF antigen or ristocetin cofactor assay, not D-dimer

The pattern described in the question — normal aggregation with ristocetin but poor response to ADP and collagen — is classic for Glanzmann thrombasthenia, a rare inherited platelet function disorder.

Option Why it’s incorrect here

B) von Willebrand disease Poor ristocetin response is expected
C) Bernard-Soulier syndrome Also shows abnormal ristocetin response due to GpIb defect
D) Storage pool disease Shows reduced response to multiple agonists, including ristocetin (variable)

• Ristocetin-induced platelet aggregation specifically tests the interaction between von Willebrand factor (VWF) and platelet glycoprotein Ib (GPIb).

• Ristocetin promotes VWF binding to GPIb, enabling platelet aggregation.

• Abnormal results indicate:

  • VWF deficiency/dysfunction (von Willebrand disease) or

  • GPIb receptor defect (Bernard-Soulier syndrome).

Why Not the Others?

• A) Fibrinogen: Binds to GPIIb/IIIa (tested with ADP/agonists, not ristocetin).

• B) Factor VIII: Carrier protein for VWF but not directly involved in ristocetin aggregation.

• D) GPIIb/IIIa: Mediates fibrinogen binding (defective in Glanzmann thrombasthenia), but ristocetin aggregation is normal in this disorder.

• Aspirin irreversibly inhibits cyclooxygenase-1 (COX-1), reducing thromboxane A2 (TXA2) production and impairing platelet aggregation.

• The PFA-100 (Platelet Function Analyzer) is highly sensitive to aspirin’s effects, showing:

  • Prolonged closure time with the collagen/epinephrine cartridge (aspirin blocks TXA2-dependent aggregation).

  • Normal closure time with collagen/ADP (ADP bypasses COX-1 inhibition) .

Why Not the Others?

• A) Platelet count: Unaffected by aspirin (alters function, not number).

• C) PT & D) aPTT: Measure plasma coagulation (unaffected by aspirin’s platelet-specific action) .

• The Bethesda assay is the gold standard for quantifying Factor VIII inhibitors (alloantibodies in hemophilia A or autoantibodies in acquired hemophilia).

• How it works:

  1. Patient plasma is incubated with normal pooled plasma (containing FVIII) at 37°C for 2 hours.

  2. Residual FVIII activity is measured and compared to a control (without inhibitor).

  3. 1 Bethesda Unit (BU) = the amount of inhibitor that neutralizes 50% of FVIII in normal plasma.

Why Not the Others?

• A) ELISA – Detects antibody presence but not functional titer (used for non-neutralizing antibodies).

• B) Clauss assay – Measures fibrinogen, not inhibitors.

• D) Mixing study – Screens for inhibitors but does not quantify them.

• The International Sensitivity Index (ISI) reflects a PT reagent’s responsiveness to vitamin K-dependent factor deficiencies (II, VII, X).

  • Lower ISI = Higher sensitivity:

    • ISI 1.0: The reagent is highly sensitive (closer to the WHO reference thromboplastin).

    • ISI 2.1: The reagent is less sensitive (requires a greater factor deficiency to prolong PT).

• Impact on INR:

  • With the same patient sample, the new reagent (ISI 1.0) will yield a lower INR than the old reagent (ISI 2.1).

Why Not the Others?

• A) Less sensitive – Incorrect; higher ISI values indicate lower sensitivity.

• C) Unusable – Reagents with ISI 0.9–1.7 are acceptable for clinical use.

• D) Equivalent – ISI differences >0.5 require recalibration of INR calculations.

• Antithrombin (AT) is a natural anticoagulant that primarily inhibits serine protease clotting factors, including thrombin (IIa) and Factors Xa, IXa, XIa, and XIIa.

• Heparin enhances this inhibition (choice D is partially correct, but heparin is a cofactor, not the target).

• Biological assays measure AT’s ability to neutralize serine proteases (e.g., thrombin or Factor Xa) in a heparin-dependent or heparin-independent manner.

Why Not the Others?

• A) Factor VIII – Not directly inhibited by AT.

• B) Protein C – An anticoagulant itself, not a target of AT.

• D) Heparin – A cofactor that boosts AT activity but is not the enzyme being inhibited.

• Vitamin K deficiency leads to reduced synthesis of vitamin K-dependent factors (II, VII, IX, X) because vitamin K is required for their γ-carboxylation (critical for clotting function).

• Antibiotics (especially broad-spectrum) can cause vitamin K deficiency by:

  • Killing gut flora that produce vitamin K₂.

  • Reducing dietary absorption (if oral intake is already low).

• Normal Factor V and VIII (non-vitamin K-dependent) rules out liver disease (which would affect all factors).

Why Not the Others?

• A) Liver disease – Would also lower Factor V and VIII (synthesized in the liver).

• B) Platelet dysfunction – Does not affect PT/aPTT or clotting factor levels.

• D) Hemophilia A – Causes isolated Factor VIII deficiency (aPTT prolonged only).

• Clopidogrel is a P2Y12 receptor antagonist that specifically inhibits ADP-induced platelet activation and aggregation.

• Platelet aggregation studies in a clopidogrel-treated patient would show:

  • Markedly reduced aggregation with ADP (due to blocked P2Y12 signaling).

  • Normal response to collagen, epinephrine, and ristocetin (these agonists act via other pathways) .

Why Not the Others?

• A) Collagen: Aggregation is mediated by GPVI and α2β1 integrin (unaffected by clopidogrel).

• B) Epinephrine: Acts via α2-adrenergic receptors (normal response expected).

• C) Ristocetin: Depends on VWF-GPIb interaction (unrelated to ADP signaling) .

• A prolonged aPTT with normal PT that does not correct with mixing (50:50 normal plasma) strongly suggests the presence of an inhibitor rather than a factor deficiency.

• Lupus anticoagulant (C) is an acquired antiphospholipid antibody that interferes with phospholipid-dependent clotting tests (e.g., aPTT) but paradoxically causes thrombosis, not bleeding.

Why Not the Others?

• A) Heparin contamination: Also causes non-correcting aPTT, but typically prolongs thrombin time (TT) and is reversed by heparinase.

• B) Factor deficiency: Would correct with mixing (e.g., Hemophilia A/B).

• D) DIC: Usually prolongs both PT and aPTT with low platelets/fibrinogen, and mixing studies are not the primary diagnostic tool.

• A mixing study involves mixing the patient’s plasma with normal pooled plasma (which contains all clotting factors).

  • If the prolonged clotting time (PT/aPTT) corrects → Indicates a factor deficiency (missing factors are replaced).

  • If it does not correct → Suggests an inhibitor (e.g., lupus anticoagulant, heparin, or factor-specific antibodies like in hemophilia).

Why Not the Others?

• A) Bleeding time → Assesses platelet function, not inhibitors.

• C) Fibrinogen assay → Measures fibrinogen levels, not inhibitors.

• D) Thrombin time → Evaluates fibrinogen conversion, not inhibitors (except heparin).

The Fibrin Degradation Product (FDP) test detects fragments that result from the breakdown of fibrin during fibrinolysis. Specifically, it reflects the degradation of cross-linked fibrin, which is formed after fibrinogen is converted to fibrin and stabilized by Factor XIII.

Key Point:

• FDPs (including D-dimer) indicate active clot formation and breakdown, such as in:

  • DIC (Disseminated Intravascular Coagulation)

  • Deep vein thrombosis (DVT)

  • Pulmonary embolism (PE)

Option Breakdown:

• A) Platelet fragments 
  → Not related to fibrinolysis; detected by other methods like flow cytometry

• B) Cross-linked fibrin 
  → Correct. FDPs are products of degraded cross-linked fibrin

• C) Thrombin inhibitors 
  → These are medications (e.g., dabigatran) or physiological inhibitors (e.g., antithrombin), not detected by FDP test

• D) Uncross-linked fibrin monomers 
  → FDP test focuses on breakdown products of cross-linked fibrin

• Lupus anticoagulant (LA) is an antiphospholipid antibody that prolongs phospholipid-dependent clotting tests (e.g., aPTT) but paradoxically causes thrombosis, not bleeding.

• It interferes with in vitro clotting assays by targeting phospholipids, but in vivo, it promotes a prothrombotic state (venous/arterial clots, recurrent miscarriages).

Why Not the Others?

• A) Bleeding: Rare unless combined with other defects (e.g., thrombocytopenia or hypoprothrombinemia syndrome).

• B) Prolonged PT: LA primarily affects aPTT/dRVVT; PT is less commonly prolonged.

• D) Increased fibrinogen: Fibrinogen levels are unrelated to LA.

• The serotonin release assay (SRA) is the gold standard for diagnosing heparin-induced thrombocytopenia (HIT) because it directly measures pathogenic platelet-activating antibodies against PF4/heparin complexes.

  • Positive SRA: >20% serotonin release with low-dose heparin and inhibition with high-dose heparin .

• It has high specificity (~95–100%) but is technically complex and limited to specialized labs .

Why Not the Others?

• A) PF4 ELISA: Detects anti-PF4/heparin antibodies but lacks specificity (many antibodies are non-pathogenic). Used as a first-line screening test due to high sensitivity (~99%) .

• B) Ristocetin cofactor assay: Diagnoses von Willebrand disease (unrelated to HIT).

• C) Reptilase time: Assesses fibrinogen conversion (prolonged in dysfibrinogenemia or thrombin inhibitors; irrelevant to HIT) .

• Factor VII is part of the extrinsic coagulation pathway, which is specifically evaluated by the PT test.

• A deficiency in Factor VII will prolong the PT, while the aPTT, thrombin time, and platelet aggregation remain normal (since they do not depend on Factor VII).

Why Not the Others?

• A) aPTT: Measures the intrinsic pathway (unaffected by Factor VII deficiency).

• C) Thrombin time: Assesses fibrinogen conversion (independent of Factor VII).

• D) Platelet aggregation: Evaluates platelet function (no relation to Factor VII).

• The chromogenic anti-Xa assay quantifies heparin (UFH/LMWH) or direct oral anticoagulants (e.g., rivaroxaban) by measuring their ability to inhibit Factor Xa activity.

• How it works:

  1. Exogenous Factor Xa is added to the plasma sample.

  2. If heparin (or anti-Xa drugs) is present, it binds antithrombin (AT) and inhibits the added Factor Xa.

  3. Residual uninhibited Factor Xa cleaves a synthetic chromogenic substrate, releasing a colored product.

  4. The color intensity (measured spectrophotometrically) is inversely proportional to heparin/anti-Xa drug levels.

Why Not the Others?

• A) Platelet aggregation – Measured by light transmission aggregometry (LTA).

• C) Clot formation rate – Assessed by PT/aPTT/TEG.

• D) Plasma fibrinogen – Measured via Clauss method (thrombin time-based).

• DIC is characterized by widespread thrombosis (consuming platelets and clotting factors) followed by fibrinolysis, leading to:

  • ↑ D-dimer (from cross-linked fibrin breakdown).

  • ↓ Platelets (thrombocytopenia due to consumption).

  • Prolonged PT/aPTT (factor depletion).

• This combination is classic for DIC.

Why Not the Others?

• A) Hemophilia A: Causes prolonged aPTT but normal D-dimer/platelets.

• B) Vitamin K deficiency: Prolongs PT (Factors II, VII, IX, X) but does not elevate D-dimer or cause thrombocytopenia.

• D) Platelet function defect: Causes bleeding but normal D-dimer (no fibrinolysis).

• Modern standard of care for hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency) involves factor replacement therapy using recombinant or plasma-derived factor concentrates.

• Extended half-life (EHL) factor concentrates (e.g., rFVIII-Fc for hemophilia A, rFIX-Fc for hemophilia B) are preferred because they:

  • Reduce dosing frequency (longer duration of action).

  • Improve prophylaxis (preventing bleeds rather than treating them episodically).

  • Enhance quality of life (fewer infusions needed).

Why not the others?

• A) Fresh frozen plasma (FFP) – Outdated for hemophilia; does not provide sufficient factor concentration and carries infection/volume overload risks.

• C) Desmopressin (DDAVP) – Only useful for mild hemophilia A (releases endogenous factor VIII), not for severe cases or hemophilia B.

• D) Antifibrinolytics (e.g., tranexamic acid) – Adjuvant therapy (helps with mucosal bleeding) but not primary treatment for major bleeds.

• The Clauss method is the gold standard for measuring functional fibrinogen levels.

• Thrombin is added directly to diluted plasma, converting fibrinogen → fibrin to form a clot.

• The clotting time is inversely proportional to fibrinogen concentration (shorter time = higher fibrinogen).

Why Not the Others?

• A) Tissue factor – Used in PT assays (extrinsic pathway).

• B) Kaolin – Activates the intrinsic pathway (aPTT).

• D) Calcium – Required for clotting but is not the primary reagent in the Clauss method.

• The primary purpose of a mixing study is to determine whether a prolonged clotting time (PT or aPTT) is caused by a coagulation factor deficiency or the presence of an inhibitor (e.g., lupus anticoagulant, heparin, or factor-specific antibodies).

• Method: Patient plasma is mixed 1:1 with normal pooled plasma (NPP).

  • Correction of the clotting time suggests a factor deficiency (NPP supplies missing factors).

  • No correction suggests an inhibitor (neutralizes factors in both patient and NPP) .

Why Not the Others?

• A) Measure fibrinolysis: Assessed by D-dimer or fibrin degradation products (FDPs), not mixing studies.

• C) Determine platelet count: Requires a CBC, not coagulation testing.

• D) Detect von Willebrand disease: Diagnosed via VWF antigen and ristocetin cofactor activity, though severe VWD with low Factor VIII may prolong aPTT and correct with mixing

• The PFA-100 (Platelet Function Analyzer) evaluates primary hemostasis by measuring:

  • Platelet adhesion (to collagen under flow conditions).

  • Platelet aggregation (in response to ADP/epinephrine).

• It is used to screen for von Willebrand disease (VWD) and platelet function disorders (e.g., aspirin effect, Bernard-Soulier syndrome).

Why Not the Others?

• A) Factor X activity → Measured by PT/aPTT or specific factor assays (not PFA-100).

• C) aPTT → Assesses the intrinsic coagulation pathway (plasma-based, not platelet function).

• D) D-dimer → Reflects fibrin degradation (e.g., DIC, VTE), not platelet activity.

• aPTT (activated Partial Thromboplastin Time) evaluates the intrinsic and common pathways (Factors XII, XI, IX, VIII, X, V, II, and fibrinogen).

• PT (Prothrombin Time) evaluates the extrinsic and common pathways (Factors VII, X, V, II, and fibrinogen).

If aPTT is prolonged but PT is normal, the defect is likely in the intrinsic pathway (not affecting the extrinsic pathway).

• Factor VIII (B) is part of the intrinsic pathway → Its deficiency (e.g., Hemophilia A) prolongs aPTT but leaves PT normal.

• Factor VII (A) → Only affects PT (extrinsic pathway).

• Factor X (C) & Factor V (D) → Affect both PT and aPTT (common pathway).

• Thrombin time (TT) measures the time for fibrinogen → fibrin conversion after adding exogenous thrombin.

• Heparin directly inhibits thrombin, prolonging TT (even at low doses).

Why Not the Others?

• A) Vitamin K: Affects Factors II, VII, IX, X (prolongs PT/aPTT, not TT).

• B) Platelet clumping: Irrelevant to TT (a plasma-based test).

• D) Factor VII deficiency: Prolongs PT, not TT (thrombin bypasses extrinsic/intrinsic pathways)

• Protein C (along with protein S and antithrombin) is a natural anticoagulant that regulates clotting by inactivating Factors Va and VIIIa.

• Deficiency of protein C leads to uncontrolled thrombin generation, increasing the risk of venous thrombosis (e.g., DVT, PE).

• It can be inherited (autosomal dominant) or acquired (e.g., warfarin-induced).

Why Not the Others?

• A) Fibrinogen deficiency → Causes bleeding, not thrombosis.

• B) Platelet dysfunction → Leads to bleeding disorders (e.g., von Willebrand disease).

• D) Factor VIII deficiency (Hemophilia A) → Causes bleeding, not clotting.

• Low molecular weight heparin (LMWH) primarily inhibits Factor Xa (with minimal effect on thrombin), making the anti-Xa assay the gold standard for monitoring its activity .

  • LMWH does not significantly prolong aPTT or PT, rendering these tests ineffective for monitoring .

  • The anti-Xa assay directly measures heparin activity by quantifying residual Factor Xa inhibition, correlating with LMWH plasma concentrations .

Why Not the Others?

• A) aPTT – Insensitive to LMWH due to its preferential anti-Xa (over anti-IIa) activity .

• B) PT – Reflects the extrinsic pathway; unaffected by LMWH .

• D) Thrombin time (TT) – Prolonged only by thrombin inhibitors (e.g., UFH), not LMWH

• Glanzmann thrombasthenia is characterized by:

  • Normal ristocetin-induced aggregation (depends on GPIb, which is intact).

  • Abnormal response to ADP, collagen, and epinephrine (due to GPIIb/IIIa deficiency, preventing fibrinogen binding).

Why Not the Others?

• B) von Willebrand disease: Shows reduced ristocetin aggregation (VWF dysfunction) but normal ADP/collagen response.

• C) Bernard-Soulier syndrome: Absent ristocetin aggregation (GPIb/IX/V defect) but normal response to other agonists.

• D) Storage pool deficiency: Reduced ADP/epinephrine aggregation (granule defects) but normal ristocetin response.

• Low molecular weight heparin (LMWH) primarily inhibits Factor Xa, and its anticoagulant effect is best measured by the anti-Xa assay, which quantifies anti-Factor Xa activity .

• The test is calibrated for LMWH (typically targeting 0.5–1.0 IU/mL for prophylaxis, 1.0–2.0 IU/mL for treatment) .

Why Not the Others?

• A) PT & B) aPTT: Insensitive to LMWH (LMWH has minimal effect on these tests due to shorter chains and preferential anti-Xa activity) .

• D) Bleeding time: Reflects platelet function; unaffected by LMWH .

• DIC is a consumptive coagulopathy triggered by sepsis, trauma, or malignancy, characterized by:

  • Prolonged PT/aPTT: Due to consumption of clotting factors.

  • Low fibrinogen: Consumed by widespread clot formation.

  • High D-dimer: Reflects fibrinolysis of microthrombi.

  • Thrombocytopenia (common but not listed in the question).

• Sepsis is the most common cause of DIC, as endotoxins activate coagulation and suppress anticoagulant pathways.