Among the listed options, the ketone test is least affected by contamination from vaginal discharge because:

• Ketones (primarily acetoacetic acid) are metabolic byproducts, not typically present in vaginal secretions.

• The ketone pad relies on the nitroprusside reaction, which is not triggered by proteins, cells, or secretions found in vaginal fluid.

Why the others are more affected:

• a) Protein → Vaginal secretions contain mucus and proteins, which can cause false positives.

• c) Blood → Menstrual contamination or vaginal bleeding can lead to a false positive blood result.

• d) pH → Vaginal discharge has a slightly acidic to neutral pH, which may alter urine pH readings.

• Creatinine is the most reliable marker for assessing hydration status and renal function because:

  • It is a waste product of muscle metabolism exclusively excreted by the kidneys.

  • Blood levels (serum creatinine) reflect glomerular filtration rate (GFR)—a key indicator of kidney function.

  • Urine creatinine (e.g., in urine osmolality or creatinine clearance tests) helps evaluate renal concentrating ability and hydration.

Why Not the Others?

• b) Hydrogen – Not clinically used for hydration/renal assessment.

• c) Sodium – Can indicate electrolyte balance but is influenced by diet, hormones (e.g., aldosterone), and non-renal factors.

• d) Glucose – Reflects glycemic control, not kidney function or hydration.

A positive leukocyte esterase test indicates the presence of white blood cells (WBCs) in urine, specifically detecting the enzyme esterase released from neutrophils.

• It’s a screening tool for pyuria (pus in urine), which can suggest a urinary tract infection (UTI).

Other options:

• a) Erythrocytes → Detected by the blood pad.

• b) Nitrite → Detected by a separate nitrite pad.

• c) Bacteria → May be inferred if both nitrite and leukocyte esterase are positive, but not directly detected by this pad.

• Protein reagent strips (dipsticks) are highly specific for albumin, the most common protein in urine. They use a pH-based color change (e.g., bromophenol blue) that primarily reacts with albumin.

• They are less sensitive for non-albumin proteins (e.g., globulins, Bence-Jones proteins).

Why Not the Others?

• a) SSA test – Detects all proteins (albumin, globulins, Bence-Jones) but is not specific for albumin.

• c) Clinitest – A reducing substances test for glucose/galactose, not protein.

• d) Ictotest  – Detects bilirubin, not protein.

• Hemoglobin (from lysed RBCs) and myoglobin (from muscle breakdown) both contain heme, which reacts with the peroxidase-like activity on the occult blood pad of urine dipsticks.

  • This produces a positive “blood” result even if no intact RBCs are present.

Why Not the Others?

• a) Protein – Hemoglobin/myoglobin are proteins but are not detected by the standard protein pad (which reacts with albumin).

• b) Ketones – Detected by nitroprusside, unrelated to heme.

• d) Leukocyte esterase – Detects WBCs, not hemoglobin/myoglobin.

False positive protein results on urine dipsticks (reagent strips) can occur due to:

1. Highly alkaline urine (pH > 8) – The pH-sensitive dye (e.g., bromophenol blue) in the protein pad can react even in the absence of significant protein.

2. Contamination (e.g., antiseptics like chlorhexidine, quaternary ammonium compounds).

3. Prolonged exposure of the dipstick to urine (overdevelopment).

Why Not the Others?

• a) High glucose – Does not directly cause false-positive protein results (though concentrated glucose can slightly increase specific gravity).

• b) High specific gravity  – Concentrated urine may increase protein detection sensitivity but does not cause false positives.

• d) Low temperature – Cold urine may slow the chemical reaction, potentially causing false negatives, not false positives.

• Reagent strip protein tests (dipsticks) are most sensitive to albumin due to their reliance on the protein error of indicators (e.g., bromophenol blue).

  • Albumin’s positive charge strongly binds to the pH-sensitive dye, triggering a color change.

  • Other proteins (e.g., globulins, Bence-Jones) lack this affinity, leading to false negatives.

Why Not the Others?

• b) Hemoglobin – Detected by the blood/heme pad (peroxidase activity), not the protein pad.

• c) Paraproteins (e.g., Bence-Jones proteins) – Missed by dipsticks (require SSA test).

• d) Mucoproteins  – Typically non-reactive on reagent strips.

• Ictotest is a specific tablet-based test for bilirubin that is more sensitive than standard urine dipstick tests.

• Dipstick tests for bilirubin can miss low levels of bilirubin or be affected by interfering substances (e.g., ascorbic acid).

• Ictotest is used when clinical suspicion of bilirubinuria exists despite a negative dipstick (e.g., early liver disease or mild biliary obstruction).

Why Not the Others?

• a) Urobilinogen  – Detected by dipstick, not Ictotest.

• b) Blood  – Detected by dipstick (not Ictotest).

• d) Ketones  – Detected by dipstick (not Ictotest).

False-positive protein results on urine dipstick tests (which primarily detect albumin via the pH indicator dye method, such as tetrabromophenol blue) can occur in:

• Highly alkaline urine (pH > 8) – The test pad is pH-sensitive, and excessive alkalinity can cause a color change mimicking proteinuria, even when protein is absent.

• Other causes of false positives:

  • Quaternary ammonium compounds (disinfectants).

  • Prolonged exposure of dipstick to urine (overdevelopment).

  • Contamination with chlorhexidine or povidone-iodine.

Why Not the Others?

• a) Glucose-rich urine – Does not interfere with protein tests.

• b) X-ray contrast media – May increase urine specific gravity but does not cause false-positive protein results.

• c) Bence Jones proteins – These are light-chain immunoglobulins seen in multiple myeloma, which are not detected by standard dipsticks (leading to false negatives, not false positives). A sulfosalicylic acid (SSA) test is needed for detection.

The ketone reagent strip (using sodium nitroprusside) primarily detects acetoacetic acid and, to a lesser extent, acetone. However, certain substances can cause false-positive results:

1. Phthalein dyes (e.g., phenolphthalein, bromsulphthalein) – These compounds can react similarly to ketones, leading to false positives.

2. Highly pigmented urine (e.g., from medications like levodopa or phenazopyridine) – May interfere with color interpretation.

Why Not the Others?

• a) Acetone – Detected by some strips (though weakly), so this is a true positive, not a false one.

• b) Beta-hydroxybutyrate – The main ketone in DKA, but not detected by standard urine strips (a common cause of false negatives, not false positives).

• d) Ascorbic acid – More likely to cause false negatives (due to its reducing properties) rather than false positives.

• Ictotest is a specific confirmatory test for bilirubin in urine.

• When bilirubin is present, the diazotized reagent reacts to form a dark purple azo dye on the mat.

• The intensity of the purple color correlates with the bilirubin concentration.

Why Not the Others?

• a) Protein → Detected by tetrabromophenol blue (dipstick) or sulfosalicylic acid test, not Ictotest.

• b) Glucose → Detected by glucose oxidase/peroxidase (dipstick) or Clinitest, not Ictotest.

• c) Ketones → Detected by nitroprusside reaction (e.g., Acetest), not Ictotest.

Key Point:

A dark purple mat is a strong positive for bilirubin, suggesting liver disease or biliary obstruction.

• Uric acid crystals typically form in acidic urine (pH < 6.0). Their presence in alkaline urine (pH 8.5) is unusual and suggests:

  • Inaccurate pH measurement (e.g., contaminated dipstick, delayed testing).

  • Mixed disorder (e.g., transient alkaline urine with residual uric acid crystals from prior acidic urine).

Why Not the Others?

• b) Protein error – Affects protein pad, not pH.

• c) Glucose interference – Does not alter pH or uric acid crystal formation.

• d) SG discrepancy – Specific gravity does not explain this pH/crystal mismatch.

Key Point:

Always repeat pH testing if uric acid crystals appear in alkaline urine. True alkaline urine should favor phosphate/calcium crystals, not uric acid.

• The protein reagent pad (based on the tetrabromophenol blue reaction) is most affected by high urine specific gravity (SG) because:

  • Concentrated urine (high SG) can cause false-positive protein results due to increased acidity or ionic strength, which alters the dye’s color response .

  • Conversely, dilute urine (low SG) may yield false negatives by reducing sensitivity .

Why Not the Others?

• a) Glucose: Unaffected by SG; relies on glucose oxidase/peroxidase reaction.

• b) Ketone: Detects acetoacetate (nitroprusside reaction), independent of SG.

• d) Bilirubin: Uses diazo coupling; SG has minimal impact.

• Occult blood tests (urine dipstick) detect heme via peroxidase-like activity (e.g., hemoglobin or myoglobin).

• Visible RBCs in urine (hematuria) should typically make the test positive, but false negatives can occur due to:

  • Vitamin C (ascorbic acid) – Acts as an antioxidant, inhibiting the chemical reaction on the dipstick, leading to a false-negative result despite RBCs.

  • High nitrite levels (UTI with certain bacteria) can also interfere.

Why Not the Others?

• a) Bleach – May cause false-positive results (oxidizes the test pad).

• c) Salicylates – No significant interference with occult blood tests.

• d) Hemoglobin – Should cause a positive result (detected as “blood” on dipstick).

A positive nitrite test depends on bacterial conversion of nitrates → nitrites by gram-negative bacteria that possess the enzyme nitrate reductase.

Bacteria That Typically Produce a Positive Nitrite Test:

✔ a) E. coli (most common UTI pathogen, strong nitrate reducer)
✔ b) Klebsiella (gram-negative, reduces nitrates)
✔ c) Pseudomonas (gram-negative, can reduce nitrates, though less consistently)

Bacteria That Do NOT Produce a Positive Nitrite Test:

d) Enterococcus (gram-positive, lacks nitrate reductase, does not convert nitrates → nitrites)

Diabetes mellitus, especially when poorly controlled, leads to:

• Hyperglycemia → causes glucosuria (positive glucose test)

• Increased fat metabolism due to lack of insulin → leads to ketone production (positive ketone test

Why the others are incorrect:

• b) Hepatitis → May affect bilirubin or urobilinogen, not glucose and ketones

• c) Urinary tract infection → May show positive nitrite and leukocyte esterase, but not typically glucose or ketones

• d) Starvation → Can cause positive ketones, but usually no glucosuria

The test detects hemoglobin (from intact or lysed red blood cells) or myoglobin (from muscle damage) using a peroxidase-like reaction:

1. Hemoglobin acts as a pseudoperoxidase—it catalyzes the reaction between hydrogen peroxide (H₂O₂) and an organic chromogen (e.g., tetramethylbenzidine).

2. This reaction causes the chromogen to oxidize, producing a color change (typically green to dark blue).

Why Not the Others?

• a) Oxidation of chromogen by peroxidase – Incorrect because the strip does not use true peroxidase; hemoglobin itself acts as a pseudoperoxidase.

• b) Reduction of peroxide – Incorrect because the reaction involves oxidation, not reduction.

• d) Enzymatic hydrolysis – Irrelevant; this principle applies to tests like leukocyte esterase, not blood detection.

• The diazo reaction on urine reagent strips (e.g., Multistix) specifically detects bilirubin.

  • Mechanism: Bilirubin couples with diazotized dichloroaniline to form a purple azo dye.

  • Clinical significance: Confirms conjugated hyperbilirubinemia (liver disease/biliary obstruction).

Why Not the Others?

• a) Ketones: Detected by nitroprusside reaction (Acetest).

• c) Glucose: Uses glucose oxidase/peroxidase reaction.

• d) Protein: Relies on tetrabromophenol blue pH shift.

• The glucose pad on urine reagent strips (e.g., Multistix) relies on a glucose oxidase/peroxidase reaction:

  1. Glucose oxidase converts glucose → gluconic acid + hydrogen peroxide (H₂O₂).

  2. Peroxidase uses H₂O₂ to oxidize a chromogen (e.g., potassium iodide), producing a color change (green to brown).

• This method is specific for glucose (unlike Clinitest, which detects all reducing sugars).

Why Not the Others?

• a) Sodium nitroprusside: Used for ketone detection (Acetest).

• b) Tetramethylbenzidine: The chromogen in blood/leukocyte esterase pads.

• d) Ehrlich’s reagent: Detects urobilinogen/porphobilinogen.

• The nitrite pad on a urine reagent strip tests for nitrite, which is produced when certain bacteria reduce nitrate (normally present in urine) to nitrite.

• This reaction is typically carried out by Gram-negative bacteria, especially:

  • Escherichia coli (E. coli)

  • Klebsiella

  • Proteus

Why other options are incorrect:

• a) White blood cells → Detected by the leukocyte esterase pad, not nitrite.

• b) Gram-positive bacteria → Do not usually reduce nitrate to nitrite.

• d) Crystals → Have no enzymatic activity, so they don’t affect the nitrite test.

• Negative bilirubin in urine means conjugated (direct) bilirubin is not elevated (it is water-soluble and would appear in urine if present).

Why This Option?

• Hemolytic anemia → Excessive RBC breakdown → Increased unconjugated bilirubin (not excreted in urine) → More bilirubin reaches the gut → Increased urobilinogen production → Elevated urobilinogen in urine.

• Since unconjugated bilirubin is not water-soluble, it does not appear in urine, explaining the negative bilirubin result.

Why Not the Others?

• a) Gallstones (biliary obstruction) → Positive bilirubin + Low/Normal urobilinogen (bilirubin can’t reach intestines).

• b) Viral hepatitis → Typically causes both bilirubinuria and elevated urobilinogen (due to liver dysfunction).

• d) Liver cirrhosis → May cause variable urobilinogen (often decreased due to reduced bile flow) and possible bilirubinuria.

• False-positive occult blood results on urine dipsticks can occur due to bleach (chlorine-based cleaners) because:

  • Bleach acts as a strong oxidizing agent, mimicking the peroxidase-like activity of hemoglobin/myoglobin in the test pad.

  • This triggers the color change (e.g., green-blue) even in the absence of blood.

Why Not the Others?

• b) Protein  – Protein does not interfere with the occult blood pad.

• c) Hemoglobin – A true positive (not false). The test detects free Hb or intact RBCs.

• d) Ascorbic acid (Vitamin C)  – Causes false negatives (inhibits the peroxidase reaction).

• The Multistix urobilinogen test uses Erlich’s aldehyde reagent (p-dimethylaminobenzaldehyde), which reacts with urobilinogen to produce a red-orange color.

  • Mechanism: The aldehyde group binds to urobilinogen, forming a colored complex.

  • Clinical relevance: Elevated urobilinogen suggests hemolysis or hepatic dysfunction.

Why Not the Others?

• b) Sodium nitroprusside: Used for ketone detection (Acetest).

• c) Tetramethylbenzidine: The chromogen in occult blood tests.

• d) Ortho-toluidine: An outdated reagent for glucose testing (now replaced by glucose oxidase).

• Urobilinogen is light-sensitive and oxidizes to urobilin when exposed to light, leading to falsely low results.

• Proper handling requires:

  • Protecting the specimen from light (e.g., using amber containers).

  • Testing fresh urine (or refrigerating if delayed).

Why Not the Others?

• a) Neutralized to pH 7 – Urobilinogen stability is not pH-dependent in this range.

• c) Stored at room temperature  – While bacterial growth over time may alter results, light exposure is the primary cause of false lows.

• d) Collected in sterile containers  – Sterility does not affect urobilinogen (unless contaminated bacteria degrade it, which is slower than light oxidation).

• Acetest is a specific tablet test that detects ketones (acetoacetate and acetone) in urine or serum, providing a more reliable confirmation than dipstick tests.

• It is often used when dipstick ketone results are unclear or when higher sensitivity is required (e.g., diabetic ketoacidosis monitoring).

Why Not the Others?

• a) Ictotest – Detects bilirubin, not ketones.

• b) SSA test  – Detects proteins, not ketones.

• d) Clinitest – Detects reducing substances (e.g., glucose, galactose), not ketones.

• Nitrite tests detect bacteria that convert nitrates → nitrites (e.g., E. coli, Proteus, Klebsiella).

• A negative nitrite result despite bacteria on microscopy suggests:

  • Non-nitrate-reducing organisms (e.g., Enterococcus, Staphylococcus, Pseudomonas), which do not produce nitrites.

  • Dilute urine (frequent voiding flushes nitrates).

  • Insufficient dietary nitrates (fasting, low vegetable intake).

Why Not the Others?

• b) Unmixed specimen – Would cause uneven results but not selectively affect nitrites.

• c) Oxidizing detergents – Can cause false-positive nitrites, not false negatives.

• d) Delayed testing – May cause false-negative leukocyte esterase but nitrites are stable for hours.

Key Point:

Leukocyte esterase + bacteria on microscopy is more reliable than nitrites for detecting UTIs caused by non-nitrate-reducing bacteria.

• Standard urine reagent strip tests (e.g., Ketostix, Multistix) detect acetoacetic acid (via nitroprusside reaction) and acetone, but do not measure β-hydroxybutyrate (BHB), the predominant ketone in diabetic ketoacidosis (DKA) and starvation .

  • Clinical impact: In severe DKA, BHB may account for >90% of ketones, leading to false-low urine ketone readings despite critical ketosis .

Why Not the Others?

• a) Acetone: Detected indirectly (converts to acetoacetate at alkaline pH).

• b) Acetoacetic acid: The primary target of nitroprusside-based strips.

• d) None of the above: Incorrect; BHB is missed.

• A false-positive protein result on urine reagent strips (e.g., Multistix) is most commonly caused by alkaline urine (pH >8). The high pH alters the color response of the tetrabromophenol blue indicator, mimicking proteinuria even when none is present .

• Other causes of false positives include:

  • Highly pigmented urine (e.g., hematuria, bilirubinuria).

  • Quaternary ammonium compounds (disinfectant contamination).

Why Not the Others?

• a) Acidic urine: Does not interfere; may cause false negatives for protein if urine is very dilute.

• c) Cold temperature: May delay color development but rarely causes false positives.

• d) Light exposure: Degrades bilirubin/urobilinogen, not protein.

• The SSA (Sulfosalicylic Acid) test is more sensitive than reagent strips for detecting low concentrations of albumin (and other proteins) in dilute urine.

  • It precipitates all proteins, including albumin, globulins, and Bence-Jones proteins, forming turbidity even at low concentrations.

• Reagent strips (dipsticks) (d) are less reliable in dilute urine because they rely on a pH-based color change (bromophenol blue), which may miss trace albumin if urine is too dilute.

Why Not the Others?

• a) Clinitest  – Detects reducing substances (e.g., glucose), not protein.

• c) Ictotest  – Detects bilirubin, not albumin.

• d) Reagent strip  – Less sensitive for trace albumin in dilute urine (optimal for concentrated samples).

• A positive nitrite test in urine occurs when certain bacteria (like E. coli, Proteus, Klebsiella, etc.) convert dietary nitrates (normally present in urine) into nitrites.

• This is a key indicator of a bacterial urinary tract infection (UTI).

Why Not the Others?

• b) Protein breakdown ❌ – Protein metabolism does not produce nitrites.

• c) Urobilinogen reduction ❌ – Urobilinogen is related to bilirubin metabolism, not nitrites.

• d) pH elevation ❌ – While some bacteria that cause UTIs can alter urine pH, pH alone does not cause nitrite formation.

Clinical Note:

• A positive nitrite test + leukocyte esterase strongly suggests a UTI.

• However, some bacteria (like Enterococcus and Staphylococcus) do not convert nitrates, so a negative nitrite test does not rule out infection.

• The ketones reagent pad on urine dipsticks contains sodium nitroprusside, which reacts with acetoacetate (a ketone body) in an alkaline medium to form a purple-colored complex.

• This reaction is the basis for detecting ketonuria (e.g., in diabetic ketoacidosis or starvation).

Why Not the Others?

• a) p-Arsanilic acid – Used in the nitrite test (diazotization reaction).

• b) Acetoacetic acid  – A ketone body detected by the test, not a reagent.

• c) Ehrlich reagent  – Used for urobilinogen detection (turns pink with urobilinogen).

• Nitrite testing is most clinically useful when combined with leukocyte esterase:

  • Nitrite (+) + Leukocyte esterase (+) = High probability of UTI (e.g., E. coli, Klebsiella).

  • Nitrite (-) + Leukocyte esterase (+) = Possible UTI by non-nitrate-reducing bacteria (e.g., Enterococcus).

• Leukocyte esterase detects WBCs, which indicate inflammation/infection, complementing nitrite’s specificity for certain bacteria.

Why Not the Others?

• a) pH – Alkaline pH may suggest Proteus but is nonspecific.

• b) Occult blood – Nonspecific (UTI, stones, trauma).

• c) Specific gravity – Reflects hydration, not infection.

• Bilirubin is highly light-sensitive and degrades rapidly when exposed to light, converting into photoisomers (e.g., lumirubin) that are not detected by standard dipstick or Ictotest methods.

• False-negative bilirubin results can occur if urine is:

  • Left uncovered in bright light (>1 hour).

  • Stored without protection (amber containers are recommended).

Why Not the Others?

• a) pH  – Stable if urine is refrigerated; light exposure has minimal effect.

• b) Protein – Unaffected by light (but can degrade with bacterial overgrowth).

• c) Ketones – Acetoacetate slowly degrades over time, but light is not the primary factor.

• Reagent strip (dipstick) protein tests are highly specific for albumin but may show false positives due to:

  • Highly alkaline urine (pH > 8)

  • Contamination (e.g., chlorhexidine, quaternary ammonium compounds)

  • Pigmented urine (rarely interferes)

• SSA test detects all proteins (albumin + globulins + Bence-Jones), so if it is negative, the dipstick’s positive result is likely not due to true proteinuria.

Why Not the Others?

• a) Bence-Jones proteins – SSA test would be positive (it detects these proteins).

• b) Non-protein error  – This is vague; the discrepancy is better explained by a dipstick false positive.

• d) Hematuria – Blood can cause a false positive dipstick protein, but RBCs do not precipitate in SSA tests (so SSA remains negative).

• A positive blood test on urine reagent strip without RBCs on microscopy strongly suggests myoglobinuria, which occurs when myoglobin (released from muscle breakdown, e.g., rhabdomyolysis) is filtered into the urine.

  • Reagent strips detect heme: Myoglobin contains heme, triggering a positive result.

  • No RBCs on microscopy: Myoglobin is freely filtered (unlike intact RBCs).

Why Not the Others?

• a) Hematuria: Would show RBCs on microscopy.

• b) Hemoglobinuria: Also heme-positive but typically associated with intravascular hemolysis (low serum haptoglobin).

• d) Pyuria: WBCs on microscopy, negative for blood unless concurrent hemorrhage.

• Acetest is a tablet-based test specifically for detecting ketones (acetoacetate) in urine or serum.

• When ketones are present, the sodium nitroprusside in the tablet reacts to form a dark purple color.

• The intensity of the purple correlates with the ketone concentration.

Why Not the Others?

• a) Protein → Detected by tetrabromophenol blue (dipstick) or sulfosalicylic acid test, not Acetest.

• b) Glucose → Detected by glucose oxidase/peroxidase (dipstick), not Acetest.

• d) Bilirubin → Detected by Ictotest, not Acetest.

Key Point:

Acetest is more sensitive than dipsticks for ketones and is used to confirm diabetic ketoacidosis (DKA) or starvation ketosis.

The glucose reagent strip (based on the glucose oxidase reaction) can yield false-negative results in the presence of:

1. Ascorbic acid (vitamin C) – Acts as a reducing agent, interfering with the hydrogen peroxide–chromogen reaction (a key step in the test).

2. High urine specific gravity or low pH – May reduce test sensitivity but are less common causes than ascorbic acid.

3. Delayed urine testing – Glucose can be metabolized by bacteria if the sample sits too long.

Why Not the Others?

• a) Bacterial contamination – Typically causes false negatives only if urine is stored improperly (bacteria consume glucose over time).

• b) Low specific gravity – Unlikely to directly cause false negatives; may dilute glucose but won’t block detection.

• d) Elevated ketones – No direct interference; ketones and glucose often coexist (e.g., in diabetic ketoacidosis).

The SSA (Sulfosalicylic Acid) test uses sulfosalicylic acid as the primary reagent to detect proteins in urine. Here’s how it works:

1. Sulfosalicylic acid is added to urine, which denatures and precipitates proteins, causing turbidity.

2. The degree of turbidity correlates with protein concentration (e.g., trace, 1+, 2+, etc.).

Why Not the Others?

• b) Sodium nitroprusside  – Used in ketone tests (e.g., Acetest) and creatinine assays (Jaffe reaction).

• c) Acetic acid  – Sometimes used in urine sediment prep or urobilinogen tests, but not for SSA protein detection.

• d) Hydrochloric acid  – Used in some chemical tests (e.g., porphobilinogen), but not the SSA test

• Urease-producing bacteria (e.g., Proteus, Klebsiella, Staphylococcus) break down urea in urine into ammonia (NH₃) and CO₂.

  • Ammonia increases urine pH (alkalinization) by accepting H⁺ ions (NH₃ + H₂O → NH₄⁺ + OH⁻).

  • This occurs rapidly at room temperature due to bacterial proliferation.

Why Not the Others?

• a) Nitrite – Produced by nitrate-reducing bacteria (e.g., E. coli) but does not affect pH.

• c) Esterase  – An enzyme released by WBCs (detected by leukocyte esterase tests), unrelated to pH.

• d) Nitrate  – A substrate for nitrite production, not pH changes.

• The blood pad on urine reagent strips (e.g., Multistix) uses a peroxidase-like reaction where hemoglobin or myoglobin catalyzes the oxidation of a chromogen (e.g., tetramethylbenzidine).

  • Positive result: The pad turns from yellow to blue-green (graded as trace, 1+, 2+, etc.).

  • Mechanism: Heme groups in hemoglobin/myoglobin act as pseudoperoxidases .

Why Not the Others?

• a) Orange to blue: Describes pH indicators (not blood).

• b) Yellow to green: Seen with leukocyte esterase (WBCs).

• d) Red to brown: Occurs with bilirubin (diazo reaction).

The Sulfosalicylic Acid (SSA) test is a qualitative/quantitative method to detect proteins in urine, including:

• Albumin (the main protein detected by dipsticks)

• Globulins (e.g., Bence-Jones protein in multiple myeloma)

• Other low-molecular-weight proteins (missed by standard dipstick tests)

Why Not the Others?

• a) Glucose – Detected by glucose oxidase tests (e.g., urine dipstick), not SSA.

• b) Bilirubin  – Detected by dipstick (Ictotest), not SSA.

• d) Ketones  – Detected by nitroprusside reaction (dipstick), not SSA.

In uncontrolled diabetes mellitus, particularly type 1 diabetes, the following occurs:

1. Hyperglycemia → Exceeds renal threshold → Glucose spills into urine (glycosuria).

2. Insulin deficiency → Body breaks down fats for energy → Ketogenesis → Ketones (acetoacetate, beta-hydroxybutyrate, acetone) appear in urine (ketonuria).

This combination (glucose + ketones in urine) is classic for diabetic ketoacidosis (DKA) or severe insulin deficiency.

Why Not the Others?

• a) Liver cirrhosis  – May cause bilirubinuria but not glycosuria/ketonuria.

• c) Acute nephritis – May cause hematuria/proteinuria but not ketones/glucose.

• d) Bacterial UTI  – May cause nitrites/leukocytes but not ketones/glucose.

A false positive nitrite test can occur due to:

1. Bacterial contamination (e.g., improper sample collection, non-sterile container) – Even if no UTI is present, contaminating bacteria can convert nitrates to nitrites.

2. Prolonged urine retention at room temperature – Allows bacteria to multiply and produce nitrites artificially.

Why Not the Others?

• a) Ascorbic acid (Vitamin C) – Causes false negative nitrite tests (interferes with the reaction).

• c) Low nitrate diet  – Leads to false negatives (no substrate for bacteria to convert into nitrites).

• d) Short bladder incubation  – Causes false negatives (insufficient time for bacteria to convert nitrates).

• Red-brown urine + Positive occult blood + No RBCs suggests myoglobinuria, which occurs due to:

  • Rhabdomyolysis (muscle breakdown from trauma, exertion, or toxins).

  • Myoglobin is a heme-containing protein released from muscle, detected as “blood” on dipsticks but no RBCs on microscopy.

Why Not the Others?

• a) Nitrates → Converted to nitrites by bacteria (UTI marker), unrelated to color or heme.

• b) Pyridium (phenazopyridine) → Turns urine orange/red but does not cause a positive occult blood.

• c) Porphyrins → May cause red urine but are not detected by occult blood tests.

• High-dose vitamin C (ascorbic acid) is a reducing agent that can interfere with urine dipstick tests, causing false-negative results for:

  • Bilirubin (vitamin C inhibits the diazotization reaction).

  • Blood (interferes with peroxidase activity).

  • Glucose (competes with the glucose oxidase reaction).

  • Nitrite (reduces nitrite to nitrogen, preventing color development).

Why Not the Others?

• a) pH  – Vitamin C does not affect pH measurement.

• b) Ketones – Vitamin C does not interfere with nitroprusside-based ketone tests.

• d) Specific gravity  – Vitamin C does not alter specific gravity (measured by ionic concentration or refractive index).

• The protein pad on urine reagent strips (e.g., Multistix) uses the protein error of indicators principle, where a pH-sensitive dye (e.g., tetrabromophenol blue) binds to albumin, causing a color change (yellow → green/blue) as the dye’s pKa shifts.

  • Mechanism: Albumin’s positive charges attract the anionic dye, altering its color independent of urine pH .

Why Not the Others?

• a) Protein-copper complex: Used in Biuret tests (quantitative serum protein), not urine strips.

• c) Barium precipitation: Outdated method for sulfosalicylic acid (SSA) testing.

• d) Enzymatic cleavage: Irrelevant; protein strips are non-enzymatic.

Ascorbic acid (vitamin C) can interfere with glucose detection on urine reagent strips because it acts as a reducing agent, competing with the glucose oxidase/peroxidase reaction used in the test. This can lead to false-negative or falsely low glucose readings.

Why the other options are incorrect:

• a) Hemoglobin – May cause interference in blood detection, but not typically in glucose.

• c) Creatinine – Does not interfere with glucose detection.

• d) High pH – May affect protein or other tests but not specifically glucose.

The nitrite test on urine dipsticks is based on the diazotization reaction, which involves the following steps:

1. Bacteria (e.g., E. coli, Klebsiella) convert nitrates → nitrites in urine.

2. Nitrites react with para-arsanilic acid (or similar aromatic amines) on the dipstick pad to form a diazonium compound.

3. This compound then couples with a tetrahydrobenzoquinoline dye, producing a pink/purple color (positive result).

Why Not the Others?

• a) Reduction of nitrate – This is what bacteria do to produce nitrites, but it is not the dipstick’s detection mechanism.

• c) Peroxidase activity  – This principle is used for blood/hemoglobin detection (not nitrites).

• d) Glucose oxidation  – This is the principle of glucose tests (glucose oxidase reaction).

• Clinitest is a copper reduction test that detects all reducing sugars (e.g., glucose, galactose, fructose, lactose), even when the glucose-specific dipstick is negative.

  • Mechanism: Reducing sugars convert cupric sulfate (blue) to cuprous oxide (orange/brown precipitate).

  • Clinical use: Suspected galactosemia, hereditary fructose intolerance, or lactose intolerance in infants .

Why Not the Others?

• a) Ictotest: Confirms bilirubin (not sugars).

• b) Acetest: Detects ketones (not sugars).

• d) SSA test (sulfosalicylic acid): Identifies proteins (not sugars).

Key Points:

• Dipstick limitation: Glucose oxidase pads miss non-glucose reducing sugars.

• False positives: High-dose vitamin C or cephalosporins may interfere.

• Genetic disorders:

  • Galactosemia: Positive Clinitest + negative glucose dipstick in neonates .

  • Renal glycosuria: Glucose in urine despite normal blood glucose.

• A positive leukocyte esterase (indicating white blood cells/WBCs) with a negative nitrite test often suggests:

  • Gram-positive bacteria (e.g., Enterococcus, Staphylococcus saprophyticus), which do not reduce nitrates to nitrites.

  • Non-bacterial causes: Interstitial cystitis, renal tuberculosis, or contaminants (e.g., vaginal WBCs).

Why Not the Others?

• a) No infection: Unlikely with positive leukocyte esterase (pyuria).

• c) Gram-negative infection: Typically nitrite-positive (e.g., E. coli, Proteus reduce nitrates).

• d) Renal failure: Causes isosthenuria/proteinuria, not isolated pyuria.

• Bilirubin is highly light-sensitive and rapidly degrades when exposed to UV light (e.g., sunlight, fluorescent lamps). This can lead to false-negative results on urine dipstick tests (e.g., Multistix) because:

  • Bilirubin oxidizes into biliverdin (green pigment), which does not react with the diazo reagent .

  • Storage tip: Collect urine in amber containers or test immediately to prevent degradation .

Why Not the Others?

• a) Bacteria: May convert bilirubin → urobilinogen but do not directly interfere with the dipstick reaction.

• b) Protein: No effect on bilirubin detection (unlike protein tests, which are pH-sensitive).

• d) Low pH: Stabilizes bilirubin (acidic urine slows degradation).

In tablet ketone tests (such as Legal’s test or Rothera’s test), glacial acetic acid is used to enhance the reactivity of acetone (or other ketones) by providing an acidic medium that facilitates the reaction with nitroprusside or other reagents.

• Lactose (a) and galactose (b) are sugars and do not play a role in enhancing acetone reactivity.

• Ascorbic acid (c) is an antioxidant and can sometimes interfere with tests rather than enhance reactivity.

• Glacial acetic acid (d) creates the optimal acidic conditions for the color reaction in ketone detection tests.

Urobilinogen is detected in urine using reagent strips that contain p-dimethylaminobenzaldehyde, which reacts with urobilinogen to produce a red-orange color (similar to the classic Ehrlich’s test).

Why Not the Others?

• a) p-Arsanilic acid – Used in some nitrite tests, not urobilinogen.

• b) Sodium nitroprusside – Used for ketone detection (Legal’s/Rothera’s test).

• c) Diazotized dichloroaniline – Used for bilirubin detection, not urobilinogen.

• Bilirubin appears in urine when conjugated (direct) bilirubin is elevated (water-soluble).

• Urobilinogen is formed in the intestines from bilirubin and is reabsorbed into the bloodstream.

Interpretation of Results:

• Positive bilirubin + Normal urobilinogen suggests:

  • Biliary obstruction (e.g., gallstones, tumor) → Bilirubin cannot reach the intestines → No urobilinogen is formed.

  • The liver may still be functioning normally, so no excess urobilinogen is produced.

Why Not the Others?

• a) Hemolytic disease → Increased unconjugated bilirubin (not excreted in urine) + High urobilinogen (from excess breakdown of RBCs).

• c) Hepatic disease → May cause both bilirubinuria and variable urobilinogen (depending on liver function).

• d) UTI → Does not directly affect bilirubin or urobilinogen levels.

• Standard urine glucose test strips (using glucose oxidase) detect glucose only and do not react with galactose.

• In infants with galactosemia, galactose accumulates in urine but glucose is normal/absent, leading to a false sense of safety if only glucose is tested.

• Clinitest (a copper reduction test) would detect galactose (and other reducing sugars).

Why Not the Others?

• a) Glucose – Already confirmed negative by the strip.

• c) Bilirubin – Unrelated to carbohydrate metabolism.

• d) Ketones – May occur in starvation but are not specific to galactosemia.

Galactosemia is a medical emergency in infants (liver failure, cataracts). Always confirm with Clinitest or blood tests if galactosemia is suspected.

• Leukocyte esterase urine dipstick tests detect granulocytic esterases (enzymes released by neutrophils).

• They are negative for lymphocytes because:

  • Lymphocytes do not produce these specific enzymes.

  • Lymphocyturia (e.g., in viral infections, transplant rejection) will not trigger a positive result.

Why Not the Others?

• a) Monocytes – Rare in urine; may weakly react but are not the primary target.

• b) Eosinophils – Seen in allergic interstitial nephritis; may produce faint positivity but are not the main source.

• d) Neutrophils – The primary cause of positive leukocyte esterase (pyuria in UTIs).

High concentrations of protein (especially albumin) in urine can interfere with leukocyte esterase test results, potentially leading to false negatives.

Why?

• The leukocyte esterase test detects enzymes released by white blood cells (WBCs).

• Excess protein can:

  • Mask the reaction by coating the reagent pad.

  • Alter urine pH, affecting the test’s sensitivity.

  • Compete with the chemical reaction, reducing color development.

Why Not the Others?

• a) Lactose – No known interference with leukocyte esterase tests.

• c) Ketones – May cause false positives in some tests but not false negatives for leukocyte esterase.

• d) Bilirubin – Can discolor urine but does not typically cause false-negative leukocyte esterase results.

The leukocyte esterase test detects white blood cells (WBCs) in urine, which indicate inflammation (often due to a UTI). When combined with a positive nitrite test, the sensitivity for detecting a UTI increases significantly because:

• Nitrite (+): Suggests bacteria that convert nitrates to nitrites (e.g., E. coli, Klebsiella).

• Leukocyte esterase (+): Indicates WBCs, a sign of immune response to infection.

Together, these two markers improve the accuracy of UTI detection compared to either test alone.

Why Not the Others?

• a) Protein  – Proteinuria can occur due to many conditions (kidney disease, fever, etc.) but does not specifically increase UTI detection sensitivity.

• c) pH  – Alkaline urine may suggest certain bacteria (e.g., Proteus), but pH alone does not enhance leukocyte esterase sensitivity.

• d) Blood  – Hematuria can occur in UTIs but is nonspecific (also seen in stones, trauma, etc.).

The pH test pad on urine reagent strips utilizes a double indicator system to measure urine pH across a broad range (typically pH 5–9). This system combines:

1. Methyl red: Changes from red to yellow in the pH range of 4–6.

2. Bromothymol blue: Changes from yellow to blue in the pH range of 6–9.

Together, these indicators produce a color gradient from orange (pH 5) to yellow/green (neutral) to dark blue (pH 9), allowing precise differentiation of urinary pH levels .

Why Not the Other Options?

• Glucose (a): Uses an enzymatic reaction (glucose oxidase/peroxidase) .

• Blood (b): Relies on the pseudoperoxidase activity of hemoglobin.

• Ketone (d): Based on the sodium nitroprusside reaction for acetoacetic acid

• Urine ketone reagent strips (e.g., nitroprusside-based tests like Ketostix) primarily detect acetoacetic acid.

• They may also detect acetone to a lesser degree, but do not react with beta-hydroxybutyric acid (the predominant ketone in diabetic ketoacidosis).

Why Not the Others?

• a) Acetone – Detected weakly by some strips but is volatile and less clinically significant.

• b) Cholesterol – Not a ketone and not detected.

• d) Beta-hydroxybutyric acid – The major ketone in DKA, but requires blood testing (urine strips miss it).

Key Limitation:

Urine ketone strips may underestimate ketosis in DKA, as beta-hydroxybutyrate dominates. Blood ketone meters are more reliable.

• Ictotest is a tablet-based test specifically designed to detect bilirubin in urine.

• It is more sensitive and specific than standard dipstick tests (Multistix), capable of detecting very low concentrations of bilirubin (as low as 0.05–0.1 mg/dL).

• The test uses a diazotization reaction (bilirubin + diazonium salt → azobilirubin, forming a purple color).

Why Not the Others?

• a) Multistix strip  – Less sensitive for bilirubin (~0.5 mg/dL threshold) and prone to false negatives (e.g., ascorbic acid interference).

• b) Acetest  – Detects ketones, not bilirubin.

• c) SSA test – Detects proteins, not bilirubin.

• Glucose oxidase-based urine dipsticks (most common method) can give falsely low glucose results in the presence of ascorbic acid because:

  • Ascorbic acid is a strong reducing agent that interferes with the peroxidase step of the reaction, preventing the color change.

  • High doses of vitamin C (e.g., supplements) are a common cause of this false-negative effect.

Why Not the Others?

• a) Bleach → May cause false-positive results (oxidizes the test pad).

• b) Galactose → Not detected by glucose oxidase strips (requires Clinitest for detection).

• d) Hydrogen peroxide → Rare in urine; if present, could cause false positives (unlikely to lower glucose results).

Key Takeaway:

If a diabetic patient has symptoms of hyperglycemia but negative/normal urine glucose, suspect vitamin C interference.

A diazonium salt-based reagent pad is commonly used in urinalysis dipsticks to detect bilirubin.

• The diazonium salt reacts with bilirubin to form a colored azo dye, typically producing a tan to pink/purple color in the presence of bilirubin.

• This reaction is highly specific for bilirubin and helps in diagnosing conditions like liver disease or bile duct obstruction.

Why Not the Others?

• b) Ketones: Detected using nitroprusside (Legal’s test/Rothera’s test), not diazonium salts.

• c) Hemoglobin: Detected using peroxidase-like reactions (with tetramethylbenzidine or orthotolidine).

• d) Urobilinogen: Detected using Ehrlich’s reagent (p-dimethylaminobenzaldehyde) or modified diazonium salts, but not the same as direct bilirubin detection.

• Dilute urine (low specific gravity) can lead to a falsely low protein reading because:

  • The protein concentration falls below the dipstick’s detection threshold (e.g., trace amounts become undetectable).

  • Reagent strips (e.g., bromophenol blue) are less sensitive in dilute urine.

Why Not the Others?

• a) Albumin-rich samples  – These yield true positives (dipsticks are highly albumin-specific).

• b) Mucus-containing samples  – May cause false positives (turbidity mimics protein in SSA tests), but not false negatives.

• c) Concentrated urine – Increases protein detection sensitivity (may reveal trace amounts).

• The pH reagent pad in urine dipsticks uses a double indicator system:

  • Methyl red (red at pH 4.2–6.3)

  • Bromthymol blue (yellow at pH 6.0–7.6)

• Together, they provide a broad pH range (5–9) with color changes:

  • Acidic urine (pH 5–6) → Orange/yellow

  • Neutral (pH 7) → Green

  • Alkaline (pH 8–9) → Blue

Why Not the Others?

• a) Diazotized dichloroaniline → Used for bilirubin detection.

• c) Glucose oxidase → Enzyme for glucose testing.

• d) Tetrabromophenol blue → Indicator for protein (not pH).

• Urine pH directly measures the concentration of free hydrogen ions (H⁺) in urine.

  • Low pH (acidic urine) = High H⁺ concentration.

  • High pH (alkaline urine) = Low H⁺ concentration.

• The kidneys regulate pH by excreting H⁺ (acid) or bicarbonate (base) to maintain systemic acid-base balance.

Why Not the Others?

• a) Sodium ions – Affects urine osmolality, not pH.

• c) Calcium ions – May influence stone formation but does not determine pH.

• d) Magnesium ions – No direct role in pH measurement.

• cetest is the confirmatory test for ketones (acetoacetate and acetone) in urine or serum.

  • It uses sodium nitroprusside, which reacts with ketones to form a purple color.

  • More reliable than dipsticks for trace ketones or when interfering substances (e.g., sulfhydryl drugs) are suspected.

Why Not the Others?

• a) SSA – Detects proteins, not ketones.

• b) Ictotest – Detects bilirubin, not ketones.

• d) Clinitest – Detects reducing substances (e.g., glucose), not ketones.

The urobilinogen reagent strip test is based on the diazo reaction, where urobilinogen reacts with para-dimethylaminobenzaldehyde (Ehrlich’s reagent) or similar diazonium salts to produce a colored compound (typically pink or red) .

• Other options:

  • Ketones (a): Detected via the nitroprusside reaction (Legal’s test), not diazo.

  • Blood (c): Relies on the pseudoperoxidase activity of hemoglobin to oxidize a chromogen (e.g., tetramethylbenzidine) .

  • Glucose (d): Uses an enzymatic reaction (glucose oxidase/peroxidase)

• Microalbuminuria refers to the excretion of small amounts of albumin (30–300 mg/day) in urine, which is below the detection limit of standard urine dipsticks.

• Specialized microalbumin reagent strips use an immunochemical (immunoassay-based) reaction, such as:

  • Antibody-coated strips (e.g., anti-human albumin antibodies) that bind specifically to albumin.

  • This method provides high sensitivity and specificity for low albumin concentrations.

Why Not the Others?

• a) Diazonium salt – Used for bilirubin detection, not albumin.

• b) Protein error – Standard urine dipsticks (e.g., tetrabromophenol blue) use this principle for total protein, but they lack sensitivity for microalbumin.

• d) p-Arsanilic acid – Used in nitrite tests, not albumin detection.

• Urine reagent strips should be stored at room temperature (20–25°C) in a cool, dry place, tightly sealed to prevent:

  • Moisture absorption (causes premature reagent activation).

  • Heat degradation (alters chemical pads).

  • Light exposure (affects bilirubin/urobilinogen pads).

Why Not the Others?

• a) Frozen (−20°C)  – Freezing damages reagents (cracking pads, denaturing enzymes).

• b) Refrigerated (5°C)  – Condensation can hydrate strips, ruining them.

• d) Incubator (37°C)  – Heat accelerates chemical degradation.

Ketones in a 2-year-old’s urine most commonly suggest:

• Prolonged vomiting (e.g., due to gastroenteritis, starvation, or metabolic disorders) → Leads to starvation ketosis as the body breaks down fat for energy.

• Dehydration (reduced oral intake) exacerbates ketone production.

Why Not the Others?

• b) Hemolytic event → Causes hemoglobinuria/bilirubinuria, not ketonuria.

• c) UTI → Typically causes leukocytes/nitrites in urine, not ketones (unless vomiting occurs).

• d) Biliary obstruction → Causes bilirubinuria, not ketones.

• Bilirubin in urine (positive test): Indicates conjugated (direct) bilirubin, which is water-soluble and appears in urine when there is biliary obstruction (e.g., gallstones, tumors, cholestasis).

• Negative urobilinogen: Normally, bilirubin is converted to urobilinogen in the intestines. If bile flow is blocked (obstruction), urobilinogen cannot form and is absent in urine.

Why Not the Others?

• a) Normal liver function – Bilirubin should not appear in urine if liver function is normal.

• b) Hemolytic disease  – Hemolysis causes increased unconjugated bilirubin (indirect), which is not excreted in urine. Urobilinogen would be high (not negative).

• d) Acute renal failure  – Renal failure does not directly affect bilirubin/urobilinogen metabolism in this way.

Urine ketones (ketonuria) are most likely to be present in conditions where the body is breaking down fat for energy due to:

• Insufficient carbohydrate intake (starvation, fasting, anorexia nervosa)

• Uncontrolled diabetes mellitus (leading to diabetic ketoacidosis)

• Prolonged vomiting

Why This Option?

• Anorexia nervosa → Severe calorie restriction → body uses fat stores → produces ketones (acetoacetate, acetone, beta-hydroxybutyrate) → excreted in urine.

Why Not the Others?

• a) Proteinuria – Indicates kidney damage/nephrotic syndrome, not directly linked to ketosis.

• c) Glucosuria – Suggests diabetes, but ketones only appear if insulin deficiency causes fat breakdown (not always present in simple glucosuria).

• d) Bilirubinuria – Reflects liver/bile duct issues, unrelated to ketone production.

The ketones reagent pad on a urine test strip primarily detects acetoacetic acid (a type of ketone body). In some cases, it may also react with acetone, but it does not detect beta-hydroxybutyrate, another ketone body.

Why not the others?

• a) Glucose – Detects glucose (sugar) in urine, not ketones.

• c) Protein – Detects albumin and other proteins, not ketones.

• d) Bilirubin – Detects bilirubin, a bile pigment, not ketones.

Ketone testing is important for monitoring conditions like diabetic ketoacidosis (DKA), fasting, or high-fat diets.

False negatives in urine dipstick blood tests (which detect hemoglobin/myoglobin) can occur due to:

1. Dilute urine (e.g., excessive water intake) – Lowers the concentration of blood cells or hemoglobin below the detectable threshold.

2. High urine specific gravity (not listed as correct here, but can sometimes mask small amounts of blood).

3. Ascorbic acid (Vitamin C) – Can interfere with the chemical reaction, causing false negatives (though not listed in options).

Why Not the Other Options?

• a) Alkaline urine  – Does not typically cause false negatives for blood detection (may affect other tests like protein).

• c) High specific gravity – While concentrated urine can sometimes make detection harder, dilute urine is a more common cause of false negatives.

• d) Strong oxidizing agents  – These are more likely to cause false positives (e.g., bleach contamination).

• Positive occult blood on a urine dipstick without RBCs on microscopy suggests hemoglobinuria, which occurs when:

  • Free hemoglobin is present in urine due to intravascular hemolysis (e.g., hemolytic anemia, transfusion reactions, severe burns).

  • The dipstick detects heme (from hemoglobin), but since RBCs are lysed, none are seen microscopically.

Why Not the Others?

• a) Oliguria – Low urine output; unrelated to occult blood.

• b) Hematuria – Requires RBCs in urine (would be seen microscopically).

• d) Hemosiderinuria – Occurs with chronic hemolysis (e.g., PNH) but is detected via Prussian blue stain, not dipstick.

Key Differential:

• Myoglobinuria (e.g., rhabdomyolysis) also gives this pattern but is distinguished clinically (muscle pain, CK elevation).

• Ictotest is a confirmatory test for bilirubin in urine.

• When bilirubin is present, the diazotized reagent reacts to form a pink-to-purple azo dye on the mat.

• A pink color is interpreted as a positive result (bilirubin detected).

Why Not the Others?

• a) Invalid → No color change or unusual colors (e.g., yellow/gray) suggest improper technique.

• c) Negative → The mat remains white or light tan (no bilirubin).

• d) Nonreactive → Not standard terminology for Ictotest results.

Key Point:
Ictotest is more specific than dipsticks for bilirubin, avoiding false positives from drugs/pigments.

The pH test pad on a urine reagent strip uses a double indicator system—typically:

• Methyl red (color change from red to yellow, pH ~4–6)

• Bromthymol blue (color change from yellow to blue, pH ~6–9)

This combination allows for a broad pH range (approx. 5.0 to 9.0) and more precise color interpretation.

Why the other options are incorrect:

• a) Ketone → Uses the nitroprusside reaction, not indicators.

• c) Protein → Based on protein error of indicators, but not a double indicator system.

• d) Bilirubin → Uses the diazo reaction, no indicator dyes involved.

• Nitrite tests detect bacteria that convert dietary nitrates → nitrites (e.g., E. coli).

• False-negative nitrite results occur when:

  • Urine dwell time in the bladder is too short (<4 hours) → Bacteria lack time to reduce nitrates.

  • Low dietary nitrates (fasting, low vegetable intake).

  • Non-nitrate-reducing bacteria (e.g., Enterococcus, Staphylococcus).

Why Not the Others?

• a) Contains nitrate-reducers → Should produce a positive nitrite.

• b) Has skin flora → Contaminants (e.g., Staph) typically do not reduce nitrates.

• c) Has cleaning agents → May cause false positives (oxidizing agents), not false negatives.

Key Point:

First-morning urine is ideal for nitrite testing (longer bladder incubation).

• Leukocyte esterase is an enzyme released by white blood cells (WBCs), indicating pyuria (WBCs in urine), a hallmark of urinary tract infection (UTI).

• A positive leukocyte esterase result strongly supports bacterial infection, especially when combined with:

  • Positive nitrites (indicating nitrate-reducing bacteria like E. coli).

  • Symptoms (dysuria, frequency, fever).

Why Not the Others?

• a) Glucose – Suggests diabetes (a risk factor for UTIs but not diagnostic).

• b) Occult blood – Nonspecific (can occur in UTI, stones, trauma, or menstruation).

• c) Urobilinogen – Reflects liver/bile duct function, not infection.

Key Point:

Recurrent UTIs require culture confirmation, but leukocyte esterase is the most reliable rapid screening marker.

• The reagent strip protein test (e.g., dipstick) uses the protein error of indicators principle:

  • pH-sensitive dyes (e.g., bromophenol blue) change color in the presence of albumin.

  • Normally, these dyes respond to pH, but when albumin binds to them, they change color at a constant pH (hence “error”).

Why Not the Others?

• a) Copper sulfate reduction – Used in Clinitest (reducing substances), not protein strips.

• c) Ehrlich reaction  – Detects urobilinogen, not protein.

• d) Double enzyme reaction  – Used for glucose (glucose oxidase/peroxidase), not protein.

• Glucose is the analyte most likely to decrease in an unrefrigerated urine sample due to glycolysis by bacteria (e.g., Enterobacter, Pseudomonas), which metabolize glucose within hours.

• Key factors:

  • Bacterial overgrowth accelerates glucose consumption.

  • Room temperature storage (>2 hours) significantly impacts accuracy.

Why Not the Others?

• a) Protein – Stable at room temperature for 24+ hours (unless urine becomes contaminated).

• c) Ketones  – Acetoacetate degrades slowly (hours to days), but beta-hydroxybutyrate is more stable.

• d) Nitrite  – May increase over time as bacteria convert nitrates → nitrites, but does not “decrease.”

• Ictotest is a tablet-based confirmatory test specifically for bilirubin in urine.

  • It uses diazotized 2,4-dichloroaniline, which reacts with bilirubin to produce a purple/pink color.

  • More sensitive and specific than urine dipsticks (which can give false positives).

Why Not the Others?

• a) Acetest → Detects ketones (acetoacetate), not bilirubin.

• b) Clinitest → A copper reduction test for reducing sugars (e.g., glucose, galactose).

• d) Guaiac test → Detects fecal occult blood, not bilirubin.

When to Use Ictotest?

• When urine dipstick is positive for bilirubin but clinical suspicion is unclear.

• To rule out false positives from medications (e.g., phenazopyridine) or pigments.

• The nitrite test on urine reagent strips can yield false-negative results due to multiple factors:

  1. High vitamin C (ascorbic acid): Acts as a reducing agent, inhibiting the Griess reaction (conversion of nitrites to a pink azo dye) .

  2. Low dietary nitrate: Nitrites are derived from dietary nitrates; low intake reduces substrate for bacteria to convert to nitrites .

  3. Short bladder incubation: Bacteria need ≥4 hours to convert nitrates → nitrites; frequent voiding may prevent detection .

Key Points:

• False positives: Rare but may occur with contaminated containers or certain medications (e.g., phenazopyridine).

• Clinical implications:

  • Negative nitrite + UTI symptoms: Still consider infection (e.g., Staphylococcus saprophyticus doesn’t reduce nitrates).

  • Confirm with microscopy/urine culture.

• The glucose oxidase/peroxidase reaction is the specific enzymatic method used in urine dipsticks to detect glucose:

  1. Glucose oxidase converts glucose → gluconic acid + hydrogen peroxide (H₂O₂).

  2. Peroxidase then uses H₂O₂ to oxidize a chromogen (e.g., potassium iodide), producing a color change (green to brown).

Why Not the Others?

• a) pH → Measured with pH indicators (methyl red/bromthymol blue).

• b) Nitrites → Detected via Griess reaction (nitrite → diazonium salt).

• d) Ketones → Detected by nitroprusside reaction (Legal’s test).

• False-positive glucose results on urine dipsticks (glucose oxidase method) can occur with bleach (chlorine) contamination due to:

  • Oxidation of the chromogen (e.g., potassium iodide) in the test pad, mimicking a glucose reaction.

  • Strong oxidizing agents can bypass the enzyme-mediated reaction, causing a color change.

Why Not the Others?

• b) Lactose – Lactose is a non-glucose reducing sugar (detected by Clinitest, not glucose oxidase strips).

• c) Galactose – Like lactose, galactose is not detected by glucose oxidase strips (requires Clinitest).

• d) Ascorbic acid (Vitamin C) – Causes false negatives (interferes with peroxide-mediated color development).

A false-negative bilirubin test (where bilirubin is present but not detected) can occur due to:

1. Light Exposure (b):

   • Bilirubin is light-sensitive and degrades when exposed to light, especially sunlight. Urine samples must be protected from light to prevent false negatives .

2. Other Causes of False Negatives:

   • High vitamin C (ascorbic acid) or nitrite concentrations in urine can interfere with the test .

   • Prolonged storage of urine samples without proper handling may also lead to bilirubin degradation .

Why Not the Other Options?

• a) Phenazopyridine: Causes false-positive results (due to reddish urine discoloration), not false negatives .

• c) Proteinuria: Does not interfere with bilirubin detection on reagent strips.

• d) Hematuria: May cause visual interference but does not typically lead to false-negative bilirubin results.

When processing a timed urine specimen (e.g., 24-hour urine), the first critical step is to:

1. Measure and record the total volume – This is essential for calculating analyte concentrations per unit time (e.g., mg/24h for protein, creatinine clearance).

2. Mix the specimen thoroughly – Ensures homogeneity before aliquoting for testing.

3. Proceed with preservative addition or testing (if required).

Why Not the Others?

• a) Subculture for bacteria  – Irrelevant for quantitative analysis (unless infection is suspected).

• b) Add preservative – Preservatives (e.g., HCl for catecholamines) are added during collection, not processing.

• c) Screen for albumin  – Performed after volume measurement and mixing.

The reagent strip test for protein is primarily sensitive to albumin due to its reliance on the “protein error of indicators” principle. This method involves pH indicators (e.g., tetrabromophenol blue) that change color at a lower pH in the presence of protein, particularly albumin. Here’s why:

1. Albumin Sensitivity:

   • The test is optimized to detect albumin, the most common protein in urine during early kidney damage. It reacts strongly with the pH indicators, producing a visible color change even at low concentrations (as low as 5–20 mg/dL) .

2. Lower Sensitivity to Other Proteins:

   • Globulins (b): Poorly detected because their structure doesn’t induce the same pH shift as albumin.

   • Bence-Jones protein (c): A low-molecular-weight protein associated with multiple myeloma; the strip test is notably insensitive to it, often yielding false negatives .

   • Mucin (d): Not a target for this test; mucoproteins are rarely detected unless present in very high concentrations

• Glucose in urine (glycosuria) normally occurs when blood glucose exceeds the renal threshold (~180 mg/dL) (e.g., uncontrolled diabetes).

• If urine glucose is positive but blood glucose is normal, this suggests renal glycosuria—a benign condition where the kidneys fail to reabsorb glucose properly, even at normal blood sugar levels.

Why Not the Others?

• a) Renal threshold exceeded  – This would imply high blood glucose (e.g., diabetes mellitus), but the question states blood glucose is normal.

• b) Diabetes insipidus  – Causes dilute urine but no glycosuria (no glucose in urine).

• d) High intake of sugar  – Would cause temporary high blood glucose, leading to glycosuria, but the question specifies normal blood glucose.

• The occult blood pad on urine dipsticks reacts with heme (from hemoglobin, myoglobin, or intact RBCs).

• Orange with green spots is a classic pattern for intact red blood cells (RBCs):

  • The orange background comes from free hemoglobin/myoglobin (if present).

  • The green spots represent unlysed RBCs trapped in the pad’s matrix, creating a speckled appearance.

Why Not the Others?

• a) Myoglobin → Causes uniform orange/brown (no green spots; RBCs absent).

• c) Hemoglobin → Also uniform orange (RBCs lysed, releasing free hemoglobin).

• d) Ascorbic acid → May cause false negatives but doesn’t create this pattern.

Key Point:

Microscopic urinalysis confirms intact RBCs (hematuria) vs. hemoglobin/myoglobin.

• pH increases (becomes more alkaline) when urine is left at room temperature due to:

  1. Bacterial growth (e.g., Proteus, Pseudomonas) → urea splits into ammonia, raising pH.

  2. Loss of dissolved CO₂ → Reduces carbonic acid, shifting pH upward.

• Clinically significant: A falsely high pH can mask acidosis or obscure stone risk analysis (e.g., uric acid stones).

Why Not the Others?

• b) Protein– Stable at room temperature for 24+ hours unless contaminated.

• c) Occult blood – RBCs lyse over time, but hemoglobin remains detectable (dipstick detects both intact RBCs and free Hb).

• d) Specific gravity – Unaffected by short-term storage (measured via refractive index or reagent strips).

The Clinitest is a copper reduction test used to detect reducing substances (e.g., glucose, galactose, lactose, fructose) in urine.

• A red-orange precipitate (or green/yellow, depending on concentration) indicates the presence of reducing sugars due to the reduction of cupric sulfate (blue) to cuprous oxide (red-orange).

• Unlike urine dipsticks (which use glucose oxidase and are specific for glucose), Clinitest reacts with any reducing sugar, making it useful for detecting rare disorders like galactosemia or hereditary fructose intolerance.

Why Not the Others?

• a) Urobilinogen – Detected by Ehrlich’s reagent or dipstick, not Clinitest.

• c) Ketones – Detected by nitroprusside tests (e.g., Acetest or dipstick).

• d) Protein – Detected by SSA test or dipstick, not Clinitest.

• The sulfosalicylic acid (SSA) test is the gold standard to confirm proteinuria when a dipstick is positive.

  • Mechanism: SSA precipitates all proteins (albumin, globulins, Bence Jones proteins), causing turbidity proportional to protein concentration.

  • Advantage: Detects non-albumin proteins missed by dipsticks (e.g., multiple myeloma light chains) .

Why Not the Others?

• a) Acetest: Confirms ketones (irrelevant to protein).

• b) Clinitest: Identifies reducing sugars (e.g., galactose).

• c) Ictotest: Specific for bilirubin.