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Direct Coombs Test

If you have been fatigued, have trouble breathing, cold hands and feet, and very pale skin, you may have an insufficient number of red blood cells. This condition is called anemia and it has many causes.

If your doctor confirms that you have a low red blood cell count, the Coombs test is one of the blood tests that your doctor may order to help determine what type of anemia you have.

Direct Coombs Test
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Also Known As:  DAT, Direct Coombs Test,  Direct Anti-human Globulin Test

Purpose Of The Test (Indications):

  1. To diagnose the hemolytic anemia of the newborn.
    1. This test is done on the cells (RBC) especially in the case of a newborn in the Rh-negative mother when the baby is Rh-positive and in case of erythroblastosis fetalis.
  2. T0  diagnose autoimmune hemolytic anemias.
  3. To diagnose blood transfusion reactions.
  4. To diagnose drug-induced hemolytic anemia.

Why Get Tested?

To help diagnose the cause of red blood cells destruction (hemolytic anemia); to investigate a blood transfusion reaction; to diagnose hemolytic disease of the newborn

When To Get Tested?

  • When you have had a blood transfusion recently and are experiencing symptoms of a transfusion reaction, such as dizziness, back pain, dark urine, shortness of breath
  • When a newborn shows signs of hemolytic disease of the newborn
  • When you have symptoms of increased destruction of your red blood cells, which may include feeling tired more often than usual, shortness of breath, headaches, paleness and your healthcare practitioner wants to find out if this happens because your immune system mistakenly makes antibodies that destroy your red blood cells

Sample Required:

  1. The blood is collected in EDTA.
  2. Separate RBC immediately to prevent absorption of the complement to RBC.
  3. Avoid clotted blood if possible. In the case of clotted blood, keep the blood at room temp. 37 °C, until the cells are separated.
  4. Can store sample at 4 °C for one week.

Result

  1. It is normally negative, there is no agglutination.
  2. Positive direct Coombs shows agglutination immediately after centrifugation indicating that RBCs are coated with antibodies.
    1. This test is read positive when the clumping on a scale of a trace to +4.
  3. False-positive results are seen in:
    1. Insufficient washing of the RBCs.
    2. Over Centrifugation of the test.
    3. Contaminated reagents.
    4. If the washed RBCs left for a longer period of time.

The Positive Test Is Seen In:

  1. Autoimmune hemolytic anemia.
    1. Warm reactive autoantibody.
    2. Cold reactive autoantibody.
    3. Cold haemagglutinin disease is seen in Lymphoma, pneumonia.
    4. Paroxysmal cold hemoglobinuria.
    5. Systemic lupus erythematosus.
  2. Drug-induced hemolytic anemia e.g penicillin, quinidine, Cephalosporin, and α-methyldopa.
  3. Hemolytic anemia: Transfusion reaction to incompatible RBC and hemolytic anemia of the newborn.
    1. Erythroblastosis fetalis.
    2. Incompatible blood transfusion.
  4. Malignant diseases:
    1. Lymphoma.
    2. Chronic lymphocytic leukemia.
  5. Infections :
    1. Infectious mononucleosis.
    2. Mycoplasma pneumoniae.

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