Indirect Coombs Test detects circulating antibodies against Red Blood Cells. The major purpose of the Indirect Coombs Test is to determine if the patient has minor serum antibodies (other than the major ABO/Rh system) to RBCs before receiving a blood transfusion. Therefore the Indirect Coombs Test is the “screening” portion of the “type and screen” routinely performed for blood compatibility testing (crossmatching in the blood bank). If the recipient of the blood transfusion has antibodies to the donor’s RBCs, agglutination occurs. The blood cannot be used for that recipient. On the other hand, if the recipient has no antibodies to the donor’s RBCs, agglutination will not occur. Transfusion should then proceed safely without any transfusion reaction.. Indirect Coombs Test is also used to detect other agglutinins, such as cold agglutinins that are associated with mycoplasmal infections.
In this test a small amount of the recipient’s serum is added to donor RBCs containing known antigens on their surfaces. This is the first stage. In the second stage of the test, Coombs serum is added after the test RBCs have been washed of any free globulins. If antibodies exist in the patient’s serum, agglutination occurs. In blood transfusion screening, visible agglutination indicates that the recipient has antibodies to the donor’s RBCs. If the recipient has no antibodies against the donor’s RBCs, agglutination will not occur; transfusion should then proceed safely without any transfusion reaction. Circulating antibodies against RBCs also may occur in an Rh-negative pregnant woman who is carrying an Rh-positive fetus.
Drugs that Cause False Positive Indirect Coombs Resutls
Drugs that may cause false-positive results include:
- Chlorpromazine (Thorazine)
- Methyldopa (Aldomet).
- Phenytoin (Dilantin).
Causes of Positive Indirect Coombs Resutls
- Incompatible Crossmatched Blood: Anti-ABO/Rh antigens in the donor blood cross-react with the patient’s serum.
- Detected antibodies can be developed due to previous exposure to fetal Rh+ Red Blood Cells. This can be detected in Maternal Anti-Rh Antibodies, Newborn Hemolytic Disease
- Acquired Immune Hemolytic Anemia.
- Presence of specific Cold Agglutinin Antibody.