Direct Coombs Test

Most of the antibodies to Red Blood Cells are directed against the ABO/Rh blood grouping antigens, such as those that occur in hemolytic anemia of the newborn or blood transfusion of incompatible blood. When a transfusion reaction occurs, the Coombs test can detect the patient’s antibodies coating the transfused RBCs. The Coombs test is therefore useful in evaluating suspected transfusion reactions.


Non–blood-grouping antigens can develop on the RBC membrane and stimulate formation of antibodies. Drugs such as levodopa or methyldopa can do this. Also, in some diseases, antibodies not originally directed against the patient’s RBCs can attach to the RBCs and cause hemolysis, which can be detected by the direct Coombs test. Examples of the latter would include:

  • Antibodies developed in reaction to drugs such as penicillin
  • Autoantibodies formed in various autoimmune diseases
  • Antibodies developed in some patients with advanced cancer (e.g., lymphoma)

Frequently the production of these autoantibodies against RBCs is not associated with any identifiable disease, and the resulting hemolytic anemia is therefore called idiopathic.

The direct Coombs test demonstrates that RBCs have been attacked by antibodies in the patient’s bloodstream. The RBCs of patients suspected of having antibodies against RBCs are washed to eliminate any excess free gamma globulins. Coombs serum is added to the RBCs. If the RBCs have antibodies on them, Coombs serum will cause agglutination. The greater the quantity of antibodies against RBCs, the more clumping occurs. This test is read as positive with clumping on a scale of trace to 4. If the RBCs are not coated with autoantibodies against RBCs (immunoglobulins), agglutination will not occur; this is a negative test. In Newborns, blood is collect from the umbilical cord in order to test it for the presence of antibodies.




Causes of False Positive Direct Coombs

  • Existance of Antiphospholipid Antibodies (Anticardiolipin Antibodies) can cause a false-positive Direct Coombs Resutls.
  • Drugs that may cause false-positive results include Ampicillin, Captopril, Cephalosporins, Chlorpromazine (Thorazine), Chlorpropamide, Hydralazine, Indomethacin (Indocin), Insulin, Isoniazid (INH), Levodopa, Methyldopa (Aldomet), Penicillin, Phenytoin (Dilantin), Procainamide, Quinidine, Quinine, Rifampin, Streptomycin, Sulfonamides, and Tetracyclines.




Causes of Positive Direct Coombs results

  • Hemolytic Disease of the newborn.
  • Adult Hemolytic Anemia (idiopathic): Autoantibodies not otherwise associated with any other disease attach to Red Blood Cells.
  • Incompatible Blood Transfusion Reaction:Antibodies to the patient’s RBCs have been created by mixing of incompatible blood grouping antigens.
  •  In some conditions Autoantibodies detected in the Direct Coombs test attach to the Red Blood Cells. Such conditions include Lymphoma, Autoimmune Hemolytic Anemia (Rheumatoid/Collagen Diseases for example Systemic Lupus Erythematosus, and Rheumatoid Arthritis).
  • Hemolytic anemia after heart bypass: Autoantibodies formed during the use of the heart/lung bypass machine attach to RBCs.
  • Infections may lead to developing antibodies that attach to Red Blood Cells for unknown reasons. This kind of antibodies can be detected using Direct Coombs Test with patients who have Mycoplasmal infection, and Infectious Mononucleosis.