Type of test Blood
Normal findings
 Negative; no agglutination
Test explanation and related physiology
This test is performed to identify immune hemolysis (lysis of RBCs) or to investigate hemolytic transfusion reactions. Most of the antibodies to RBCs are directed against the ABO/Rh blood grouping antigens, such as those that occur in hemolytic anemia of the newborn or transfusion of incompatible blood. When a transfusion reaction occurs, the Coombs test can detect the patient’s antibodies or complement components coating the transfused RBCs. Therefore the Coombs test is very helpful in evaluating suspected transfusion reactions.
Non–blood grouping antigens can develop on the RBC membrane and stimulate formation of antibodies. Such drugs as levodopa or penicillin cause this. Also, in autoimmune diseases, antibodies not originally directed against the patient’s RBCs can attach to the RBCs and cause hemolysis that is detected by the direct Coombs test. Frequently, the inciting factor for the production of these autoantibodies against RBCs is not associated with any identifiable disease, and the resulting hemolytic anemia is called idiopathic.
The direct Coombs test demonstrates if the patient’s RBCs have been attacked by antibodies in the patient’s own blood stream. Coombs serum is a solution containing antibodies to human globulin (antibodies). Coombs serum is mixed with the patient’s RBCs. If the RBCs have antibodies on them, agglutination of the patient’s RBCs will occur. The greater the quantity of antibodies against RBCs, the more clumping occurs. This test is read as positive with clumping on a scale of micropositive to +4. If the RBCs are not coated with autoantibodies against RBCs (immunoglobulins), agglutination will not occur; this is a negative test result.
Interfering factors
 • Antiphospholipid antibodies (p. 61, anticardiolipin antibodies) can cause a false-positive DAT.
* Drugs that may cause false-positive results include ampicillin, captopril, cephalosporins, chlorpromazine, chlorpropamide, hydralazine, indomethacin, insulin, isoniazid, levodopa, methyldopa, penicillin, phenytoin, procainamide, quinidine, quinine, rifampin, streptomycin, sulfonamides, and tetracyclines.
Procedure and patient care
 • See inside front cover for Routine Blood Testing.
 • Fasting: no • Blood tube commonly used: red or lavender
 • Use venous blood from the umbilical cord to detect the presence of antibodies in the newborn
• List on the laboratory slip all medications and any transfusions that the patient has had in the last few days.
Abnormal findings
- Autoimmune hemolytic anemia
- Transfusion reaction
- Hemolytic disease of the newborn
- Lymphoma
- Systemic lupus erythematosus
- Mycoplasmal infection
- Infectious mononucleosis