Coombs’ Test:- Part 2 – Coombs’ Indirect test, Anti-globulin indirect test
Coombs’ Test Indirect
- This is done on mother serum.
- To diagnose the presence of antibodies in the pregnant mother serum.
- Detection of certain weak antigens in RBCs like Du.
- To find weak antibodies or incomplete types of antibodies like found in Duffy or Kidd blood groups.
- Detection of incomplete antibodies in serum, before pretransfusion screening or may do titration of the antibody.
- Detection of cold agglutinins autoantibodies.
- To diagnose autoimmune hemolytic anemia.
- To find the red cell sensitization by drugs.
Precaution of Coombs’ test indirect
- This should be done on fresh serum.
- Some of the drugs that give false-positive results are cephalosporin, antiarrhythmic, insulin, methyldopa, Dilantin, sulphonamides, and tetracycline.
- The false-positive or negative results are seen due to:
- Clerical mistakes.
- Technical errors like not washing RBCs, failure to add reagents.
- Contamination by 5% or 10% glucose in water but not glucose in saline from intravenous tubing.
- Poor quality of commercial coombs’ reagents.
- The indirect Coombs test detects the circulating antibodies in the serum, these are the free antibodies.
- The major purpose of this test is to detect if the recipient or the patient has serum antibodies other than the ABO / Rh system to RBC before receiving the blood transfusion.
- This is the test for screening purposes in routine blood transfusion.
- This test can diagnose other agglutinins like cold agglutinins which are usually associated with Mycoplasmal infection.
- Circulating antibodies against RBC may be seen in the Rh-negative mother who is carrying an Rh-positive fetus.
- There is a mixing of the feto-maternal blood during the first delivery.
- Fetal blood Rh-Positive when goes into mother circulation will sensitize the mother and there is the formation of IgG type of antibodies.
- These antibodies can cross the placental barrier and enter fetal circulation.
- In the second pregnancy, there is a Type II cytotoxic reaction that will destroy the fetal RBCs and give rise to hemolytic anemia.
- Coombs’ serum is prepared by injecting IgG immunoglobulin (antibody) to the rabbit, then from his serum anti-IgG antibody is separated, which is called Coombs serum.
The procedure of coombs’ indirect test
- This test is done in the serum.
- Add recipient serum (patients) to donor RBC (known blood O) in a test tube.
- Incubate and then wash the RBC three times with saline.
- Now add Coombs’ serum and again centrifuge, then see for agglutination.
- If the test is positive then you will see clumping or agglutination.
- Negative and there is no agglutination.
A positive coombs’ indirect test indicates:
- Incompatible blood transfusion.
- Mother has anti-Rh antibodies.
- Erythroblastosis fetalis.
- Acquired hemolytic anemia.
- due to drugs and cold antibodies.