Coombs Test - Indirect
What is ICT?
The Indirect Coombs Test, also called Indirect Antiglobulin Test (IAT) is a laboratory procedure. It is performed to identify the presence or absence of antibodies in blood directed against antigens found on red blood cells. This procedure helps to determine compatibility for blood transfusion, Rh incompatibility between a pregnant woman and her child, and the possibility of hemolytic anemia occurring due to any autoimmune disorder.
Why is ICT done?
· At the time of donating blood, tissue, organ, or bone marrow or while getting registered as a potential donor
· Before receiving a blood transfusion or blood components transfusion to determine compatibility with donated blood
· Before pregnancy and during pregnancy to determine the risk of incompatibility between the mother and fetus
· To determine if hemolytic anemia is caused due to an autoimmune disorder
What does ICT Measure?
The red blood cells have specific antigen markers present on their surface. These marker antigens are glycoproteins and help the body to recognize its own type of red blood cells (RBCs). Antibodies are naturally produced against incompatible RBC antigens when there is exposure to incompatible red blood cells. Three most common antigen markers found on RBCs are A, B, and Rh. The presence or absence of these antigen markers is used to determine the blood group of a person.
Blood transfusion between incompatible blood types causes an antigen-antibody reaction which causes the RBCs to clump together (agglutination) and this results in their destruction. Hence, blood groups of donor and recipient must match to ensure the success of the blood transfusion or organ transplant. Agglutination may also occur due to Rh incompatibility between Rh-negative pregnant woman (who is having Rh antibodies because of previous sensitization) and her Rh-positive child which may cause antibodies in the mother to cross the placenta and attack the RBCs of the fetus. In some autoimmune disorders, antibodies are produced against antigens on the body’s own RBCs. These antibodies may attack the ‘self’ RBCs and cause agglutination.
The Indirect Coombs Test is performed on the principle of the antigen-antibody reaction between incompatible blood groups. The collected blood specimen is treated with a large range of collected or synthetic human RBC antigens and observed under a microscope. If agglutination (clumping) of RBCs is seen upon treatment with a particular antigen, antibodies against that antigen are already present in the blood specimen. Antibodies against RBC antigens A and B occur naturally, while antibodies against other RBC antigens are produced upon exposure to the corresponding antigen. A similar test called the Direct Coombs Test is performed to detect the presence of antibodies (bound to RBCs) against the body’s own RBCs (autoimmune antibodies) that may be the cause of autoimmune hemolytic anemia.
Interpreting ICT results
Agglutination: Positive result, antibodies present
No agglutination: Negative result, antibodies absent
No antibodies detected
Positive in undiluted serum up to 1:16 titre
Positive in titre of 1:32 or above
Positive in increasing titre in subsequent tests
Antibodies are produced in the blood upon exposure to incompatible antigens as during a blood transfusion. Antibodies produced in the first incompatible transfusion produce severe reactions upon subsequent transfusions of the same blood type.