Indirect Coombs Test (IAT)


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Subject: Indirect Coombs Test (IAT)

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  • The IAT uses the patient's serum (or plasma), which is incubated with reagent or donor RBCs. Subsequently, this mixture is washed to remove unbound globulins and then incubated with Coombs reagent. Agglutination is seen if the patient's serum contains antibodies against the RBCs.

  • In about 80% of patients with autoimmune hemolytic anemia, the autoantibodies are also present in serum.

  • Alloantibodies to RBC antigens induced by previous blood transfusions or fetal–maternal incompatibility are also detected by this assay. These alloantibodies are usually present only in serum because they do not bind to the patient's RBCs, and the DAT is negative in these cases.


  • The utility of the IAT in blood banking stems from its great sensitivity in detecting various IgG antibodies in a patient's serum. It is used to detect the presence of alloantibodies directed against non-ABO blood group antigens.

    • Antibody screening and cross-matching prior to blood transfusions.

    • Prenatal testing of pregnant women.

  • In some cases of autoimmune hemolytic anemia, the IAT and the DAT may be positive because some autoantibody attaches to the RBCs and some (excess) antibody may not be bound to red cells and be present in the serum.


  • The IAT is positive in the presence of serum alloantibodies in patients previously transfused and immunized against non-self–red cell antigens.


  • The IAT may be unable to detect low titer antibodies.

  • A positive IAT requires further investigation to identify more precisely the offending antibody.