The Indirect Coombs Test or Indirect Antiglobulin Test

Indirect Coombs test (also known as the indirect antiglobulin test or IAT) is used to detect very low concentrations of antibodies present in a patient's plasma/serum prior to a blood transfusion.

The IAT is a two-stage test.

Stage 1 : Test RBCs washed and incubated with a test serum. If the serum contains antibodies to antigens on the RBC surface, the antibodies will bind onto the surface of the RBCs.

Stage 2: RBCs are then washed well with saline and then incubated with antihuman globulin. If antibodies have bound to RBC surface antigens in the first stage, RBCs will agglutinate when incubated with the antihuman globulin and the indirect Coombs test will be positive. By diluting the serum different titrations can be measured.

Most common uses of the IAT are : 1) To screen pregnant women for antibodies that may cause hemolytic disease of the newborn. 2) To test donor blood for compatibility.