Coombs' test

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Described by Robin Coombs et al.[1]

The Coombs' test, also known as the anti-globulin test (AGT), is used to test for a response to antibodies against red blood corpuscles surface antigens.

Identifying the blood group relies on a principle similar to the Coombs' test. In the 4 columns on the left, erythrocytes from the patient are added to the columns which contain antibodies to the red cell membrane antigens. The patient's blood group is B (Rhesus positive), so agglutination occurs and the resulting complex does not pass through the gel column on centrifugation. On the 2 rightmost columns, serum from the patient is added to the columns which contain antigens.

Coombs' reagent consists of antibodies against human globulin together with complement factors. These animal-derived (polyclonal) antibodies bind to the globulin part of human immunoglobulin (usually IgG).

Direct Coombs Test

  • Direct antiglobulin test (DAT)
  • Blood is taken from patient with suspected haemolysis, in particular antibody-mediated haemolysis.
  • If antibodies are present, these will still be bound to the surface of the red blood corpuscles. For instance, in a Rhesus-negative baby carried by a mother previously exposed to Rhesus antigen, the mother's anti-Rhesus antibodies will bind to the baby's red cells.
  • The binding of these antibodies to the Rhesus antigen is not enough to cause them to clump together.
  • However, addition of Coombs reagent causes clumping of the cells as the anti-human globulin antibodies cause cross-linking of the red blood corpuscles - this is a positive result.

Indirect Coombs Test

  • Indirect antiglobulin test (IAT)
  • This is the method used to group and cross-match blood types.
  • Also can be used in mothers' suspected to have developed a reaction
  • Serum from the a potential recipient is taken.
  • This is added to the donors blood.
  • If there is antibodies are present, these bind to the red blood corpuscles.
  • Addition of Coombs reagent causes agglutination - a positive result - implying the presence of reactive antibodies.


  1. Coombs RRA, Mourant AE, Race RR. A new test for the detection of weak and "incomplete" Rh agglutinins. Brit J Exp Path 1945;26:255-66.