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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.
- ↑ 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.