Antiglobulin test (direct Coombs' test)
The direct antiglobulin test is a laboratory test that detects antibodies (most often IgG) or complement fragments (e.g., C3d) attached to the surface of red blood cells within a patient's body. It is an important diagnostic tool for identifying immune forms of haemolytic anaemia and other conditions associated with the destruction of red blood cells.
Unlike the indirect Coombs test, which detects free antibodies in plasma, the direct test shows whether there is already sensitisation (i.e. antibody attachment) on the surface of the red blood cells themselves.
A sample of venous blood collected in a tube with an anticoagulant (usually EDTA) is used for the test. Antiglobulin serum is added to the patient's red blood cells. If antibodies or complement components are present on the cell surface, agglutination will occur - the cells will stick together, which is visualised and interpreted as a positive reaction