Antiglobulin (Coombs) Test
Definition
A two-stage assay that detects IgG antibodies bound to red blood cells. The direct test detects in-vivo cell-bound IgG; the indirect test detects free IgG in serum by first incubating serum with red cells and then adding anti-human globulin to crosslink any IgG that has attached. Used when IgG antibodies are present but do not cause direct agglutination because of electrostatic repulsion.
- Direct test
- Detects IgG already bound to red blood cells in vivo
- Indirect test
- Detects free IgG in serum against test red cells
- Reagent
- Anti-human globulin, crosslinks bound IgG to cause agglutination
- Used when
- IgG is present but too weak electrostatically to agglutinate directly
Common questions
Why doesn't IgG cause visible agglutination on its own the way IgM does?+
IgG is a smaller molecule than IgM and cannot physically bridge the gap between red cells created by their surface charge, so cells coated only with IgG stay in suspension until the anti-human globulin reagent is added to link the IgG molecules together.
What is the practical difference between running the direct and indirect Coombs test?+
The direct test is run straight on the red cells to find antibody already attached to them, while the indirect test first incubates serum with reference red cells to let free antibody bind before the antiglobulin reagent is added, so it detects circulating antibody rather than cell-bound antibody.
Related terms
- Agglutination Titre
- The reciprocal of the highest dilution of a serum or antigen solution at which visible agglutination still occurs. A titre of 1:64...
- Hemagglutination
- Agglutination specifically involving red blood cells as the antigen-bearing particles. Direct hemagglutination occurs when antibodies bind surface antigens naturally present on red...
- Low-Ionic-Strength Saline (LISS)
- A reaction medium with reduced salt concentration used to enhance agglutination sensitivity, particularly for IgG antibodies. Lowering ionic strength reduces the electrostatic...
- Passive (Indirect) Agglutination
- An agglutination format in which a soluble antigen or antibody is adsorbed or covalently linked to a carrier particle (latex bead, tanned...
- Prozone (Hook) Effect
- A false-negative agglutination result caused by antibody excess. When antibody molecules outnumber antigen sites, each site is occupied by a single antibody...