indirect Coombs test

The indirect Coombs test identifies unbound, circulating antibodies in a patient's plasma that have the potential to bind to RBC antigens. Test procedure: patient serum is mixed with reagent RBCs of known antigen profile; if antibodies in the serum bind to these RBCs, then anti-human globulin (Coombs reagent) is added, causing agglutination if antibody-coated RBCs are present. A positive result indicates the presence of alloantibodies (e.g., anti-D, anti-Kell) or other RBC antibodies in the serum. Clinical applications: (1) Prenatal screening in Rh-negative mothers to detect anti-D antibodies that could cause hemolytic disease of the fetus and newborn (HDFN); (2) Pre-transfusion compatibility testing (type and screen/crossmatch) to detect antibodies against donor RBC antigens before transfusion, reducing risk of hemolytic transfusion reactions; (3) Antibody identification panels in patients with unexplained hemolysis or transfusion reactions. This contrasts with the direct Coombs test (direct antiglobulin test, DAT), which detects antibodies already bound to the patient's own RBCs in vivo—used in autoimmune hemolytic anemia, HDFN diagnosis in the newborn, and drug-induced hemolytic anemia. Mnemonic: Indirect test = In-vitro (antibody in serum, tested against known RBCs); Direct test = Detects antibody Directly on patient's RBCs.