Coombs Test - Indirect

The indirect Coombs test (indirect antiglobulin test) detects free antibodies in a person’s serum that can bind red blood cells. It’s used for prenatal screening to find maternal alloantibodies that could cause hemolytic disease of the newborn, and for pre‑transfusion compatibility testing to identify unexpected antibodies that might react with donor red blood cells.

An indirect Coombs test is done to screen for free circulating red‑cell antibodies: routinely in pregnancy (early and around 28 weeks, and when the mother is Rh‑negative) to detect alloantibodies that can cause hemolytic disease of the fetus/newborn; before blood transfusion as part of pre‑transfusion antibody screening and crossmatching; and when investigating suspected hemolytic transfusion reactions or unexplained hemolytic anemia.

If an indirect Coombs test is negative, no free red‑cell antibodies are detected in the blood. That indicates a low immediate risk of immune‑mediated hemolysis for transfusion or the fetus/newborn. Routine care continues, but repeat antibody screening is recommended later in pregnancy or after transfusion/exposure because antibodies can develop later or be below initial detection thresholds.

A positive direct Coombs (direct antiglobulin) test means antibodies or complement are bound to the patient’s red blood cells, indicating immune-mediated hemolysis. It triggers evaluation for causes—autoimmune disease, drugs, transfusion reaction, or hemolytic disease of the newborn—and labs (bilirubin, LDH, haptoglobin, smear). Treatment targets the cause and may include stopping offending drugs, corticosteroids or IVIG, specialist referral, and transfusion if needed.

The normal Coombs (direct antiglobulin) test result is negative—no antibody or complement bound to red blood cells. Some labs report a 0 to 4+ grading; 0 (negative) is normal. Any positive result (1+ to 4+) or a positive indirect Coombs indicates antibodies are present and requires further evaluation for immune‑mediated hemolysis, transfusion compatibility, or antibody screening.

The indirect Coombs (indirect antiglobulin) test requires the patient's serum — blood allowed to clot and centrifuged to obtain serum — which contains circulating antibodies that may react with red‑cell antigens. That serum is incubated with reagent red cells to detect allo‑ or autoantibodies, commonly used for prenatal antibody screening and pre‑transfusion compatibility testing.