direct Coombs test
Summary
The direct Coombs test (direct antiglobulin test, DAT) detects antibodies or complement proteins bound directly to the surface of a patient's red blood cells in vivo. A positive result indicates immune-mediated hemolysis, as seen in autoimmune hemolytic anemia, hemolytic disease of the newborn, or drug-induced hemolysis.
Detail
The direct Coombs test works by adding anti-human globulin (Coombs reagent) directly to a patient's washed RBCs. If the RBCs are already coated with IgG antibodies or complement (C3b) in vivo, the anti-human globulin will cross-link these antibodies, causing visible agglutination—a positive result. This test detects antibodies already attached to RBCs within the patient's body, distinguishing it from the indirect Coombs test, which detects free antibodies in the patient's serum (used for prenatal testing and blood crossmatching). Clinical applications include: (1) Autoimmune hemolytic anemia (AIHA)—warm AIHA (IgG-mediated, associated with SLE, CLL, drugs like methyldopa) and cold AIHA (IgM-mediated, associated with Mycoplasma pneumoniae, infectious mononucleosis, lymphoma); (2) Hemolytic disease of the fetus and newborn (HDFN)—maternal anti-Rh(D) or anti-ABO antibodies crossing the placenta and coating fetal RBCs; (3) Drug-induced immune hemolytic anemia—drugs like penicillin, cephalosporins, or methyldopa inducing antibody formation against RBC antigens or drug-RBC complexes; (4) Transfusion reactions—delayed hemolytic transfusion reactions where recipient antibodies attack transfused donor RBCs. A positive direct Coombs test with evidence of hemolysis (elevated LDH, indirect bilirubin, decreased haptoglobin, reticulocytosis, schistocytes on smear) confirms immune-mediated hemolysis. Management depends on the underlying cause—corticosteroids and rituximab for warm AIHA, cold avoidance for cold AIHA, discontinuation of offending drugs for drug-induced hemolysis.
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- UpToDate: Diagnosis of hemolytic anemia in adults
- Harrison's Principles of Internal Medicine
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