A 40-year-old woman with autoimmune hemolytic anemia has a positive direct Coombs test. The direct Coombs test detects:
- A Free antibody in the patient's serum against donor RBCs
- B IgG or complement (C3d) already bound to the patient's RBCs ✓
- C Cold agglutinins reactive at 4°C in the serum
- D Osmotic fragility of RBCs in hypotonic saline
Correct answer: B. IgG or complement (C3d) already bound to the patient's RBCs
Explanation
The direct antiglobulin (Coombs) test adds anti-human globulin reagent directly to patient's washed RBCs; agglutination indicates antibody or complement already coating the RBCs in vivo. This is positive in autoimmune hemolytic anemia, hemolytic transfusion reactions, and drug-induced hemolysis. The indirect Coombs test detects free antibody in serum. Cold agglutinin titer and osmotic fragility are separate tests.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.