A neonate born to a Rh-negative mother is found to be Rh-positive. To assess whether maternal anti-Rh IgG antibodies have crossed the placenta and are coating fetal red blood cells, a direct Coombs (direct antiglobulin) test is performed. What does this test detect?
- A Free anti-Rh antibodies in the fetal serum
- B Anti-Rh antibodies bound to fetal RBC surface ✓
- C Complement C3d on fetal RBCs
- D Maternal B cells in fetal circulation
Correct answer: B. Anti-Rh antibodies bound to fetal RBC surface
Explanation
The direct antiglobulin test (direct Coombs test) detects antibodies or complement proteins already bound to the surface of red blood cells in vivo. Anti-human globulin (Coombs reagent) is added to washed patient RBCs; agglutination indicates IgG or C3 coating the cells. The indirect Coombs test detects free antibodies in serum by incubating patient serum with known antigen-positive RBCs.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.