A newborn develops severe hemolytic disease of the fetus and newborn due to Rh incompatibility. A direct antiglobulin test (Coombs test) is performed on the infant's red blood cells. What does a positive direct Coombs test detect?

• A Maternal anti-Rh antibodies circulating in the infant's serum
• B Anti-Rh antibodies bound to the infant's red blood cells ✓
• C Rh antigens on the infant's red blood cells
• D Complement C3 bound to maternal red blood cells

Explanation

The direct Coombs test detects antibodies or complement already bound to the surface of red blood cells in vivo. In hemolytic disease of the newborn, maternal IgG crosses the placenta and coats fetal Rh-positive red blood cells. The indirect Coombs test detects free antibodies circulating in the serum, not those bound to the cells themselves.

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.