Which test detects Rh sensitization of an infant's red blood cells?

• A. Indirect antiglobulin test
• B. Crossmatch
• C. Antibody screen
• D. Direct antiglobulin test

Answer: (D)

The direct antiglobulin test detects antibodies or complement that are already attached to the patient’s red blood cells. In Rh sensitization of an infant, maternal IgG anti-D crosses the placenta and coats the infant’s D-positive red cells. Using anti-human globulin, the DAT reveals this in vivo coating, indicating that maternal antibodies are bound to the infant’s red cells and can cause hemolysis.

Other tests don’t show this cell-bound coating. The indirect antiglobulin test looks for antibodies in the patient’s serum that could react with red cells but doesn’t demonstrate antibodies bound to the patient’s own cells. A crossmatch evaluates compatibility between donor cells and recipient plasma, not sensitization of the infant’s red cells. An antibody screen detects antibodies in maternal serum but isn’t evidence of in vivo coating on the infant’s red cells.