The direct antiglobulin test is most useful for which purpose?

• A. Detecting hemolytic disease of the newborn
• B. Screening for ABO incompatibility
• C. Identifying iron deficiency
• D. Detecting alloantibodies in serum

Answer: (A)

The direct antiglobulin test (DAT) detects antibodies or complement that are bound directly to a person’s red blood cells in vivo. This makes it especially useful for identifying immune-mediated hemolysis, such as hemolytic disease of the newborn, where maternal IgG antibodies cross the placenta and coat fetal red cells, leading to their destruction. A positive DAT on the newborn’s red cells indicates this coating and helps confirm HDFN.

Screening for ABO incompatibility relies on detecting antibodies in the patient’s serum or reactions in crossmatching, not on antibodies/complement already attached to red cells, so the DAT isn’t the primary tool there. Iron deficiency is a nonimmune issue and isn’t diagnosed by looking for cell-bound antibodies or complement. Detecting alloantibodies in serum is done with the indirect antiglobulin test, which looks for free antibodies in the serum, not coating on the patient’s red cells.