Which statement best describes the direct antiglobulin test?

• A. Detects soluble antibodies in plasma
• B. Detects hemolysis markers
• C. Detects antibodies on red cell surfaces
• D. Detects Hb in cells

Answer: (C)

The direct antiglobulin test looks for antibodies or complement components that are already bound to red blood cell surfaces in the patient. In the test, the patient’s washed red cells are exposed to anti-human globulin (Coombs reagent). If antibodies (often IgG) or complement (like C3d) are attached to the red cells, the anti-human globulin binds to them and causes agglutination, yielding a positive result. This indicates that immune components are coating the red cells, which can underlie immune-mediated hemolysis.

This is different from detecting soluble antibodies in plasma—that’s the indirect antiglobulin test, used for antibody screening or crossmatching. It also doesn’t assess hemoglobin inside cells, so detecting Hb in cells isn’t what the DAT measures.