In a direct antiglobulin test, what is being tested?

• A. Donor red blood cells against patient serum
• B. Reagent antisera against donor red blood cells
• C. Patient red blood cells against reagent antisera
• D. Patient serum against donor red blood cells

Answer: (C)

Direct antiglobulin testing looks for antibodies or complement already attached to the surface of a patient’s red blood cells. The procedure uses the patient’s washed red cells and adds anti-human globulin (reagent antisera). If there are antibodies or C3 on the cell surface from in vivo sensitization, the antihuman globulin binds to them and causes agglutination, producing a positive result. This means the test is assessing the patient’s red blood cells against reagent antisera to detect coating, rather than testing serum against cells. This distinction also helps separate it from the indirect antiglobulin test, which screens serum for antibodies by reacting it with donor red cells and then adding the antihuman globulin.