Which statement best describes the principle of reverse grouping?

• A. Manufactured RBCs are used to detect naturally occurring antibodies
• B. Reagents used to detect antigens on RBCs
• C. Serum tested against donor RBCs
• D. Reagent antibodies used to detect A and B antigens

Answer: (A)

Reverse grouping tests the patient’s serum for antibodies by exposing it to reagent red blood cells that have known ABO antigens. If the serum contains naturally occurring antibodies against A or B, those antibodies will bind to the corresponding antigens on the reagent RBCs and cause agglutination. The use of manufactured reagent RBCs to reveal these antibodies is the essence of reverse grouping.

In contrast, forward typing uses antisera to detect antigens on the patient’s red cells, which is described by options about detecting A and B antigens on cells. Statements about testing serum against donor cells or about reagents detecting antigens on RBCs refer to forward typing or are too vague to describe reverse grouping.