How is ABO blood group determined in pretransfusion testing?

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Determining ABO blood group in pretransfusion testing relies on two complementary serologic steps: forward typing and reverse typing. Forward typing uses anti-A and anti-B reagents to detect which A or B antigens are present on the patient’s red blood cells. The pattern of agglutination identifies the antigens on the cell surface and defines the ABO type. Reverse typing then tests the patient’s serum or plasma against known group A and group B cells to see which antibodies are present. If the plasma contains antibodies against A or B antigens in a pattern that matches the forward typing, the ABO designation is confirmed. Using both tests together is essential because it increases accuracy and helps reveal discrepancies that can arise from subgroups, recent transfusions, or other factors. Genetic sequencing is not used routinely for ABO typing in pretransfusion testing because serologic methods are faster, cheaper, and directly assess blood group antigens.