When laboratories are determining someone’s ABO type, they are required to do so in two ways. First, they check to see what ABO antigens are present on the person’s red blood cells (this is “red cell” or “forward” grouping). However, the red cell type must be confirmed by the person’s ABO antibody reactions, so we test the patient’s serum or plasma against reagent (known) group A and group B red blood cells. The reactions tell us whether a person has the appropriate antibodies to confirm their red cell type (for example, a person who appears to be blood group A on red cell grouping should carry anti-B antibodies and not anti-A antibodies).
Blood banks are required to perform serum grouping on pretty much every person that they test for ABO type, with two exceptions: 1) When confirming an ABO type on units of labeled red cells that were collected elsewhere, and 2) When doing ABO typing for babies less than 4 months of age.
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