A blood specimen types as A, Rh-positive with a negative antibody screen, but one unit is incompatible. What should be done first?

When a blood specimen types as A, Rh-positive with a negative antibody screen, yet one unit is found to be incompatible, the first step is to perform a direct antiglobulin test (DAT) on the donor unit. The rationale behind this approach lies in determining whether there are any unexpected antibodies or immune complexes present in the donated blood that could lead to incompatibility.

The direct antiglobulin test provides crucial information about the presence of antibodies that may be coating the red blood cells. If such antibodies are detected, it indicates a hemolytic reaction could occur, thereby justifying the incompatibility observed in the crossmatch. Conducting this test first allows for an understanding of the cause behind the incompatibility before considering further steps like repeating the ABO grouping or testing for low-incidence antigens, which may not directly address the issue at hand.

This initial assessment is essential for ensuring safe blood transfusion practices, as it can reveal underlying factors causing the incompatibility, allowing for appropriate action to be taken to prevent potential adverse reactions in the recipient.