In a major crossmatch situation with a positive screening cell that has not been investigated, what is the next essential laboratory step?

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When a major crossmatch screening cell is positive and hasn’t been investigated, the essential next laboratory step is to identify the antibody responsible for the positive screen. This step determines the specificity of the alloantibody (or autoantibody) in the patient’s plasma, which is crucial for selecting compatible donor units and preventing a hemolytic transfusion reaction. Without knowing the antibody, you can’t reliably choose antigen‑negative units or anticipate reactivity with potential donors.

Once the antibody is identified, you can proceed to test donor units for the corresponding antigen(s) and perform a compatible crossmatch, or request antigen‑matched or autologous options if appropriate. Verifying patient identity and order is important, but it does not address the transfusion risk posed by an uninvestigated positive screen. Releasing units immediately without identifying the antibody would be unsafe.