Perform an antibody identification with a group O panel to identify the antibody present.
Do the panel in saline phases at RT (or 4°C) since these antibodies are IgM and react best in the cold.
As QC on the antibody identification, include an autocontrol and cord cells in case the patient has an autoanti-I reactive at 4°C.
When the antibody is identified, antigen type the A₁ cells, the B cells and the patient's own cells for the corresponding antigen. Expect the patient to be antigen-negative. The discrepant A₁ (or B) cells should be antigen-positive.
As QC on the antigen typing, include positive and negative antisera controls.
Redo the ABO group using A₁ and B cells that are negative for the corresponding antigen.