A 42-year-old female requires crossmatching for surgery, and one donor shows incompatible results. What is the most likely cause?

The situation presented involves crossmatching for a patient requiring blood for surgery, where one donor shows incompatible results. The most likely cause of this incompatibility is the presence of single alloantibodies in the patient.

Single alloantibodies typically arise when a patient has been previously sensitized to a specific antigen through transfusion or pregnancy. These antibodies can interact specifically with corresponding antigens on donor red blood cells, leading to incompatible crossmatch results. This scenario is quite common, especially in individuals who have had previous blood transfusions.

On the other hand, multiple alloantibodies would suggest a broader range of sensitization, where the patient has developed antibodies against more than one blood group antigen, making it less likely that only one donor would show incompatibility unless that donor expresses one of the specific antigens the patient is reacting against. Rh incompatibilities are more specific to D antigen recognition and would not typically result in incompatibility for all donors. A positive Direct Antiglobulin Test (DAT) in the donor might point to other immunological issues related to the donor's blood, but it does not directly account for incompatibility in crossmatching since the focus is on recipient and donor interaction, not the characteristics of the donor blood alone.

Therefore