A patient is typed as group O, Rh-positive and both antibody screening cells and crossmatched units are incompatible. What is the most likely cause of the incompatibility?

In this scenario, the patient is typed as group O, Rh-positive, and there is a compatibility issue with both antibody screening cells and crossmatched units. The most likely cause of this incompatibility is the presence of recipient alloantibodies.

Alloantibodies are antibodies that develop in response to antigens from transfused blood or pregnancy, which are not present on the individual's own red blood cells. Given that the patient is group O, it is possible they have developed antibodies against A or B antigens, which would explain the incompatibility seen during crossmatching. The presence of these antibodies would inhibit the binding of compatible blood cells in both the antibody screening phase and the crossmatch, resulting in a lack of compatibility.

The other options, while they may lead to incompatibility in different scenarios, do not best fit the information outlined in the question. For instance, recipient autoantibodies may affect the crossmatch but typically would not cause incompatibility against screening cells that are designed to detect such issues. Positive DATs in donors mainly indicate that the donor's red blood cells are coated with antibodies, which might cause a reaction if the recipient's serum was also reactive, but this is not the most likely cause given a broader antibody response