An incompatible unit should not be transfused into this patient with a history of alloantibodies. Nineteen out of 20 units are compatible and should be transfused if needed.
Antibodies to low-prevalence antigens are typically identified as a nonspecific positive result on an antibody panel or at the full crossmatch stage of testing. Because the incompatible antigen is of low prevalence, very few red cells in inventory will be positive for this antigen and further screening will identify compatible units. Furthermore, this unit can be re-issued to another patient who does not have this specific antibody.
Most of the units (19/20) were crossmatch compatible with this patient; thus, surgery should proceed without delay. The nature of this specific antibody may never be resolved, however an attempt to identify it with selected antibody panels can be performed.
If the patient had an autoantibody, it would have been seen on pretransfusion testing, not at the crossmatch step. In addition, in the presence of a warm autoantibody, all fully crossmatched units would be expected to be incompatible.
If the patient had an antibody to a high-prevalence antigen then almost all of the 20 crossmatched units would be crossmatch incompatible. Almost all units contain high-prevalence antigens. Fortunately, most patients do not make antibodies to these high-prevalence antigens because they express them too.