When a patient shows positive results with screening cells and multiple donor units, but has a negative autocontrol, what is the most likely antibody present?

In this scenario, the patient exhibits positive reactions with screening cells and multiple donor units but shows a negative autocontrol. This finding suggests the presence of an alloantibody rather than an autoantibody. The autocontrol being negative implies that the patient's own red blood cells are not reacting with the serum, indicating that any antibody present is likely directed against antigens not present on the patient’s red blood cells.

The presence of anti-k (also known as anti-Kell) as the likely antibody is consistent with the observed positives. Anti-k antibodies are known to cause reactivity with donor units if those units are positive for the k antigen, which is present on around 91% of the population. The fact that multiple donor units showed positivity suggests that they likely carry the corresponding antigen.

In contrast, anti-H is less commonly associated with such reactions because it is predominantly an ABO-related antibody and does not fit the pattern observed here. Anti-S would be less likely to cause widespread reactivity unless many donors carried that antigen, which is less common. Anti-Kpa is rarer and would less commonly cause reactions with multiple units unless specifically tested against units known to express that antigen.

Therefore, the most logical conclusion given the test results is the presence of anti