If five out of six crossmatched units are compatible and one is incompatible with a negative antibody screen, what could be the problem?

The scenario describes a situation in which five out of six crossmatched blood units are compatible, but one unit shows incompatibility despite a negative antibody screen. This suggests that there might be an issue not detected by the standard antibody screening methods.

In this context, the most plausible explanation for the incompatibility is that the donor unit has a positive Direct Antiglobulin Test (DAT), which indicates the presence of antibodies bound to the red blood cells of the donor. If the donor unit has antibodies against antigens that are present on the patient’s red blood cells, this would lead to incompatibility during the crossmatch process. A positive DAT would not necessarily trigger a reaction in the antibody screen if the specific antibodies involved are not routinely tested for or are not present in significant amounts, leading to the false reassurance of compatibility based on the screening.

By contrast, options discussing the patient having an alloantibody to a high-frequency antigen or the donor having a high-frequency antigen suggest scenarios where the antibody or antigen might not be accounted for in routine testing, creating complexities. However, these would not adequately explain the direct incompatibility seen in crossmatching when a negative antibody screen was performed. Similarly, the mention of the patient having an abnormal protein, while it can be