A patient has been grouped as O Rh negative. All screening cells and antibody panels are positive with no rouleaux suspected. All crossmatches are incompatible. What is the likely explanation?

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The key idea is that some individuals lack the H antigen that underlies the ABO system. In Bombay phenotype (hh), red cells do not express H, and the person’s serum contains a strong anti-H antibody. This antibody reacts with any red cells that have H antigen, leading to panreactivity on antibody screening and always incompatible crossmatches with most donor units.

In this scenario, the patient is grouped as O Rh negative, but all screening cells and the antibody panel show reactivity, and every crossmatch is incompatible. That pattern fits anti-H reacting with cells that express H antigen, which would be present on normal donor units but absent on Bombay donor units. Therefore only hh (Bombay) donor units would be compatible. This explains why the tests show widespread positivity and incompatibility across crossmatches despite typing as O Rh negative.

Other possibilities don’t fit as well. Autoimmune hemolysis would typically show a DAT positive result and might not produce the same universal incompatibility pattern across all donor units. Mixed-field reactions imply a mix of cell populations rather than a universal, panreactive antibody. The universal donor concept does not apply here because Bombay individuals are not universally compatible donors themselves; they require hh donor units due to their lack of H antigen and presence of anti-H in their serum.