Rouleaux mimicking anti-H in molecularly confirmed para-Bombay siblings: a diagnostic pitfall.

The para-Bombay phenotype is a rare H-deficient blood group variant characterized by markedly reduced or absent expression of H on red blood cells (RBCs) because of mutations in FUT1, with variable secretor status influenced by FUT2. The Bombay phenotype is relatively more common in the Indian population, but the para-Bombay phenotype remains exceedingly rare and is frequently underrecognized because of diagnostic challenges during routine serologic testing. We report molecularly confirmed para-Bombay siblings from India in whom rouleaux-induced pseudoagglutination mimicked anti-H reactiv ity. The index case demonstrated absent reactivity with anti-H lectin and weak reactivity with pooled group O RBCs during reverse grouping and thermal amplitude testing, initially raising suspicion of clinically significant anti-H. However, detailed serologic evaluation, including microscopic examination, saline replacement, adsor ption-elution studies, and saliva inhibition testing, favored rouleaux-induced pseudoagglutination over true anti-H-mediated agglutination. Molecular analysis identified homozygous FUT1 c.328G>A and FUT2 c.357C>T mutations in both affected siblings, consistent with para-Bombay secretor phenotype and similar to previously reported variants in the Telangana population. This case highlights rouleaux-induced pseudoagglutination as an impor tant diagnostic pitfall in para-Bombay phenotypes. Integration of conventional serologic methods with molecular characterization was essential for accurate diagnosis, appropriate transfusion planning, and avoidance of unnecessary restrictions on compatible blood components.