Effect of Bombay phenotype on plasma-VWF glycan expression, plasma VWF:Ag level, and plasma-VWF multimer composition. (A) The amount of H antigen expressed per unit VWF was measured in a series of healthy individuals (group O, n = 72; group A, n = 64; group AB, n = 15) and in Bombay (n = 30) or para-Bombay (n = 17) individuals using a modified sandwich ELISA. Each plasma sample was tested in duplicate at 3 dilutions, and results represent means ± SEM. Using similar methodologies, no A or B antigen expression was detected on Bombay or para-Bombay VWF (data not shown). In some cases the SEM cannot be seen due to its small size. (B) Plasma VWF:Ag levels were measured by ELISA. Median values for each group are shown. VWF:Ag levels were significantly lower in Bombay compared with groups AB, A, and B (^***P < .001, ^**P < .01, and ^*P < .05, respectively). Among the blood group A individuals, genotype (A₁A₁, A₁O₁, or A₂O₁) at the ABO locus was determined by polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) analysis as previously described.¹²  Bombay VWF:Ag levels were significantly lower than those in A₁A₁ homozygotes or A₁O₁ heterozygotes (P < .01). Although previous studies have demonstrated an effect of Secretor blood group on plasma-VWF levels, we found no difference in plasma VWF:Ag levels between para-Bombay (Secretor) and Bombay (non-Secretor) individuals (data not shown). (C) Plasma multimer analysis of 4 Bombay individuals (B1 to B4) compared with 2 healthy controls. No loss of HMW-VWF multimers was apparent in the Bombay individuals. (D) Plasma VWF:CB levels were also significantly reduced in Bombay plasmas (median VWF:CB = 71 IU/dL), compared with group O (median VWF: CB = 88 IU/dL; P = .04, Mann-Whitney). However, as shown for the 47 Bombay individuals, there remained a good correlation between VWF:Ag and VWF:CB. NP indicates normal plasma.