Bleeding time, blood groups and von Willebrand factor.

The bleeding time in healthy volunteers was determined according to both the Ivy and the Simplate II techniques. A significantly longer bleeding time in people with blood group O than in people with non-O blood groups was demonstrated with both techniques. This difference could not be attributed to a difference in sex ratio, platelet count or haematocrit. The mean level of von Willebrand factor in blood group O is lower than in non-O blood groups, but we found no association between the level of von Willebrand factor and the bleeding time, despite the very broad range of von Willebrand factor levels in the subjects examined.