Blood group O and black race are independent risk factors for thrombotic thrombocytopenic purpura associated with severe ADAMTS13 deficiency.

BACKGROUND It has been postulated that blood group O subjects may be partially protected against thrombotic thrombocytopenic purpura (TTP) because they have lower plasma levels of von Willebrand factor. STUDY DESIGN AND METHODS The Oklahoma TTP Registry enrolled 301 consecutive patients from November 13, 1995 (when systematic ADAMTS13 measurements began), through 2009; 281 (93%) patients had ADAMTS13 measurements. Patients were designated as having severe ADAMTS13 deficiency when the activity measurement by either method was less than 10%. ABO blood group was determined in all 281 patients. The observed frequency of blood group O was compared to the expected frequency. The association between severe ADAMTS13 deficiency and blood group, race, sex, and age were analyzed by logistic regression. RESULTS The frequency of blood group O was unexpectedly and significantly greater than the race-ethnicity-adjusted expected frequency in 65 patients with severe ADAMTS13 deficiency (60.0% vs. 47.4%, p = 0.042) but not in 216 patients without severe ADAMTS13 deficiency (44.9% vs. 46.5%, p = 0.639). Blood group O and race-ethnicity were independently associated with severe ADAMTS13 deficiency among patients with TTP. The probability for severe ADAMTS13 deficiency was 45.8% with O and 32.1% with non-O blood groups for black patients and 24.1% with O and 15.1% with non-O blood groups for white patients. CONCLUSION Among patients with TTP and severe ADAMTS13 deficiency the relative frequency of patients with blood group O was greater than expected, suggesting that blood group O may be a risk factor for TTP associated with severe ADAMTS13 deficiency.