While beginning a registry for immune thrombocytopenic purpura (ITP) in Oklahoma, we noted that the number of black patients seemed smaller than expected. At the same time we read a report suggesting that ITP is rare among black Africans and people of African ancestry in other parts of the world.1 However, in standard textbooks and in major review articles on ITP, race is not mentioned. Therefore, we systematically searched the Medline database for all publications on ITP from the United States to identify articles describing 10 or more patients that identified patients by race; we found only 6 articles.2-7 These articles report data from different regions of the United States and across many years.
In each of the 6 articles,2-7 the proportion of blacks among patients with ITP was lower than the proportion of blacks in the population (Table 1). In 5 of the published studies, the 95% confidence interval (CI) for the observed percent of blacks among the ITP patients did not overlap with the percentage of blacks in the population.2-4,6,7 One of these articles commented on their observation that only 4 (6%) of 67 patients with ITP were black, compared with 25% of their hospital patients.2
A potential limitation of this study is our selection of census data. Although our calculations were based on United States census data for the cities from which the patients were reported and from a time near to patient accrual (Table 1), the percentage of blacks in the census data that we used may not accurately reflect the geographic origin of the reported patients if referral areas were large.
Our awareness of potential racial disparities was stimulated by our recent demonstration, from the Oklahoma Thrombotic Thrombocytopenic Purpura Hemolytic Uremic Syndrome (TTP-HUS) Registry, of an increased proportion of blacks among patients with TTP and severe ADAMTS13 deficiency. The age- and sex-standardized incidence rate ratio of blacks to nonblacks was 9.3 (95% CI, 4.3-19.9).8 The apparent opposite racial disparity among patients with ITP and TTP is intriguing, but different racial disparities also occur among other autoimmune disorders. For example, the prevalence of systemic lupus erythematosus (SLE) is greater among blacks,9 while the prevalence of anticardiolipin antibodies preceding SLE is greater among whites.10
If a racial disparity among patients with ITP is confirmed, it may indicate a genetic influence on the etiology of ITP. Alternative explanations may be that the diagnosis of ITP is made less often among blacks because of disparities in health care11 or because the presence of petechiae and ecchymoses may not be appreciated.
