The Incidence of TTP-HUS: Racial Disparity among Patients with Severe ADAMTS13 Deficiency.
Authors: Williams Lauren A. , Terrell Deirdra R. , Lammle Bernhard , Kremer-Hovinga Johanna , George James N. , Vesely Sara K. 
Published: October 2019

Abstract
Previous studies have estimated the U.S. incidence of TTP to be 3.7/106/year, based on U.S. death certificates (Am J Hem1995;50:84), and 3.8/106/year, based on claims submitted to a national health insurer (Epidemiol2004;15:208). These methods are indirect and require extrapolation to determine incidence rates. We estimated the incidence directly from the Oklahoma TTP-HUS Registry. The Registry accrues all patients treated with plasma exchange (PE) for a clinical diagnosis of TTP or HUS (therefore excluding only children with typical HUS) in central, western, and southeastern Oklahoma since the Oklahoma Blood Institute (OBI) is the sole provider for PE in this region of 2.3 million people. Our estimates are based on patients who had their initial episode from 1/1/1996, after routine ADAMTS13 measurements began, to 6/30/2004. During these 8.5 years ADAMTS13 activity was measured in 189 (90%) of 209 consecutive patients. Prior to 1996, the number of patients accrued in the Oklahoma TTP-HUS Registry increased each year, similar to other data (JAMA2003; 290:1351); since 1996, there has been no consistent change in the annual number of patients accrued.
Categories Incidence (new cases/10 6 population/year) 1 All patients Black race 1 Other race 1 Cases Incidence 1 Cases Incidence 1 Cases Incidence 1 1Population data are from the 2000 US Census for the Oklahoma counties served by the OBI. Black race includes Black alone and in combination with one or more other races. Other race refers to all other racial designations. 2Urban or rural designations were determined by county of residence. Urban is the Oklahoma City Metropolitan Statistical Area; all other counties are rural. 3Excludes patients after BMT, pregnant/postpartum, drug-associated, bloody diarrhea prodrome, and additional or alternative diagnoses. All patients 209 10.64 40 24.08 169 9.40     Urban2 117 12.71 29 27.63 88 10.79     Rural2 92 8.82 11 17.99 81 8.25 Idiopathic 3 77 3.92 21 12.64 56 3.12 ADAMTS13 deficient (<5%) 27 1.37 12 7.22 15 0.83
The incidence of all patients with TTP-HUS from the Oklahoma Registry (10.64/106/year) was greater than previous reports, perhaps because of our inclusion of all patients in a defined geographic region who were treated with PE. The incidence of patients with idiopathic TTP-HUS (3.92/106/year) was similar to the previous reports for TTP. The incidence of patients with severe ADAMTS13 deficiency (<5% activity) was 1.37/106/year. For all patients with TTP-HUS the incidence of patients living in an urban area was 1.4-fold the incidence of patients living in rural areas. The rate of urban excess was greater for patients with idiopathic TTP-HUS (1.8-fold) and patients with ADAMTS13 deficiency (3.2-fold) (data not shown). Part of the urban excess may reflect better recognition of TTP-HUS because of familiarity with the Registry. However much of the urban excess can be attributed to the racial disparity that is greatest among patients with severe ADAMTS13 deficiency. Although the number of patients with severe ADAMTS13 deficiency in the Oklahoma TTP-HUS Registry is small, the incidence among the Black population was 8.7-fold the incidence among the non-Black population (95% CI, 4.1–18.5, p<0.0001). The explanation for why Black race is a risk factor for TTP is unknown, but may be related to the reasons why there is a greater prevalence of other vascular and thrombotic diseases among Blacks.