Relative incidence of thrombotic thrombocytopenic purpura and haemolytic uraemic syndrome in clinically suspected cases of thrombotic microangiopathy

Background. Data are lacking on the relative incidence of thrombotic thrombocytopenic purpura (TTP), haemolytic uraemic syndrome (HUS) caused by Shiga toxin-producing Escherichia coli (STEC) and atypical HUS (aHUS) in patients presenting with thrombotic microangiopathies (TMAs). Methods. This was a prospective, cross-sectional, multicentre and non-interventional epidemiological study. Patients fulfilling criteria for TMAs (platelet consumption, microangiopathic haemolytic anaemia and organ dysfunction) were included in the study. The primary objective was to assess the relative incidence of TTP, STEC-HUS, aHUS and 'other' physician-defined diagnoses. The secondary objective was to develop an algorithm to predict a severe deficiency in ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) activity (<= 10%) using routine laboratory parameters. A post hoc classification using the recent Kidney Disease: Improving Global Outcomes diagnostic criteria was then undertaken to further classify patient groups. Results. aHUS was diagnosed with a relative incidence of 61%, whereas TTP, STEC-HUS and 'other' were diagnosed in 13, 6 and 20% of patients, respectively. In the post hoc analysis, 27% of patients with a TMA were classified as 'primary aHUS' and 53% as 'secondary aHUS'. Multivariate analysis revealed that severe deficiency in ADAMTS13 activity (<= 10%) was unlikely to underlie TMA if platelet and serum creatinine were above threshold values of 30 x 10(9)/L and 1.8 mg/dL, respectively (negative predictive value of 92.3 and 98.1, respectively, if one or both values were above the threshold). Conclusions. In this study, aHUS was the most common single diagnosis among patients presenting with a TMA. In the absence of an ADAMTS13 activity result, platelet count and serum creatinine may aid in the differential diagnosis.