Biomarkers of terminal complement activation confirm the diagnosis of aHUS and differentiate aHUS from TTP.

Atypical hemolytic uremic syndrome (aHUS) is characterized by dysregulated complement activity, the development of a thrombotic microangiopathy (TMA), and widespread end organ injury. aHUS remains a clinical diagnosis without an objective laboratory test to confirm the diagnosis. We performed a retrospective analysis of 103 patients enrolled in the Ohio State University TTP/aHUS Registry presenting with an acute TMA. Nineteen patients were clinically categorized as aHUS based on the following criteria: (1) platelet count <100 × 10(9)/L, (2) serum creatinine >2.25 mg/dL, and (3) a disintegrin and metalloprotease with thrombospondin type 1 motif, 13 (ADAMTS13) activity >10%. Sixteen of 19 patients were treated with plasma exchange (PEX) therapy, with 6/16 (38%) responding to PEX. Nine patients were treated with eculizumab with 7/9 (78%) responding to therapy. In contrast to thrombotic thrombocytopenic purpura (TTP) patients, no aHUS patients demonstrated ultralarge von Willebrand factor multimers at presentation. Median markers of generalized complement activation (C3a), alternative pathway (Bb), classical/lectin pathway (C4d), and terminal complement activation (C5a and C5b-9) were increased in the plasma of these 19 patients. Compared with a cohort of ADAMTS13-deficient TTP patients (n = 38), C5a and C5-9 were significantly higher in the 19 patients clinically characterized as aHUS, suggesting that pretreatment measurements of complement biomarkers C5a and C5b-9 may confirm the diagnosis of aHUS and differentiate it from TTP.