Utility of Complement and ADAMTS13 Testing in Diagnosis of Thrombotic Microangiopathy: A Single Centre Retrospective Review

Background: A syndrome of TMA presenting with MAHA and thrombocytopenia can result from a variety of etiologies. Making an accurate diagnosis is important to guide treatment decisions and to inform prognosis. Our centre is one of the two adult apheresis units in the Greater Toronto Area, Ontario, Canada with a catchment area of over 5 million people. Our centre maintains a database of all patients who were referred for investigation and treatment of TMA. We present our initial experience with complement testing (protein, function and genetics) in identifying patients with aHUS.

Materials and Methods: Complement genetics studies included screening CFI, CFH, CFB, MCP/CD46, CFHR5, C3, APLN and THBD/CD141 genes and were performed at The Hospital for Sick Children, Toronto, Ontario, Canada. Complement protein and function studies (CH50, AH50, C3d, sC5b-9, C3, factor H, factor I, and anti-factor H antibody) were performed in the laboratory of Michael Kirschfink, University of Heidelberg, Germany. ADAMTS13 testing was performed at St. Michael’s Hospital by a qualitative collagen binding assay before 2011 and after 2011 by ELISA (Technoclone GmbH, Vienna, Austria).

Results: Between Jan 1 2010 and Dec 31 2013, 63 patients were referred with a presumed diagnosis of TTP for plasmapheresis. On presentation, all patients had evidence of MAHA and thrombocytopenia. Two additional patients had a history of TMA and were referred for a second opinion.

One patient died before any diagnosis could be established and was excluded from further analysis.

Based on diagnostic work-up, the 64 patients could be divided into 4 groups.

Group 1: 32 patients were found to be ADAMTS13 deficient (ADAMTS13 activity less than 5%) and were diagnosed with TTP. One additional patient was diagnosed with TTP relapse even though her ADAMTS13 was more than 5% at the time of presentation.

Group 2: 11 patients had clearly established alternative diagnoses: malignancy (4); STEC-HUS (3); scleroderma renal crisis (2); Clopidogrel induced TMA (1); hemaphagocytic syndrome (1). None of these patients underwent further testing.

Group 3: In 10 patients, TMA was associated with (and perhaps caused by) other factors including postpartum state (2), splenectomy (1), EtOH binge (1), sepsis, post alloBMT (1), dyskeratosis congenita, post alloBMT (1), autoimmune disease (4). Six out of ten had evidence of acute kidney injury and underwent complement genetics testing. No disease causing mutations were identified in 5 patients and a possible disease causing mutation was identified in CFH gene in one patient. Five patients were eventually given a diagnosis of aHUS. Only 3 of these patients had complement protein studies done and the results were as follows: no abnormalities (1), elevated sC5b-9 (2), low C3 (2), low factor I (1).

Group 4: 11 patients were diagnosed with aHUS. They all presented with TMA and acute kidney injury and no precipitant or alternative diagnosis could be identified. Out of these 11 patients, 9 patients had complement genetics done and the results were as follows: no disease causing mutation identified (6), mutation in CFHR5 (2), mutation in CFI (1). 5 patients had complement protein studies done. Notable findings included no abnormalities in any of the parameters (2), anti-H antibody (1), elevated sC5b-9 (2), low C3 (1), and elevated AH50 (3).

Conclusion: From all patients referred to our apheresis unit with TMA, 50% were eventually diagnosed with TTP and 25% with aHUS. Careful work-up is warranted to assist with therapeutic decisions, however, at present no one test can accurately identify patients with aHUS. Utilizing the current genetic screen, only 25% (4/16) of patients clinically diagnosed with aHUS had genetic abnormalities detected. Half of patients diagnosed with aHUS underwent complement antigen and function testing and the most common results were elevated sC5b-9 (4), elevated AH50 (3), low C3 (3) and no abnormalities (3). The study is ongoing.