ADAMTS13 testing plays a critical role in confirming the clinical diagnosis of acquired idiopathic thrombotic thrombocytopenic purpura (TTP) and distinguishing it from other forms of thrombotic microangiopathies (TMA). Serial measurements of ADAMTS13 activity and inhibitor levels are also helpful in determining response to treatment and/or subsequent relapses. Numerous ADAMTS13 assays have been developed recently, including some with rapid turnaround times. Despite the good inter-assay correlation of different ADAMTS13 methodologies in published case studies, discrepancies have been shown to occur. Here we present a case where discrepant results were obtained using two different assays, posing a clinical treatment dilemma.