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Context.--Acquired thrombotic thrombocytopenic purpura (A-TTP) is a rare but significant disease requiring rapid diagnosis and treatment. The diagnosis is often difficult because of variability in the presence of specific clinical criteria. The primary etiology of A-TTP involves inhibitors directed against ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). Literature has shown that the ADAMTS13 activity assay is sensitive and specific for identifying cases of A-TTP, and application of this test as an on-site screening method has not been fully explored. Objective.--Our objective is to determine if the ADAMTS13 activity assay can be used as a successful, on-site diagnostic modality to rapidly identify cases of ATTP and prevent unnecessary use of prophylactic therapeutic plasma exchange. Design.--A retrospective analysis was performed including 152 patients with clinically suspected A-TTP, screened using the ADAMTS13 activity assay. Results were correlated with potential therapeutic plasma exchange treatment for all cases highly suspicious for A-TTP and evaluated for unnecessary patient morbidity and financial cost. Results.--The ADAMTS13 activity assay had an overall sensitivity and specificity of 100% and 99%, respectively. The positive predictive value was 91% and the negative predictive value was 100%. In 95% of the studies ordered, A-TTP was ruled out, leading to decreased patient morbidity and $1.7 million of potential treatment costs avoided. Conclusion.--Implementation of the fluorescence energy transfer-based ADAMTS13 activity assay as a point-of-care laboratory study decreased patient morbidity while also directing more efficient employment of therapeutic plasma exchange in cases of suspected A-TTP. [ABSTRACT FROM AUTHOR]