The aim of the present study was to perform an economic evaluation of two alternative assays of ADAMTS13 activity (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats, member 13) for diagnosing thrombotic thrombocytopenic purpura (TTP) in the Hospital of Lyon (France). The study approach was more economic than clinical. We retrospectively calculated the prescription costs of ADAMST13 activity from January to December 2019 for patients depending on the assay: manual ELISA (Technozym) or automated assay (AcuStar Werfen, Instrumentation Laboratory). Then, we compared the cost of therapeutic plasma exchange (TPE) consumption awaiting ADAMTS13 activity assay results. From an economic point of view, the automated assay was more cost-effective. From a clinical one, we supposed that the faster results given by AcuStar could improve patient care by reducing the number of TPEs. Automated assay could improve patient care without increasing costs in our institution.