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Introduction Heparin-induced thrombocytopenia ( HIT) is a life-threatening condition, in which the anticoagulant heparin, platelet factor 4 ( PF4), and platelet-activating antibodies form complexes with prothrombotic properties. Laboratory tests to support clinical diagnosis are subdivided into functional, platelet activation assays, which lack standardization, or immunological assays, which have moderate specificity toward HIT. In this study, clinical performance of HIT A lert, a novel in vitro diagnostic ( IVD) registered platelet activation assay, was tested in a large cohort of HIT-suspected patients and compared with immunological assays. Methods From 346 HIT-suspected patients (single center), clinical data including 4T pretest probability results, citrated platelet-poor plasmas, and sera were collected, allowing direct comparison of clinical observations with HIT A lert results. HIT A lert performance was compared with PF4 IgG ELISA (246 patients, three centers) and PF4 Pa GIA (298 patients, single center). Results HITAlert showed high sensitivity (88.2%) and specificity (99.1%) when compared with clinical diagnosis. Agreement of HIT A lert with PF4 ELISA- and PF4 Pa GIA-positive patients is low (52.7 and 23.2%, respectively), while agreement with PF4 IgG ELISA- and PF4 Pa GIA-negative patients is very high (98.1 and 99.1%, respectively). Conclusion HIT A lert performance is excellent when compared with clinical HIT diagnosis, making it a suitable assay for rapid testing of platelet activation due to anticoagulant therapy. [ABSTRACT FROM AUTHOR]