Isolated HIT is defined as the initial recognition of HIT because of thrombocytopenia alone, rather than because symptoms or signs of thrombosis draw attention to the possibility of underlying HIT. A large retrospective cohort study (Warkentin and Kelton, 1996) suggests that the subsequent frequency of new, progressive, or recurrent thrombosis is relatively high in such a patient population with serologically confirmed HIT (Fig. 2). Although these data are retrospective, the investigators attempted to minimize bias. First, the date that the HIT assay was ordered was used as an objective marker of first suspicion of the diagnosis of HIT. Second, patients were excluded from analysis if there was any evidence in the medical records to suggest the possibility of new signs or symptoms of thrombosis that may have caused the physician to suspect HIT. In other words, efforts were made to identify patients in whom HIT was suggested because of thrombocytopenia alone. Finally, only objectively documented new, progressive, or recurrent thrombotic events were analyzed.
The study identified 62 patients who met the definition of isolated HIT. The 30-day cumulative risk for thrombosis in this study was 52.8% (Fig. 2).
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