It is important to incorporate clinical information into the interpretation of any laboratory result for HIT. This is because thrombocytopenia, whether or not caused by HIT, is common in hospitalized patients receiving heparin, and because nonpathogenic HIT antibodies are often detected by sensitive assays in patients who have received heparin for 5 or more days.
Several clinical scoring methods have been described to help estimate the probability of HIT independently of the HIT antibody test results (Greinacher et al., 1994a; Pouplard et al., 1997; Warkentin, 2003a; Warkentin and Heddle, 2003; Lo et al., 2006). Some include assessing the platelet count recovery upon stopping heparin, and so may be more useful when reviewing a case after its clinical evolution. Chapter 2 provides an example of one scoring system to estimate the pretest probability of HIT that can be applied at the time of initial diagnostic assessment.
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