If the Test Is Positive the Patient Has HIT

Serologic tests are important in confirming the diagnosis and should be ordered whenever the diagnosis is reasonably suspected. Nevertheless, the practical limitations of currently available tests must be appreciated by clinicians. In terms of sensitivity-specificity tradeoff, reproducibility, and availability of results in "real time," no assay is highly satisfactory. The commercially available enzyme(-linked) immunosorbent assays (EIAs or ELISAs) are the most widely used, and have the advantages of standardization, ease of performance, wide availability, and high sensitivity, but a major problem is the very high rate of "false positives" for diagnosing clinical HIT. A question has emerged of where appropriate cut-off values should be between positive and negative defined in differing clinical situations. For example, 1-2 wk after heart surgery, 25-70% of patients will have a "positive" EIA test, but only 3% to 5% of these will actually have clinical HIT. Interpretation of the EIA is greatly aided if the EIA optical density (a proxy for antibody titer) is taken into account, as most true positives are high titer, or if a functional (platelet activation) assay, such as the platelet serotonin release assay (SRA), is positive. It is an unfortunate fact of life that interpreting HIT serologic tests is much like interpreting many other tests obtained in clinical medicine: one has to understand the disease process, its likelihood, and the limitations of testing (see Chapter 10). As always, the interpretation of the test begins with clinical judgment and assessment of pre-test probability. Accordingly, information on the temporal course of platelet counts in relation to heparin exposure, or a scoring system such as the 4 T's, can be invaluable (Lo et al., 2006; see Chapter 3). There is little question that the overly sensitive EIAs can lead to HIT overdiagnosis by the unwary, sometimes producing its own untoward consequences.

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