Advantages and Disadvantages of Washed Platelet Assays

High sensitivity and specificity is the major advantage of the washed platelet activation assays. In our hands, utilizing "weak positive" control HIT sera for quality control, the sensitivity of the SRA for clinical HIT is very high (>95%) (Warkentin et al., 2000, 2005a), and only reduced by the small number (<5%) of patient samples that yield "indeterminate" results (Smith et al., 2006). The diagnostic specificity is also high in most clinical settings, especially if the test gives a very strong result (>80% serotonin release) and all the controls react as expected.

The major disadvantage of washed platelet assays for detecting HIT antibodies is that they are technically demanding and labor intensive. A workshop that compared a washed platelet assay (the HIPA test) and an antigen assay showed greater variability in activation assay results among the participating laboratories (Eichler et al., 1999). Washed platelet activation assays are best suited for reference laboratories assessing many HIT sera, as this facilitates acquisition of sufficient technical experience to perform the assay successfully on a consistent basis. Assay-specific disadvantages include the requirement for radioactivity (SRA), the use of a subjective, visual endpoint (HIPA), and expensive equipment (flow cytometry-based assays).

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