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Heparin-induced thrombocytopenia (HIT) is a life-threatening and limb-threatening, immune-mediated, prothrombotic disease resulting from an interaction between heparin and platelet factor 4 (PF4). Due to the many causes of thrombocytopenia in critically ill patients, the diagnosis of HIT is difficult, requiring both a high clinical suspicion and confirmatory testing. The ELISA test is the most commonly performed method to detect anti-PF4 antibodies; however, the ELISA results generally take one or more days to report. The particle immunofiltration assay (PIFA) test has the advantage of being done rapidly; with results generally available within 1 hour.
Starting in July 2009, patients in our MICU were screened daily for thrombocytopenia; defined as either a platelet count that decreased by at least 30% from baseline or an absolute platelet count less than 150,000. Patients with thrombocytopenia underwent both PIFA and GTI ELISA testing for anti-PF4 antibodies. PIFA is a qualitative test reporting results as positive or negative. The GTI ELISA test reports an optical density (OD), with an OD greater than 0.40 considered positive. Patients were followed through hospitalization.
One hundred and thirty-eight patients were admitted to the MICU, with 47 developing thrombocytopenia. The PIFA results were negative in 22 patients, positive in 21 patients and inconclusive in four patients. For all patients with a negative PIFA result, the GTI ELISA test was negative. All four inconclusive PIFA tests had negative GTI ELISA tests. In patients with PIFA-positive results, two GTI ELISA tests were positive and 19 tests were negative. A serotonin release assay (SRA) was done in 13 patients with discordant PIFA/GTI ELISA tests, of which one SRA result was positive. None of the 47 patients with thrombocytopenia developed thrombotic complications.
Our analysis suggests that a negative PIFA test can quickly exclude the presence of anti-PF4 antibodies and therefore HIT as the cause of the thrombocytopenia. The PIFA test can be used as a rapid screening procedure for patients with possible HIT, and, when negative, decrease the exposure of thrombocytopenic MICU patients to alternative anticoagulation. The clinical significance of a positive PIFA test requires further study and must be evaluated in conjunction with clinical judgment and, as appropriate, additional testing for anti-PF4 antibodies.