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Laboratory testing for heparin-induced thrombocytopenia (HIT) includes the highly sensitive, though less specific, heparin/platelet factor 4 (PF4) ELISA. A confirmatory test with excess heparin is routinely performed on positive ELISA results to improve test specificity; the significance of a negative confirmatory result is unknown. The aim was firstly to evaluate the clinical utility of the PF4 ELISA confirmatory assay, secondly to examine the relationship between ELISA optical density (OD) value and clinical diagnosis of HIT, and thirdly to assess current practice at a tertiary care medical centre regarding patients with anti-heparin/PF4 antibodies. Patients with anti-heparin/PF4 antibodies detected by commercial ELISA during 2005 were identified. A confirmatory test was performed on positive ELISA results. Patients were labeled confirmatory positive (confirm+) or confirmatory negative (confirm-). Patients were classified as HIT+ (met criteria for HIT), HIT? (HIT possible), and HIT- (did not meet criteria for HIT) utilizing ACCP guidelines. One hundred fifteen patients with anti-heparin/PF4 antibodies were identified. Ninety-eight patients were confirm+; 17 were confirm-. The majority of confirm+ patients were HIT+ or HIT?(72%); the majority of confirm- patients were HIT-(81%). Patients who were HIT+/confirm+ had higher ELISA OD values than patients who were HIT?/confirm+ or HIT-/confirm+ (p = 0.031, p = 0.001). Two confirm- patients were HIT+, one was HIT?; all had high ELISA OD values. Although confirm+ status correlated with clinical HIT, the confirmatory procedure misclassified some patients by yielding a confirm- result despite clinical HIT with high ELISA OD values. Future studies should compare higher ELISA OD values with the confirmatory procedure as strategies to improve ELISA diagnostic specificity for HIT.

Anti-heparin/platelet factor 4 antibody optical density values and the confirmatory procedure in the diagnosis of heparin-induced thrombocytopenia