Improving the specificity of the PF4 ELISA in diagnosing heparin-induced thrombocytopenia.

Heparin induced thrombocytopenia (HIT) is a serious complication of heparin therapy. The PF4 ELISA is a serologic assay that provides laboratory support for the clinical diagnosis of HIT, but it is often positive in patients who do not have the syndrome. We examined whether the specificity of the PF4 ELISA can be improved by 1) taking antibody potency into consideration, 2) by measuring only IgG antibodies, and 3) by utilizing a high concentration heparin inhibition step. We reviewed clinical information on 116 patients whose samples were referred for HIT antibody testing and assigned each a clinical score related to the likelihood of the patient having HIT. The scores were then correlated with serologic findings. Patients with strongly positive PF4ELISA results (OD ≥ 1.0) using both versions of the assay (IgG/A/M and IgG only) had clinical scores and SRA activity that were significantly higher than those having reactive or negative results. When the IgG-only PF4 ELISA was used, only the strongly positive result group had significantly higher clinical scores and SRA release, and fewer samples were classified as weakly positive or reactive, suggesting that detection of IgG only in the PF4 ELISA improves the assay's specificity. The heparin inhibition step identified "reactive" samples that were associated with clinical scores and SRA release indistinguishable from the "negative" result groups, confirming that this step further improves specificity of the test. This study supports utilizing these 3 modifications of the PF4 ELISA to improve specificity in supporting the clinical diagnosis of HIT.