Overdiagnosis of heparin-induced thrombocytopenia in surgical ICU patients.

BACKGROUND: Heparin use in surgical patients places them at increased risk for developing heparin-induced thrombocytopenia (HIT). The false positive rate of HIT using the current standard criteria is unknown in surgical ICU patients, who often have confounding factors that cause thrombocytopenia.

STUDY DESIGN: Surgical ICU patients, admitted over a 2-year period with a positive screening test for HIT (platelet factor [PF] 4 ≥ 0.4 optical density [OD]), were reviewed retrospectively at a single institution. Correlation of the Warkentin 4-T score and commercial heparin PF4 ELISA with serotonin releasing assay (SRA) was performed. Logistic regression was used to determine independent risk factors associated with the development of HIT.

RESULTS: PF4 tests were requested in 643 patients based on a clinical suspicion of HIT. Of these, 104 patients had a PF4 result, an SRA value (%), and a 4-T score available. Twenty patients (19%) had true positive HIT, defined as a positive PF4 and positive SRA (SRA ≥ 20%). Eighty-four patients (81%) were false positive, defined as a positive PF4 and negative SRA. Five of 58 patients with Warkentin score of 0 to 3, and 6 of 14 patients with Warkentin score of 6 to 8 were HIT positive by SRA.

CONCLUSIONS: In surgical ICU patients, clinical suspicion for HIT necessitates PF4 and SRA analysis. Testing for HIT or treatment with a direct thrombin inhibitor should not depend on the Warkentin 4-T score alone. Although a PF4 ≥ 0.4 OD is considered a positive screening test for HIT, a PF4 ≥ 2.0 OD is preferable in surgical ICU patients.