Heparin resistance as detected with an antifactor Xa assay is not more common in venous thromboembolism than in other thromboembolic conditions

Abstract

Study Objective. To identify whether heparin resistance when measured with an antifactor Xa assay is more frequent in patients with venous thromboembolic disease than in patients with other thromboembolic conditions. Design. Prospective observational cohort study. Setting. A 625-bed, adults-only, private teaching hospital. Patients. Three hundred seventy-two patients receiving intravenous unfractionated heparin for a variety of indications classified as either venous thromboembolism or arterial thromboembolism. Intervention. All patients received an initial unfractionated heparin dosage according to a protocol based on age and estimated blood volume. Targeted antifactor Xa was 0.35-0.65 U/ml. Measurements and Main Results. A modest increase in heparin dosage requirement was noted in patients with venous thromboembolism that was entirely related to lower hemoglobin concentration in that group compared with the arterial thromboembolism group. Low hemoglobin concentration was significantly associated with increased heparin resistance regardless of treatment indication. Conclusions. True heparin resistance seen with an antifactor Xa assay is not more common in venous thromboembolism than in other conditions. Low hemoglobin concentration, rather than treatment indication, appears to be associated with increased heparin resistance.