Background: Up to 40% of patients treated for acute venous thromboembolism (VTE) with a standard dose of unfractionated heparin (UFH) never reach therapeutic anticoagulation within the first 48 hours of treatment. Heparin binds and activates the enzyme inhibitor antithrombin III (AT), which then inactivates thrombin, factor Xa and other proteases. Low AT activity in the setting of VTE could explain the difficulty in achieving therapeutic UFH anticoagulation for some patients. However, the proportion of patients with low AT levels immediately after VTE diagnosis is not known.

Aims: Quantify the proportion of patients with VTE who have low AT activity immediately after diagnosis.

Methods: We performed a secondary analysis of a multicenter study of patients with VTE diagnosed in emergency departments of two university hospitals between 2016-2018. The study was approved by the Human Research Committee of Partners Healthcare. All patients had hypercoagulability testing performed < 24h after VTE diagnosis to avoid confounding by heparin treatment. We used descriptive statistics to calculate proportions of patients with AT activity < 80U/dL.

Results: We enrolled 378 patients. The mean age was 54 (SD=17) years and 223 (59%) were male. Overall, 46/378 (12%) VTE, including 17/161 (10%) deep vein thrombosis and 29/210 (14%) pulmonary embolism patients had AT levels below < 80U/dL.

Conclusions: AT deficiency is relatively common in patients with acute VTE; and >200 times more prevalent than hereditary AT deficiency. Low AT levels associated with acute VTE may contribute to the difficulty of therapeutic anticoagulating VTE patients using heparin.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/antithrombin-iii-levels-immediately-following-acute-venous-thromboembolism/