Anti-Factor Xa levels correlate with recurrent venous thromboembolism and clinically relevant bleeding in patients receiving low-molecular-weight heparin

Background: The aim of our study was to investigate whether anti-Xa levels assessed in selected groups of LMWH-treated patients with VTE correlate with the risk of developing thromboembolic or bleeding complications. Methods: Retrospective study, in adult patients treated with anti-Xa-adjusted LMWH analysing the association between anti-Xa activity and recurrent VTE or bleeding. Primary outcomes were symptomatic fatal or nonfatal objectively diagnosed VTE and clinically relevant bleeding (ISTH criteria). Results: From 2007-2017, 192 patients were recruited. Mean age was 62 (SD 18) years and 48% were men. Indications for anti-Xa monitoring were: VTE recurrence under anticoagulation (20.8%), prior bleeding (20.3%), renal dysfunction (45.3%), or obesity (9.9%). A total of 33 (17.2%) patients had a recurrent VTE event and 34 (17.7%) experienced clinically relevant bleeding. In the period elapsing between the first (performed after a median of 6 days from index VTE [IQR 1-39]) and the second anti-Xa determination, 9 patients developed a clinical outcome (4 recurrent and 5 bleeding events). An anti-Xa value below reference (versus within + above) correlated with recurrence (relative risk 3.0), whereas an anti-Xa value above (versus within + below) reference correlated with bleeding (relative risk 2.5). Conclusions: In selected subgroups of patients with VTE, anti-Xa levels may represent a marker for the risk of developing recurrent VTE or bleeding events. Whether adjustment of the LMWH dosage would minimize this risk remains to be investigated in management studies.