Can an anti-Xa assay for low-molecular-weight heparin be used to assess the presence of rivaroxaban?

Background Due to the convenience afforded by the lack of required laboratory monitoring, direct oral anticoagulants (DOACs) are increasingly used as alternatives to Vitamin-K antagonists for certain medical conditions. However, there are circumstances in which assessment of DOAC plasma concentrations may be helpful in guiding clinical decisions, including patients presenting with either bleeding or thrombosis, or patients requiring urgent invasive procedures. Evaluating the anticoagulant effects of DOACs is often difficult because of the limited availability of DOAC-specific assays in most laboratories. Objective To evaluate the correlation between ex vivo plasma concentrations of rivaroxaban and a chromogenic anti-Xa assay for low-molecular-weight heparin (LMWH) routinely used in our coagulation laboratory. Materials and Methods Twenty-nine blood samples from 20 patients anticoagulated with rivaroxaban (dose; 10–20 mg/day) were evaluated using an anti-Xa assay for LMWH and results were correlated with rivaroxaban plasma concentrations using a rivaroxaban specific assay. Results A linear dose-dependent relationship was demonstrated between plasma concentrations of rivaroxaban and the chromogenic anti-Xa assay for LMWH (R2 = 0.92). PT and PTT demonstrated poor correlations (R2 = 0.03; and R2 = 0.01, respectively) with rivaroxaban plasma concentrations. Conclusion Findings from this study suggest that if specific assays for rivaroxaban are unavailable, then the chromogenic anti-Xa assay for LMWH may be useful for assessing the anticoagulant effects of rivaroxaban.