Can an anti-Xa assay for unfractionated heparin be used to assess the presence of rivaroxaban in critical situations?

OBJECTIVE: Although rivaroxaban has recently become widely used for thrombosis treatment, it is difficult for clinicians to make clinical decisions in critical situations, such as emergent surgery or interventions, because a specific anti-Xa assay is not available in many laboratories. This study assessed the relationships between rivaroxaban-specific anti-factor Xa activity (AXA) and unfractionated heparin (UFH)-specific AXA and determined the cutoff level for UFH-specific AXA in critical situations for patients undergoing rivaroxaban therapy. METHODS: Thirty-eight blood samples were collected from patients with cancer-associated thrombosis receiving rivaroxaban therapy. All samples were assessed using both rivaroxaban-specific and UFH-specific anti-Xa assays. Routine coagulation studies, including prothrombin time (PT) and activated partial thromboplastin time, were also conducted on the samples. RESULTS: A positive dose-dependent correlation between rivaroxaban-specific and UFH-specific AXA was evident (R = 0.97; P < .0001). Rivaroxaban-specific AXA was also positively correlated with PT (R = 0.95; P < .0001) but only weakly with activated partial thromboplastin time (R = 0.67; P < .0001). Patients with plasma rivaroxaban concentrations <100 ng/mL were found to have UFH-specific AXA <1.41 IU/mL and PT <17.3 seconds, with sensitivities of 100% and 93.3% and specificities of 87.0% and 95.7%, respectively. CONCLUSIONS: Our study demonstrates that UFH-calibrated AXA correlates strongly with plasma rivaroxaban concentration. This assay appears to be sensitive to the presence of rivaroxaban, which may be advantageous in the setting of assessing drug levels for critical events. These findings suggest that if a rivaroxaban-specific anti-Xa assay is unavailable, the chromogenic anti-Xa assay for UFH may be useful to assess the anticoagulant effects of rivaroxaban.