Assessment of Rivaroxaban Plasma Concentration Using Chromogenic Anti‐Xa Assays and UHPLC–MS/MS in Chinese Patients With Atrial Fibrillation

ABSTRACT Accurate quantification of rivaroxaban concentration is essential in some specific clinical situations. A prospective study was conducted to compare the rivaroxaban concentration measured by Zhenyuan anti‐Xa assay with that by reference methods (ultrahigh performance liquid chromatography with tandem mass spectrometry [UHPLC–MS/MS] and Biophen DiXal) in 243 plasma samples from 182 patients with non‐valvular atrial fibrillation. Zhenyuan anti‐Xa assays demonstrated less bias versus reference methods in samples with concentrations exceeding 50 μg/L compared to those in the < 50 μg/L group. Strong correlations were observed between Zhenyuan anti‐Xa assays and both reference methods (Pearson's correlation coefficient 0.976 and 0.988, respectively). However, Bland–Altman analysis revealed systematic underestimation by Zhenyuan anti‐Xa assays, with mean biases of 41.87 μg/L (vs. UHPLC–MS/MS) and 26.76 μg/L (vs. Biophen DiXal), particularly pronounced at higher levels. Rivaroxaban concentration in patients from clinical settings was with greater variability compared to the expected ranges. Patients taking underdose of rivaroxaban are more likely to have a trough concentration falling below the targeted therapeutic range.