Comparison of calibrated chromogenic anti-Xa assay and PT tests with LC-MS/MS for the therapeutic monitoring of patients treated with rivaroxaban

Possibilities to monitor rivaroxaban therapy could be useful in certain circumstances. Prothrombin time (PT) or chromogenic anti-Xa assays such as the Biophen Direct Factor Xa Inhibitor (R) (DiXal) have been proposed to estimate rivaroxaban concentrations but are mainly based on in vitro studies. The study aim was to compare PT and Biophen DiXal (R) measurements with liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurements in plasma samples from patients treated with Xarelto (R). Fifty-two plasma samples were included. PT was performed using Innovin (R) and Triniclot PT Excel S (R). Biophen DiXal (R) was performed according to instructions from the manufacturer. The rivaroxaban plasma concentration ranged between 0 and 485 ng/ml as measured by LC-MS/MS. The limits of quantification were 30 ng/ml and 5 ng/ml for Biophen DiXal (R) and LC-MS/MS, respectively. The linear correlation between Biophen DiXal (R) and LC-MS/MS analyses was high for all rivaroxaban concentrations (r(2) = 0.95). For concentrations <= 100 ng/ml, r(2)-value was 0.83. The Bland-Altman analysis showed a mean difference of -16 ng/ml (SD: 25 ng/ml). The, PT methods did not correlate well with plasma concentrations measured by LC-MS/MS (r(2) approximate to 0.60). In conclusion, the important inter-individual variability and the poor correlation with LC-MS/MS preclude the use of PT to estimate rivaroxaban concentrations. Thanks to its small inter-individual variability and good agreement with LC-MS/MS measurements, we recommend the use of Biophen DiXal (R) assays to estimate concentrations of rivaroxaban >30 ng/ml. Quantification of low rivaroxaban levels (<30 ng/ml) requires the LC-MS/MS method.