Rivaroxaban concentrations were measured in 127 inpatient samplesusing an HPLC-MS/MS assay.Methods: We compared this measurement with a calibrated anti-Xaassay and performedPT, aPTT and dilute PT tests to assess the value of clot-basedassays in clinicaldecision-making.Results: The correlation between the anti-Xaassay and the HPLC-MS/MS at therapeuticconcentrations was strong (R2 = 0.98). The PT, RecombiPlasTin 2G, and aPTT,Actin FS, showed a linear dose-responsebut poor correlation (R2 = 0.32 and 0.44,respectively) and at dilutions of 1 in 150 to 1 in 750 the dilute PT assay also showedpoor correlation with rivaroxaban concentrations measured by specific assays. A normalPT or aPTT alone did not identify a likely safe rivaroxaban concentration to allowsurgery or invasive procedures, but the combination of normal PT and aPTT identifieda group of patients with rivaroxaban levels less than 90 ng/mL. Combined normalPT and aPTT had specificity and sensitivity of 0.97 (95% CI 0.92-0.99)and 0.37(95% CI 0.1-0.74)for a rivaroxaban concentration < 32 ng/mL.Conclusions: The PT and aPTT show poor correlation with rivaroxaban levels measuredby calibrated anti-Xaand HPLC-MS/MS assays. A normal combined PT andAPTT identified low rivaroxaban levels with high specificity but lacked sensitivity.The dPT assay at several dilutions could not be used to quantify rivaroxaban in clinicalsamples. The utility of these PT, aPTT and dilute PT assays in a clinical setting isvery limited, and results generated must be interpreted with caution.
- activated partial thromboplastin time
- anti‐Xa assay
- prothrombin time