Evaluation of the Anti-Factor Xa Chromogenic Assay for Measuring Rivaroxaban Plasma Concentrations Using Calibrators and Controls

Rivaroxaban is an oral, direct Factor Xa inhibitor in an advanced stage of clinical development for the prevention and treatment of thromboembolic disorders. It inhibits Factor Xa activity without requiring cofactors (such as antithrombin) and has been found to have predictable pharmacokinetics and pharmacodynamics in humans. Routine coagulation monitoring is not required, but a quantitative determination of rivaroxaban concentrations might be useful in some cases, such as severe overdose or to measure compliance. Although the prothrombin time assay may be of assistance for the assessment of peak rivaroxaban plasma concentrations using calibrators and controls as previously shown, alternative methods such as the anti-Factor Xa chromogenic assays may allow the measurement of a wider rivaroxaban plasma concentration range.

A multicenter study was initiated to evaluate the suitability of the anti-Factor Xa chromogenic assay for the measurement of rivaroxaban plasma concentrations (ng/mL) using rivaroxaban calibrators and controls, and to assess the inter-laboratory precision of the measurement. A total of 25 centers were provided with sets of rivaroxaban calibrators (0, 41, 209, and 422 ng/mL) and a set of rivaroxaban pooled human plasma controls (20, 199, and 662 ng/mL). The concentrations of the human plasma controls were unknown to the participating laboratories. The evaluation was carried out over 2 consecutive weeks (10 days) by each laboratory using its own anti-Factor Xa reagents as well as 1 provided centrally (modified STA® Rotachrom, Diagnostica Stago, Asnières-sur-Seine, France). A calibration curve was produced each day and the day-to-day precision was evaluated by testing in duplicate 3 human plasma controls. The plasma control sample was diluted (1:3 dilution) and re-tested if the measured level was above the highest concentration of the calibration curve.

Preliminary results from 8 centers showed that the mean rivaroxaban concentrations obtained were 21, 199, 549, and 708 ng/mL when using the modified STA Rotachrom method, or 19, 200, 556, and 676 ng/mL when using the local anti-Factor Xa reagents for the human plasma controls (19, 160, and 643 [undiluted and diluted samples] ng/mL, respectively). The mean coefficients of variation were 17.7%, 4.6%, 1.9%, and 8.9%, respectively, when using the modified STA Rotachrom method, or 33.7%, 4.7%, 2.2%, and 7.7%, respectively, when using the local anti-Factor Xa reagents.

The preliminary results of this multicenter field trial suggest that the anti-Factor Xa chromogenic method, using rivaroxaban calibrators and controls, is suitable for measuring a wide range of rivaroxaban plasma concentrations.

Samama:Bayer Healthcare: Consultancy. Spiro:Bayer Healthcare Pharmaceuticals Inc.: Employment. Perzborn:Bayer Schering Pharma AG: Employment.