Comparison of Anti-Xa and Dilute Russell Viper Venom Time Assays in Quantifying Drug Levels in Patients on Therapeutic Doses of Rivaroxaban

Abstract

Context.-Rivaroxaban is a new oral anticoagulant that functions as a direct anti-Xa inhibitor. Although routine monitoring is not required, measurement of plasma concentrations may be necessary in certain clinical situations. Routine coagulation assays, such as the prothrombin time and, to a lesser degree, activated partial thromboplastin time, correlate with drug concentration, but because of reagent variability, these methods are not reliable for determining rivaroxaban anticoagulation.Objective.-To compare different methods and calibrators for measuring rivaroxaban, including the chromogenic anti-Xa assay, which, when calibrated with a rivaroxaban standard, may be more appropriate for determining anticoagulation.Design.-We compared measured rivaroxaban concentrations with the same anti-Xa kit but used different calibrators, with different anti-Xa kits but the same calibrators, with antithrombin-supplemented anti-Xa kit versus nonsupplemented kits, and with 2 methods based on rivaroxaban-calibrated, high-phospholipid, dilute Russell viper venom time. Regression and paired t test statistics were used to determine correlation and significant differences among methods and calibrator sources.Results.-Although there was strong correlation, statistically significant biases existed among methods that report rivaroxaban levels. A single-source calibrator did not alleviate those differences among methods. High-phospholipid Russell viper venom reagents correlated with rivaroxaban concentration but were not better than chromogenic anti-Xa methods.Conclusions.-Rivaroxaban-calibrated, anti-Xa measurements correlate well, but the clinical significance of the variation with rivaroxaban measurements is uncertain. The antithrombin-supplemented, anti-Xa method should be avoided for measuring rivaroxaban.

DOAC plasma levels measured by chromogenic anti‐Xa assays and HPLC‐UV in apixaban‐ and rivaroxaban‐treated patients from the START‐Register

“…In line with previous reports, our data showed that the antithrombin‐supplemented anti‐Xa assay (BerHep‐SI) had the highest LOD and LOQ levels, and overestimated apixaban and rivaroxaban concentrations compared to HPLC‐UV, thus limiting its use for an accurate quantitative measure of drug levels, especially at low concentrations.…”

Direct Oral Anticoagulants (DOACs) in the Laboratory: 2015 Review

“…Mass spectrometry, when calibrated with each drug to be measured, is considered the gold-standard method and demonstrates good accuracy and precision over a broad concentration range although this test is not widely available [28]. More rapid methods including the dilute thrombin time, ecarin methods and chromogenic anti-Xa assays are potentially suitable means to measure DOACs, but must employ calibrators and controls specific for (or referenced against) the DOAC being measured [19,20,22,28,30]. Despite their availability, problems associated with existing assays used to quantitate DOACs include lack of: 1) FDA-approved DOAC calibrators or kits, 2) validated expected therapeutic plasma concentrations, and 3) knowledge of plasma concentrations associated with increased thrombotic or hemorrhagic risk.…”

“…Despite their availability, problems associated with existing assays used to quantitate DOACs include lack of: 1) FDA-approved DOAC calibrators or kits, 2) validated expected therapeutic plasma concentrations, and 3) knowledge of plasma concentrations associated with increased thrombotic or hemorrhagic risk. Furthermore, clot-based and chromogenic assays demonstrate variation between instrument/reagent systems, and also lack specificity [30,31].…”

“…[ Table 2] None-the-less, some heparin calibrated anti-Xa assays may have sufficient lower limits of quantitation to determine that rivaroxaban concentration is 5 ng/mL or lower and therefore can be used to evaluate patients prior to an intervention, such as surgery or the use of thrombolytics [31,52]. The use of a high phospholipid dRVVT assay calibrated with the specific drug to be measured may also be an alternative means for quantifying anti-FXa DOACs [30,48].…”

Practical Value of Anti-Xa Activity in the Evaluation of Extracorporeal Circuit Anticoagulation during Haemodialysis: Results of a Cross-Sectional Single-Centre Study

“…Apart from the larger patient cohort analysed in this study, a possible explanation for the lower anti-Xa activity found during dialysis could be the different antiXa reagent used. An important factor that introduces variability in the anti-Xa activity measured is the addition versus omission of exogenous antithrombin to the anti-Xa assay, which has been previously shown in the estimation of UFH and rivaroxaban concentrations [13,14] . Some anti-Xa reagents contain antithrombin to correct for in vivo antithrombin deficiency, as LMWHs need to complex with antithrombin to be able to bind and deactivate factor Xa [15] .…”