Accurate determination of rivaroxaban levels requires different calibrator sets but not addition of antithrombin
Helen ManiGabriele RohdeGertrud StratmannChristian W. HesseNatalie HerthStephan SchwersElisabeth PerzbornEdelgard Lindhoff‐Last
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Summary Rivaroxaban is a direct factor Xa inhibitor, which can be monitored by anti-factor Xa chromogenic assays. This ex vivo study evaluated different assays for accurate determination of rivaroxaban levels. Eighty plasma samples from patients receiving rivaroxaban (Xarelto®) 10 mg once daily and 20 plasma samples from healthy volunteers were investigated using one anti-factor Xa assay with the addition of exogenous antithrombin and two assays without the addition of antithrombin. Two different lyophilised rivaroxaban calibration sets were used for each assay (low concentration set: 0, 14.5, 59.6 and 97.1 ng/ml; high concentration set: 0, 48.3, 101.3, 194.2 and 433.3 ng/ml). Using a blinded study design, the rivaroxaban concentrations determined by the assays were compared with concentrations measured by HPLC-MS/MS. All assays showed a linear relationship between the rivaroxaban concentrations measured by HPLC-MS/MS and the optical density of the anti-FXa assays. However, the assay with the addition of exogenous anti-thrombin detected falsely high concentrations of rivaroxaban even in plasma samples from controls who had not taken rivaroxaban (intercept values using the high calibrator set and the low calibrator set: +26.49 ng/ml and +13.71 ng/ml, respectively). Plasma samples, initially determined by the high calibrator setting and containing rivaroxaban concentrations <25 ng/ml, had to be re-run using the low calibrator setting for precise measurement. In conclusion, anti-factor Xa chromogenic assays that use rivaroxaban calibrators at different concentration levels can be used to measure accurately a wide range of rivaroxaban concentrations ex vivo. Assays including exogenous antithrombin are unsuitable for measurement of rivaroxaban.Keywords:
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To the Editor.—
In a letter to the editor, Roger L. Bick, MD (239:296, 1978), commented on the clinical determination of antithrombin III, pointing out the usefulness of chromogenic substrate and that an increased predisposition to thromboembolic phenomena is associated with a decreased antithrombin III level. In reply, Harry L. Messmore, MD, and Jawed Fareed, PhD (240:345, 1978), questioned the use of autolytic forms of thrombin and the chromogenic substrate in the antithrombin III assay. The methods employed for antithrombin III assay can be roughly classified into two categories: immunological and functional. There are two types of functional assays available: the rate determination procedure and the end-point analysis. The rate of the inhibition of thrombin by antithrombin III depends on (1) the amount of antithrombin III present, (2) the amount of heparin or heparin-like substance present in the sample, and (3) the quantity of thrombin employed. It was first pointedChromogenic
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Abstract Introduction To measure direct factor Xa inhibitor (apixaban, edoxaban, rivaroxaban) concentrations, dedicated chromogenic anti‐Xa assays are recommended as suitable methods to provide rapid drug quantification. Moreover, the high‐performance liquid chromatography with ultraviolet detection (HPLC‐UV) is reported as a reliable quantitative technique. We investigated seven anti‐Xa assays and an HPLC‐UV method for measurement of apixaban and rivaroxaban levels in patients enrolled in the START‐Register. Methods A total of 127 apixaban and 124 rivaroxaban samples were tested by HPLC‐UV and the following anti‐Xa assays: Biophen DiXaI and Heparin LRT (Hyphen BioMed), Berichrom and Innovance Heparin (Siemens), STA‐Liquid Anti‐Xa (Stago Diagnostics), Technochrom anti‐Xa (Technoclone), and HemosIL Liquid Anti‐Xa (Werfen). Each method was performed in one of the participating laboratories: Bologna, Cremona, Florence, and Padua. Results Our data confirmed the overestimation of apixaban and rivaroxaban levels by the antithrombin‐supplemented anti‐Xa method (Berichrom). Performances and reproducibility of the six anti‐Xa assays not supplemented with antithrombin and the HPLC‐UV method were good, with limits of quantification from 8‐39 ng/mL (apixaban) and 15‐33 ng/mL (rivaroxaban). The six chromogenic methods showed good concordances with the quantitative HPLC‐UV [bias: −26.9‐22.3 ng/mL (apixaban), −11.3‐18.7 ng/mL (rivaroxaban)]. Higher bias and wider range between limits of agreement were observed at higher concentrations [<100 ng/mL: bias −21.3‐4.1 ng/mL (apixaban) and −6.2‐3.8 ng/mL (rivaroxaban); >200 ng/mL: bias −42.2‐36.8 ng/mL (apixaban) and −20.1‐68.9 ng/mL (rivaroxaban)]. Conclusion Overall, the anti‐Xa assays not supplemented with antithrombin and the HPLC‐UV method proved to be suitable for apixaban and rivaroxaban quantification.
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Spectrophotometric heparin assays which are based on the catalytic effect of heparin on either the inactivation of thrombin or that of factor Xa by antithrombin III, were adapted for use in a laboratory batch analyzer. Optimal conditions were determined for assays using the chromogenic substrates Chromozym-Th and S-2238 with thrombin, and S-2222 with factor Xa. Inactivation of the clotting enzyme by antithrombin III was stopped by addition of chromogenic substrate. Assays thus obtained appeared to be applicable in a wider range of heparin concentrations and were less dependent on plasma antithrombin III concentration that known manual spectrophotometric methods. The best results were obtained with the methods based on thrombin inactivation and applying a logarithmic reference curve.
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Summary Rivaroxaban is a direct factor Xa inhibitor, which can be monitored by anti-factor Xa chromogenic assays. This ex vivo study evaluated different assays for accurate determination of rivaroxaban levels. Eighty plasma samples from patients receiving rivaroxaban (Xarelto®) 10 mg once daily and 20 plasma samples from healthy volunteers were investigated using one anti-factor Xa assay with the addition of exogenous antithrombin and two assays without the addition of antithrombin. Two different lyophilised rivaroxaban calibration sets were used for each assay (low concentration set: 0, 14.5, 59.6 and 97.1 ng/ml; high concentration set: 0, 48.3, 101.3, 194.2 and 433.3 ng/ml). Using a blinded study design, the rivaroxaban concentrations determined by the assays were compared with concentrations measured by HPLC-MS/MS. All assays showed a linear relationship between the rivaroxaban concentrations measured by HPLC-MS/MS and the optical density of the anti-FXa assays. However, the assay with the addition of exogenous anti-thrombin detected falsely high concentrations of rivaroxaban even in plasma samples from controls who had not taken rivaroxaban (intercept values using the high calibrator set and the low calibrator set: +26.49 ng/ml and +13.71 ng/ml, respectively). Plasma samples, initially determined by the high calibrator setting and containing rivaroxaban concentrations <25 ng/ml, had to be re-run using the low calibrator setting for precise measurement. In conclusion, anti-factor Xa chromogenic assays that use rivaroxaban calibrators at different concentration levels can be used to measure accurately a wide range of rivaroxaban concentrations ex vivo. Assays including exogenous antithrombin are unsuitable for measurement of rivaroxaban.
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To investigate whether the characteristics of a commercial test kit for antithrombin III (Berichrom Antithrombin III) could be influenced by surfactants such as Tween-80 or polyethylene glycol (PEG), we performed some experiments with the original kit reagents and with the reagents dissolved in surfactants. Neither the reliability of the calibration curve nor the data for precision and assay kinetics were amended by the addition of either PEG to the (human) thrombin reagent or Tween-80 to the chromogenic substrate. In the same test system, assays with some other chromogenic substrates and with bovine thrombin showed comparable behavior. Evidently, if one follows the working scheme proposed for this kit, the use of surfactants is not warranted.
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Azocalix[4]arene derivatives 1–5, with one or two azophenol chromophores and different binding sites, were found to allow not only for high selective and sensitive colorimetric detection but also easy colorimetric differentiation of F−, AcO− and H2PO4−, of similar basicity, depending upon the azocalix[4]arene structure, guest basicity and conformational complementarity between the host and guest.
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