Rivaroxaban and other oral direct factor Xa inhibitors (ODiXa) are currently developed for prophylaxis and treatment of thromboembolic diseases using fixed doses. Although routine monitoring is not required, assessing the intensity of anticoagulation may be useful under certain clinical conditions. ODiXa prolong coagulation times of several clotting assays and, thus, their concentration may be determined in factor Xa specific chromogenic substrate assays. So far, no standardized and validated assay is commercially available. Here, five methods (A through E) are studied and optimized to reduce interassay variability. Human pooled plasma was spiked by a serial dilution of rivaroxaban (25–900 ng/ml). The release of para-nitroaniline from the chromogenic substrates was measured by the optical density (OD) at 405 nm. Method B was identified to yield the lowest sum of deviations from the mean value of the OD concentration curve calculated from all assays. Spline functions were developed for OD versus concentration curves for all methods. The calculated OD versus concentration curves overlapped for all methods. The coefficient of variation for all assays and concentrations of rivaroxaban decreased from 25.3 ± 11.4% using the original data to 3.8 ± 2.2% using the calculated data (P < 0.0001). The robustness of the chromogenic assay (method B) remains to be corroborated in interlaboratory comparisons.

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Summary. [14 C]-Serotonin release assay (14C-SRA) from platelets is considered to be the most sensitive test for laboratory confirmation of heparin-induced thrombocytopenia (HIT). This study compared 14C-SRA with an enzyme immunoassay (EIA) to determine the release of serotonin from platelets in the presence of heparin and serum from HIT patients. The normal range (median, 2´5 and 97´5 percentiles) of serotonin release from platelets in healthy subjects (n 149) is 38 ng/ml (19 and 62) measured by EIA-SRA. Serum from HIT patients (n 42) released 2548 ng/ml (244 and 7987) serotonin in the presence of 0´1 IU/ml heparin and 29 ng/ml (13 and 76) in the presence of 100 IU/ml heparin. One hundred per cent and 15% of HIT samples exhibited an elevated serotonin release from platelets in the presence of 0´1 IU/ml low molecular weight (LMW) heparin, 2100 ng/ml (869 and 5968), or danaparoid, 272 ng/ml (143 and 403), respectively. The sensitivity and specificity of the EIA-SRA was 100% and 97´4% and of the 14C-SRA 100% and 92´9% in HIT patients. No falsepositive results were found in patients receiving heparin (n 28), in patients with elevated levels of bilirubin (n 5), in patients with antiphospholipid antibody syndrome (n 10) or in non-HIT patients (n 78) with both assays. The EIA technique to quantify serotonin release from platelets provides a reliable non-radioactive method to diagnose heparin-induced thrombocytopenia and to assess in vitro crossreactivity of low molecular weight heparins and heparinoid.

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Rivaroxaban and other direct factor Xa inhibitors are used at fixed doses without drug monitoring and dose adjustment. Patients may require determination of the anticoagulant effect during treatment. The aim of this study was to develop a method to reduce the differences between thromboplastin reagents and coagulation analysers for determination of the anticoagulant effect of rivaroxaban in human plasma. Purity of rivaroxaban extracted from commercially available drug was confirmed by mass spectrometry, elemental analysis and 1H-NMR spectroscopy. Coagulation times of pooled human plasma spiked with 50-900  ng/ml rivaroxaban were analysed. Thromboplastin reagents, WHO RBT/90, Innovin, RecombiPlasTin 2G, STA Neoplastin Plus, Technoclot PT Plus and Thromborel S, the manual Kolle-Hook method and the KC10 analyser were used. An international sensitivity index (ISI) was determined for each reagent and coagulation method using the RBT/90 thromboplastin reagent as reference. The orthogonal, used for the determination of the ISI of coumarin plasmas, and ordinary regression analyses were compared. The results showed than increasing concentrations of rivaroxaban prolonged coagulation values of all thromboplastin assays linearly (r (2)= 0.96 and r(2) = 0.99, respectively). The coefficient of variation between the slopes of the dilution curves and the ratios of the thromboplastin reagents were reduced using the international normalized ratio (INR) and ISI calculated for rivaroxaban. The ISIs of the thromboplastin reagents ranged from 0.73 to 1.67 as compared with the WHO reagent using the manual technique. The coefficient of variations between the thromboplastin reagents comparing the orthogonal and the ordinary regression analysis were 6.8 versus 3.7% (Kolle-Hook method, P = 0.0011) and 8.5 versus 4.8% (KC10 method, P < 0.0001). Using ISI for vitamin-K antagonist and rivaroxaban, the INRs for the rivaroxaban-containing samples were significantly different for one of five commercial thromboplastin reagents. In conclusion, the determination of an ISI for rivaroxaban using a WHO thromboplastin reagent is required for commercial thromboplastin reagents. The manual Kolle-Hook method and an ordinary linear regression analysis should be adopted.