Summary
Although there is no need for routine coagulation monitoring with rivaroxaban – an
oral, direct factor Xa inhibitor – a haemostasis assay might be valuable to measure
its pharmacodynamic effects. This study aimed to find assays, among those commercially
available, to measure rivaroxaban pharmacodynamics. Several global conventional clotting
tests, as well as clotting or chromogenic assays to measure anti-factor Xa activity,
were studied. A thrombin generation test using calibrated automated thrombogram was
also done. Tests were performed with the indirect factor Xa inhibitor fondaparinux
for comparison. A concentration-dependent prolongation of prothrombin time (PT), dilute
PT, and activated partial thromboplastin time was observed with rivaroxaban. The results
varied depending on the reagents. This variability cannot be standardised with the
international normalised ratio system commonly used for vitamin K antagonists. Using
a standard calibration curve, PT test results can be expressed in plasma concentrations
of rivaroxaban rather than PT seconds or ratio. Standard methods for HepTest and two-step
prothrombinase-induced clotting time (PiCT) resulted in a paradoxical response, with
low concentrations of rivaroxaban reducing clotting times. This was not observed with
shorter incubation times, or when antithrombin-deficient (immunodepleted) plasma was
used. The chromogenic tests found a dose-dependent relationship between anti-factor
Xa activity and rivaroxaban concentration. Modified specific factor Xa chromogenic
assays are being further investigated. One-step PiCT and HepTest with shortened incubation
times, as well as the widely available PT assay (using a rivaroxaban calibrator) could
be useful to monitor the pharmacodynamic effects of rivaroxaban accurately. Finally,
all clotting and chromogenic assays showed a concentration-dependent effect induced
by rivaroxaban.
Keywords
Clotting assays - factor Xa inhibitor - fondaparinux - pharmacodynamics - rivaroxaban
- venous thromboembolism