Patients on treatment with vitamin-K-antagonists (VKA) are monitored by the international normalized ratio (INR). The international sensitivity index (ISI) standardizes the differences between prothrombin assays (PT) through a comparison with a WHO thromboplastin reagent. Rivaroxaban prolongs PT assays dose dependently. The steepness of these curves differs between the PT assays. PT assays have not been standardized for rivaroxaban using a WHO Thromboplastin reagent.
Rivaroxaban was extracted from commercially available drug with dichloromethane. Identity and purity of the isolated compound were confirmed by mass spectrometry (exact mass 436,0734), elementary analysis (carbon, hydrogen, nitrogen content) and 1H-NMR spectroscopy. Pooled plasma from healthy persons was spiked with 25 ng/ml to 900 ng/ml rivaroxaban. Thromboplastin reagents were WHO RBT/90, neoplastinPlus, recombiplastin, innovin and thromborel. All assays were performed with the manual kolla-hook method and with the KC coagulometer.
Increasing concentrations of rivaroxaban prolonged coagulation values of all PT assays linearly (coefficient of correlation between r=0.97 and r=0.99). The steepness of the dilution curves differed substantially between assays. Accordingly, the ratios for the rivaroxaban concentrations varied (ratio=PTriva(xi)/PTriva(i0)). The coefficient of variation CV of the ratios between methods ranged from 7% to 33%. The ISI for rivaroxaban was calculation for each method by x=(y-a)/b (x=ratio WHO-kolla hook manual method, y=ratio method new, a=intercept, b=steepness). The ISIRiva was 1 for the WHO reagent using the manual method and ranged from 0.94 to 1.67 for the other PT-methods compared to the WHO RBT/90 PT reagent. The equation INRriva=ratiorivaISIriva was used to calculate INRriva. The CVs of INRriva between methods decreased to 1.1% to 7.2% (mean 3.9%) as compared to the ratio without ISIRiva.
The differences of the CV between TP-reagents were reduced to about 4% using the WHO RBT/90 thromboplastin reagent manual method as standard and an ISI for rivaroxaban.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.