Monitoring of warfarin therapy: Preliminary results from a longitudinal pilot study

Abstract The aim of this study was to investigate the relationship between warfarin dosage, international normalized ratio, plasma and oral fluid concentrations of warfarin, and its metabolites, namely RR/SS- and RS/SR-warfarin alcohols. Nine patients on long-term warfarin therapy (4 with stable and 5 with unstable international normalized ratio values) were longitudinally monitored for over two months by recording warfarin dosage and measuring international normalized ratio, warfarin and warfarin alcohols concentrations in oral fluid and in plasma. At equivalent dose (20–30 mg week − 1 ), the international normalized ratio was in the range 2.0–2.5 and 1.5–3.1 for stable and unstable patients, respectively. Moreover, stable patients showed nearly double total and unbound plasma warfarin concentration, and nearly triple oral fluid warfarin concentrations compared to unstable patients. Correlations between warfarin dosage and total plasma concentration of warfarin ( r  = 0.65, p r  = 0.66, p r  = 0.72, p r  = 0.95, p