A pilot study of the association of pharmacokinetic and pharmacodynamic parameters of warfarin with the dose in patients on long-term anticoagulation.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: Warfarin is a widely used anticoagulant with a low therapeutic index. There is wide interindividual variation in the pharmacokinetics and pharmacodynamics of warfarin which is also reflected in the warfarin dose requirement. CYP2C9 and VKORC1 polymorphisms have been shown to affect warfarin dose requirement. However a large amount of the variation in warfarin dose remains unaccounted for. WHAT THIS STUDY ADDS: Our findings suggest that in patients who are on long-term warfarin therapy, INR : plasma 7-hydroxywarfarin concentration correlates well with warfarin requirement and also accounts for a large amount of variation in warfarin dose. AIMS: To assess the correlation between plasma total warfarin concentration, plasma 7-hydroxywarfarin concentration and INR and the weekly doses of warfarin in patients on long-term anticoagulation. METHODS: Twenty-five patients on long-term anticoagulation with warfarin were studied. Plasma total warfarin and 7-hydroxywarfarin concentrations and INR were determined. Equations were derived with the weekly warfarin dose as the dependent variable and plasma total warfarin concentration : plasma 7-hydroxywarfarin concentration, INR : plasma total warfarin concentration and INR : plasma 7-hydroxywarfarin concentration as independent variables. RESULTS: There was a good correlation between INR : plasma total warfarin concentration and the weekly dose of warfarin (y = 46.73e(-0.30x), r(2) = 0.65). There was a better correlation between INR : plasma 7-hydroxywarfarin concentration and the weekly dose of warfarin (y = 156.52x(-0.63), r(2) = 0.74) CONCLUSIONS: Pharmacokinetic parameters along with INR seem to correlate with the weekly doses of warfarin in patients on long-term anticoagulation. These parameters may therefore be useful for predicting warfarin doses.