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

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.