Prediction of the international normalized ratio and maintenance dose during the initiation of warfarin therapy

Aims A pharmacokinetic/pharmacodynamic model, with Bayesian parameter estim ation, was used to retrospectively predict the daily International Normaliz ed Ratios (INRs) and the maintenance doses during the initiation of warfari n therapy in 74 inpatients. Methods INRs and maintenance doses predicted by the model were compared wit h the actual INRs and the eventual maintenance dose. Cases with drugs or me dical conditions interacting with warfarin or receiving concurrent heparin therapy were not excluded. As the study was retrospective, model prediction s of the maintenance dose were not those that were administered. Mean predi ction error (MPE) and percentage absolute prediction errors (PAPE) were use d to assess the model predictions. Results INR MPE ranged from -0.07 to 0.06 and median PAPE from 10% to 20%. Dose MPE ranged from -0.7 to 0.17 mg and median PAPE from 16.7% to 37.5%. A ccurate and precise dose predictions were obtained after 3 or more INR feed back's. Conclusions This study shows that the model can accurately predict daily IN Rs and the maintenance dose in this sample of cases. The model can be incor porated into computer decision-support systems for warfarin therapy and may lead to improvement in the initiation of warfarin therapy.