A NEW REGIMEN FOR STARTING WARFARIN THERAPY IN OUT-PATIENTS

Aims Oral anticoagulation is increasingly used in elderly patients wit h atrial fibrillation to prevent embolic phenomena. The use of anticoa gulants in this population is prophylactic rather than therapeutic and so there is no urgency to establish anticoagulation within the des:re d therapeutic range. The aim of the study was to develop an out-patien t regimen for initiation of oral anticoagulation with warfarin which r equires only weekly monitoring of the International Normalized Ratio ( INR). Methods The study was undertaken in two phases. In the first pha se, factors which predict the final maintenance dosage of warfarin wer e defined and used to build a decision tree and dosage algorithm. In t he second study the algorithm was tested. Patients were given 2 mg war farin daily for 2 weeks and the INR at this time was used to predict t he maintenance dose. Patients then attended for weekly measurements of the INR until steady state had been reached. Dosage adjustments were not made unless the INR was >4.0 or <1.5 for 2 consecutive weeks. The accuracy of the prediction was measured by calculating the mean INR of weeks 6-8 and the number of patients in the target range 2.0-3.0 was determined. Results One hundred and seven consecutive out-patients (me an age 70 years range 64-86) completed the first study. The age, sex, height, weight, alcohol intake, number of cigarettes smoked, concomita nt medication, clinical evidence of right heart failure,:liver failure , abnormalities in Liver enzyme estimations, baseline INR and INR afte r 2 weeks of 2 mg warfarin daily were used in a polytomous logistic re gression analysis with stepwise inclusion of factors to determine whic h factors influenced the eventual maintenance dosage of warfarin. The INR after 2 weeks of 2 mg warfarin therapy predicted 70% of the variab ility of the maintenance dose. Of other factors only the sex of the pa tient had a large enough effect to be included in the prediction algor ithm. One hundred and six patients (mean age 71 years range 50-85 year s) completed the second study. Only one patient needed a dose adjustme nt in the first 2 weeks of warfarin 2 mg daily (INR 4.4). Overall, 60% patients were in the narrow target range (INK 2.0-3.0) at steady stat e. In five patients the INR was >4.0 at any visit after the second wee k and needed dosage adjustment. In four patients the IT JR was <1.5 at steady state. Conclusions We have developed a method of predicting th e maintenance dose of warfarin in an elderly population based on the I NR after 2 weeks of warfarin 2 mg daily, and the sex of the patient. T his is a safe and convenient way of initiating warfarin therapy as an out-patient which requires only weekly INR checks.