Oral anticoagulation in older people: prospective evaluation of a new warfarin regimen and prediction of maintenance dose

OBJECTIVE: Warfarin is highly effective in preventing thromboembolism and m ore recent clinical trials have established that adjusted dosing is highly effective in reducing the risk of ischemic stroke in patients with nonvalvu lar atrial fibrillation. Fear of major hemorrhage frequently dissuades phys icians from use of anticoagulants in older people. In addition, the time ne eded to reach the therapeutic range may be excessively long and delicate in this population. PATIENTS AND METHODS: This study was undertaken in two phases, In the first phase, 20 patients (mean age 84 years) were given 5 mg of warfarin once da ily for 3 consecutive days. During the following days, the dose of warfarin was adjusted to reach an International Normalized Ratio (INR) in the thera peutic range (between 2 and 3). The good correlation (r = - 0,77, p < 0.01) between the INR on day 4 and the daily maintenance dose was used to establ ish an algorithm to predict the maintenance dose of warfarin, In the second phase, this algorithm was successfully tested in 94 elderly patients, mean age 84 year; (range 74-99). RESULTS: The predicted dose on day 4 was effective in 56% within <plus/minu s>0.5 mg and in 92% within +/- 1 mg of the original predicted dose. No hemo rrhagic complication occurred during the study. The therapeutic range was r eached on day 4 in 63.5% and on day 1 in 91% of the patients. CONCLUSION: We have developed a method of predicting the maintenance dose o f warfarin in a very old population based on the INR. This method is safe a nd easy to use.