[Oral anticoagulation in older patients. Establishment and validation of a new posologic warfarin regimen]

Warfarin is highly effective in preventing thromboembolism and more recent clinical trials have established that adjusted dosing is highly effective in reducing the risk of ischemic stroke in patients with nonvalvular atrial fibrillation. Fear of major hemorrhage frequently dissuades physicians from use of anticoagulants in older people. In addition, the time needed to reach the therapeutic range may be excessively long and delicate in this population.

This study was undertaken in two phases. In the first phase, 20 patients (mean age 84 years) were given 5 mg of warfarin once daily for 3 consecutive days. During the following days, the dose of warfarin was adjusted to reach an International Normalized Ratio (INR) in the therapeutic 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 establish 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 years (range 74-99).

The predicted dose on day 4 was effective in 56% within +/- 0.5 mg and in 92% within +/- 1 mg of the original predicted dose. No hemorrhagic complication occurred during the study. The therapeutic range was reached on day 4 in 63.5% and on day 1 in 91% of the patients.

We have developed a method of predicting the maintenance dose of warfarin in a very old population based on the INR. This method is safe and easy to use.