Oral anticoagulants in the elderly.

Over a 1-year period, 34,998 prothrombin time tests were performed on 2,379 patients aged 40-89 years with a recommended target International Normalized Ratio (INR) value of 2.5. At least one INR value of >/= 5 was found in 507/2,379 patients (21.3%). The elderly demonstrated higher maximum and lower minimum INR values, lower warfarin doses, and an increased number of tests performed. After adjustment for these factors, the risk of INR values >/= 5 increased with age by 15% every 10 years (95% CI 4-28%). We conclude that age is a risk factor for more unstable prothrombin time results.