The interval between prothrombin time tests and the quality of oral anticoagulants treatment in patients with chronic atrial fibrillation

Background: The incidence of stroke in patients with atrial fibrillation (AF) can be significantly reduced with warfarin therapy especially if optimally controlled. Objectives: To evaluate the effect of the interval between consecutive prothrombin time measurements on the time in therapeutic range (INR 2-3) in a cohort of patients with AF on chronic warfarin treatment in the community. Methods: All INR measurements available from a relatively large cohort of patients with chronic AF were reviewed and the mean interval between consecutive INR tests of each patient was correlated with the time in therapeutic range (TTR). Results: Altogether 251,916 INR measurements performed in 4408 patients over a period of seven years were reviewed. Sixty percent of patients had their INR measured on average every 2 to 3 weeks and most others were followed at intervals of 4 weeks or longer. A small proportion (3.6%) had their INR measured on average every week. A significant decline in the time in therapeutic range was observed as the intervals between tests increased. At one to three weeks interval the TTR was 48%, at 4 weeks interval 45% and at 5 weeks 41% (P < 0.0005). A five percent increment in TTR was observed if more tests were performed at multiplications of exactly 7 days (43% vs 48% P < 0.0001). Conclusions: A better control with an increase in the TTR was observed in patients with atrial fibrillation if prothrombin time tests are performed at regular intervals of no longer than 3 weeks.