The rate of thromboembolism in patients with atrial fibrillation (AF) who interrupt ongoing warfarin treatment is unclear.

This retrospective, cohort study of 149,151 patients with a first-time hospitalisation for AF in Denmark between 1997and 2008, analysed the link between interruption of warfarin therapy and risk and timing of thromboembolic complications. Incidence rate ratios (IRRs) of thromboembolic events and all-cause mortality were calculated.

The following findings were reported for 48,989 AF patients receiving warfarin, of whom 35,396 had at least one episode of warfarin treatment interruption:

• 8255 deaths or thromboembolic events occurred during treatment interruption showing an initial clustering of events with 2717, 835, 500, and 427 events occurring during 0–90, 91–180, 181–270, and 271–360 days after treatment interruption respectively.

• The corresponding crude incidence rates for the above were 31.6, 17.7, 12.3, and 11.4 events per 100 patient-years.

• The first 90-day interval of treatment interruption was associated with a markedly higher risk of death or thromboembolism (IRR 2.5; 95% CI, 2.3 to 2.8) vs. the interval of 271 to 360 days.

The researchers conclude from these findings that “in an unselected nationwide population of patients with AF, almost three out of four patients on warfarin treatment had one or more periods of treatment interruption. Interruption of warfarin treatment was associated with a significantly increased short-term risk of thrombo-embolism or death during the first 90 days of interruption.” They add that this “underlines the importance of careful and individual consideration of the balance between risk of thrombosis and bleeding before interrupting warfarin treatment.”