Is there a period of liability with initiation of warfarin in patients with atrial fibrillation?

This editorial refers to ‘Initiation of warfarin in patients with atrial fibrillation: early effects on ischaemic strokes’†, by L. Azoulay et al., on page 1881. Patients with atrial fibrillation (AF) have a high risk of ischaemic stroke. This risk can be dramatically reduced by anticoagulation with vitamin K antagonists (VKAs; e.g. warfarin) or novel anticoagulants (direct thrombin inhibitors or factor Xa inhibitors). Theoretically, warfarin at initiation could lead to a hypercoagulable state with an increased risk of thrombo-embolic events. Azoulay et al. have performed a post-hoc nested-control analysis using the UK Clinical Practice Research Datalink in a cohort of 70 766 patients with AF.1 The hypothesis of an increased early stroke risk after the initiation of warfarin in patients with AF was based on the early effect of warfarin preventing production of the natural anticoagulants protein C and S, and the suspicion that the early labile nature of warfarin's effect could result in thrombotic events. Azoulay et al. observed a 71% increase of stroke in the first 30 days of warfarin use, while a decreased stroke risk was observed afterwards.