title: Rates of ischemic stroke during warfarin treatment for atrial fibrillation.
abstract: OBJECTIVE
  Recent evidence suggests that there may be an increased risk of ischemic stroke immediately after warfarin initiation. We examined the rate of ischemic stroke among patients with atrial fibrillation newly started on warfarin therapy.
  
  
  METHODS
  We conducted a population-based cohort study among Ontario residents aged ≥66 years with atrial fibrillation who received warfarin between April 1, 1997, and March 31, 2010. Each patient was followed up for ≤5 years in 30-day intervals. For each interval, we determined the rate of ischemic stroke.
  
  
  RESULTS
  After 5 years, the cumulative incidence of ischemic stroke among new users of warfarin (n=148,446) was 4.0% (n=6006). The risk was highest during the first 30 days after initiation (6.0% per person-year; 95% confidence interval, 5.5%-6.4%) compared with the remainder of follow-up (1.6% per person-year; 95% confidence interval, 1.5%-1.6%), and increased with higher baseline CHADS2 (congestive heart failure, hypertension, age ≥75 years, diabetes, previous stroke) scores. Less frequent monitoring may have contributed.
  
  
  CONCLUSIONS
  In a large cohort of older patients with atrial fibrillation, we observed the highest rate of ischemic stroke in the first 30 days after warfarin initiation. Although causation cannot be established given the observational nature of this study, our findings highlight the need for future research in this population.
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.).
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Tung
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.).
authors.forename: Muhammad M
authors.initials: MM
authors.lastname: Mamdani
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.).
authors.forename: David N
authors.initials: DN
authors.lastname: Juurlink
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.).
authors.forename: J Michael
authors.initials: JM
authors.lastname: Paterson
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.).
authors.forename: Moira K
authors.initials: MK
authors.lastname: Kapral
authors.affiliation: From the Institute for Clinical Evaluative Sciences (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), Sunnybrook Research Institute (D.N.J.), Department of Medicine (M.K.K.), Institute of Health Policy, Management, and Evaluation (M.M.M., D.N.J., J.M.P., M.K.K., T.G.), and Leslie Dan Faculty of Pharmacy (J.M.T., M.M.M., T.G.), University of Toronto, Toronto, Ontario, Canada; Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada (M.M.M., T.G.); Division of General Internal Medicine and Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada (M.K.K.); and Department of Family Medicine, McMaster University, Hamilton, Ontario, Canada (J.M.P.). GomesT@smh.ca.
authors.forename: Tara
authors.initials: T
authors.lastname: Gomes
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000925:Anticoagulants / Q000627:therapeutic use*; D001281:Atrial Fibrillation / Q000188:drug therapy / Q000453:epidemiology*; D002545:Brain Ischemia / Q000453:epidemiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008297:Male; D009864:Ontario / Q000453:epidemiology; D018570:Risk Assessment; D020521:Stroke / Q000453:epidemiology*; D013997:Time Factors; D014859:Warfarin / Q000627:therapeutic use*
chemical_list: D000925:Anticoagulants; D014859:Warfarin
keywords: anticoagulants; epidemiology