• Assess the person's risk of stroke using the following criteria:
• High risk of stroke
• Previous ischaemic stroke/transient ischaemic attack or thromboembolic event.
• 75 years of age or more with risk factors (hypertension, diabetes, coronary artery disease, peripheral artery disease).
• Clinical evidence of valve disease or heart failure, or impaired left ventricular function on echocardiography.
• Moderate risk of stroke
• 65 years of age or more without risk factors.
• Less than 75 years of age with risk factors.
• Low risk of stroke
• Less than 65 years of age without risk factors.
• The CHADS₂ tool can also be used to calculate the risk of stroke (see clarification for more details).
• Risk factors have a cumulative effect on stroke risk; this should be considered when discussing treatment options (see Which antithrombotic treatment).
• If uncertain about the person's risk of stroke, consider referral to a cardiologist.

• Another method of stroke risk stratification is the CHADS₂ criteria. The acronym CHADS₂ is derived from individual stroke risk factors. Adding together the points allocated to each risk factor yields the total CHADS₂ score:
• Congestive heart failure = 1.
• Hypertension (or treated hypertension) = 1.
• Age older than 75 years = 1.
• Diabetes mellitus = 1.
• Previous stroke or transient ischaemic attack = 2.
• The following treatment is recommended:
• Aspirin if the total score is 0 (low risk) or 1 (moderate risk).
• Warfarin (if there are no contraindications) if the total score is 2 or more.

These recommendations are based on the National Institute for Health and Clinical Excellence (NICE) guideline Atrial fibrillation: national clinical guideline for management in primary and secondary care [National Collaborating Centre for Chronic Conditions, 2006].

• On the basis of cohort studies, the Scottish Intercollegiate Guidelines Network published guidance showing the risk of stroke in people with atrial fibrillation (AF) considered to be at low, moderate, and high risk. For more information, see the antithrombotic therapy section in the SIGN guidance [SIGN, 1999].

Risk factors for stroke

• A systematic review (search date October 2005, seven studies) suggests that prior stroke or transient ischaemic attack (TIA), advancing age, hypertension, and diabetes are independent risk factors for stroke in people with AF [Stroke Risk in Atrial Fibrillation Working Group, 2007].
• Absolute stroke rates were in the range 6–9% per year for prior stroke or TIA, 1.5% to 3% per year for history of hypertension, 1.5–3% per year for age more than 75 years, and 2.0–3.5% per year for diabetes.
• In the systematic review, evidence for heart failure and coronary artery disease as independent risk factors for stroke is inconclusive. However, NICE includes heart failure and vascular disease as independent risk factors for stroke on the basis that left ventricular dysfunction is associated with an increased risk of stroke. Atherosclerotic vascular disease is also a risk factor for stroke, with a poor prognosis when associated with AF [Goto et al, 2008].

CHADS₂ for stroke risk

• The CHADS₂ score was designed to provide a simple approach to assessing stroke risk in primary care. The score was validated by a study on people 65–95 years of age with nonrheumatic AF who were not prescribed warfarin [Gage et al, 2004].
• The CHADS₂ criteria have been assessed against risk factors refined to form the NICE criteria, and both were found to be similar for predicting event rates in a cohort prospectively followed up for stroke and vascular events [Lip et al, 2006].