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S econdary prevention in patients with a prior ischemic stroke or a transient ischemic attack; most experts also include patients with a systemic embolic event 2 CHADS2 score Events per 100 person-years* NNT Warfarin No warfarin 0 0.25 0.49 417 1 0.72 1.52 125 2 1.27 2.50 81 3 2.20 5.27 33 4 2.35 6.02 27 5 or 6 4.60 6.88 44 NNT: number needed to treat to prevent one stroke per year with warfarin. * The CHADS2 score estimates the risk of stroke, which is defined as focal neurologic signs or symptoms that persist for more than 24 hours and that cannot be explained by hemorrhage, trauma, or other factors, or peripheral embolization, which is much less common. Transient ischemic attacks are not included. All differences between warfarin and no warfarin groups are statistically significant except for a trend with a CHADS2 score of 0. Patients are considered to be at low risk with a score of 0, at intermediate risk with a score of 1 or 2, and at high risk with a score ≥3. One exception is that most experts would consider patients with a prior ischemic stroke, transient ischemic attack, or systemic embolic event to be at high risk even if they had no other risk factors and therefore a score of 2. However, the great majority of these patients have some other risk factor and a score of at least 3. Data from: Go AS, Hylek EM, Chang Y, et al. JAMA 2003; 290:2685; and CHADS2 score from Gage BF, Waterman AD, Shannon W. JAMA 2001; 285:2864.