Impact of Completeness of Ascertainment of Minor Stroke on Stroke Incidence Implications for Ideal Study Methods

Background and Purpose-Reliable comparisons of stroke incidence are important. To determine the impact of systematic assessment of patients referred with transient ischemic attack on the measured incidence and severity of stroke, we compared 2 population-based studies. Methods-Patients with first-ever stroke ascertained during 2006 through 2010 from the Dijon Stroke Registry and the Oxford Vascular (OXVASC) Study were studied. Both studies comply with the criteria for ideal incidence studies, but the OXVASC Study also systematically assessed all patients referred with transient ischemic attack. Stroke severity was measured by the National Institutes of Health Stroke Scale. Results-Among 902 incident strokes in Dijon and 748 cases in the OXVASC Study, age and gender distribution were comparable, but severity was lower in the OXVASC Study (median National Institutes of Health Stroke Scale, 2 versus 6; P<0.001). Although overall incidence of ischemic stroke was higher in the OXVASC Study (157 versus 98 of 100 000/y; incidence rate ratio, 1.59; 95% confidence interval, 1.24-2.05; P<0.001), this was accounted for by a 3-fold excess incidence of stroke with National Institutes of Health Stroke Scale <= 2 in the OXVASC Study (90 versus 29/100 000/y; P<0.001), with no difference in incidence of more severe ischemic stroke (incidence rate ratio, 0.95; 95% confidence interval, 0.68-1.33). Of all 660 incident ischemic strokes in the OXVASC Study, 375 (56.8%) cases had an National Institutes of Health Stroke Scale <= 2, of which 232 had been ascertained in the transient ischemic attack clinic. Of these 232 minor strokes, only 71 cases had a diagnosis of definite stroke documented in the medical records by the referring physician. Conclusions-Reliance on routine clinical coding underestimates the incidence of minor stroke. To improve comparability of incidence studies, researchers should assess patients referred with transient ischemic attack, and all studies should stratify incidence rates by stroke severity.