The effect of patient attrition on estimates of the frequency of dementia following stroke.

BACKGROUND Given that prevalence surveys may underestimate the magnitude of the association between an exposure and a disease with high morbidity or mortality, we investigated the effects of patient attrition on estimates of the frequency of dementia following ischemic stroke. PATIENTS AND METHODS We examined 251 patients 3 months after stroke and diagnosed dementia in 66 (26.3%) based on the results of neuropsychological and functional assessments and modified criteria from the Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised. Those 251 patients were drawn from a larger cohort of 297 patients, with the majority of the remaining 46 patients being unavailable for assessment due to death, severe stroke, or comorbid medical disorders. Using the coefficients in a logistic model of the clinical determinants of dementia based on the 251 patients who were examined, we calculated the probability of dementia for each of the 46 patients who were not examined. We considered a patient to have dementia when that probability was higher than the mean of the median probabilities of dementia in the groups of patients with and without dementia who completed the examinations. RESULTS The sensitivity and specificity of our diagnostic method were 75.8% and 72.4%, respectively. We recognized dementia in 21 (45.7%) of the 46 unavailable patients, a significantly higher frequency than among examined patients. Additional analyses determined that the factors that increased the risk of becoming unavailable for follow-up, which included more severe stroke, left and right hemisphere infarct locations, and a history of prior stroke, are similar to the factors that increase the risk of dementia after stroke. CONCLUSION Our findings suggest that dementia is differentially associated with early patient attrition, potentially resulting in the underestimation of its frequency and underrecognition of its importance as an outcome of ischemic stroke.