Background: Few longitudinal studies have verified the clinical diagnosis of dementia based on clinical examinations. We evaluated the consistency of the clinical diagnosis of dementia over a period of 3 years of follow-up in a population-based, cohort study of older people in central Spain. Methods: Individuals (N = 5278) were evaluated at baseline (1994-1995) and at follow-up (1997-1998). The evaluation included a screening questionnaire for dementia and a neurological assessment. Results: Dementia screening consisted of a 37-item version of the Mini-Mental State Examination (MMSE) and the Pfeffer Functional Activities Questionnaire (FAQ). Study neurologists investigated those participants who screened positively (N = 713) as well as 843 who had screened negatively to test the sensitivity of the screening instruments or because they had a positive screening for other chronic neurological diseases. We detected 295 patients among those who screened positive and 13 among those who screened negatively. Three years follow-up evaluation demonstrated 14 diagnostic errors at baseline (4.5%) leading to a final number of 306 patients with dementia. The corrected prevalence of dementia was 5.8% (95% confidence interval [CI] 5.2-6.5). Conclusions: The diagnosis of dementia was highly accurate in this population-based, Spanish cohort study, and our prevalence figures agree with other European surveys. Given the high cost and difficulties of population rescreening and its relatively low yield, we conclude that a single 2-phase investigation (screening followed by clinical examination) provides accurate information for most population-based prevalence studies of dementia.
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