Prevalence rates of dementia in an ageing population: are low rates due to the use of insensitive instruments?

The varying prevalence rates of dementia reported in elderly populations may be partly due to the use of different diagnostic measures. In a recent study in which diagnosis was based on the CAPE, a 12-item questionnaire, the prevalence rate for severe cognitive impairment for the age group 75 years or over was lower than previously reported. In the present study, the performance of the CAPE was examined in an elderly general-practice sample with a higher than usual risk of dementia. The study diagnosis was based on a combination of the diagnosis made by the computer program AGECAT and a clinical diagnosis made by the interviewing psychiatrist. Forty-five per cent of patients with definite or probable dementia, as defined, and 100% of those with possible dementia had scores above the cut-point on the CAPE. The sensitivity of the CAPE was low compared with that of other rating scales. It is concluded that the low reported rate with the CAPE is probably due to only the more severe cases being identified. For comparative purposes it is important to know the level of dementia that the instruments used are detecting.