Sources of variability in prevalence rates of Alzheimer's disease.

OBJECTIVE: To investigate potential methodological reasons for the differences in published Alzheimer's disease (AD) prevalence rates.
BACKGROUND: Studies reporting prevalence rates of AD have been published worldwide. These rates differ considerably, but may greatly reflect methodological differences.
METHODS: All studies published between 1984 and 1993 that reported age-specific AD rates and sample sizes were included. Logistic regression identified variables that contribute to the variation in rates. Estimates of extrabinomial variation were also calculated.
RESULTS: Studies characterized by the following features yielded significantly higher rates: inclusion of mild cases, use of laboratory studies, ascertainment of a sample rather than the total population, inclusion of both urban and rural populations, non-use of computerized tomography (CT) scans, non-use of the Hachinski Ischemic Score, and no adjustment for false negatives. The odds of having AD increased 18% for every year of age. The variation in the age-specific prevalence rates of AD was approximately 15 times that expected by sampling variation. However, approximately 76% of this excess variation in rates could be accounted for by methodological differences.
CONCLUSIONS: After accounting for age, much of the variability in prevalence rates of AD in the published literature may be explained by differences in methodology. Some unexplained variation in prevalence rates, however, still remains.