SOURCES OF VARIABILITY IN PREVALENCE RATES OF ALZHEIMERS-DISEASE

Objective. To investigate potential methodological reasons for the dif ferences in published Alzheimer's disease (AD) prevalence rates. Backg round. Studies reporting prevalence rates of AD have been published wo rldwide. These rates differ considerably, but may greatly reflect meth odological differences. Methods. All studies published between 1984 an d 1993 that reported age-specific AD rates and sample sizes were inclu ded. Logistic regression identified variables that contribute to the v ariation in rates, Estimates of extrabinomial variation were also calc ulated. Results. Studies characterized by the following features yield ed significantly higher rates: inclusion of mild cases, use of laborat ory studies, ascertainment of a sample rather than the total populatio n, inclusion of both urban and rural populations, non-use of computeri zed tomography (CT) scans, non-use of the Hachinski Ischemic Score, an d no adjustment for false negatives. The odds of having AD increased 1 8% for every year of age. The variation in the age-specific prevalence rates of AD was approximately 15 times that expected by sampling vari ation. However, approximately 76% of this excess variation in rates co uld 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 method ology. Some unexplained variation in prevalence rates, however, still remains.