International Journal of Epidemiology, Vol 28, 492-497, Copyright © 1999 by International Epidemiological Association
MI Geerlings, B Schmand, C Jonker, J Lindeboom and LM Bouter
BACKGROUND: It is still not clear whether a low level of education
increases the risk of developing Alzheimer's disease (AD). Two common
problems in cohort studies involving an elderly population and a two- step
diagnostic procedure are the loss to follow-up without data on the presence
of AD, and the fact that, in general, people with higher levels of
education perform better on traditional cognitive tests, such as the
Mini-Mental State Examination (MMSE). Both phenomena may lead to
misclassification, resulting in a biased association between level of
education and AD. This study investigated to what extent these selection
mechanisms may influence this association. METHODS: In the community-based
Amsterdam Study of the Elderly (AMSTEL) a cohort at risk for AD was
selected of 3778 people aged 65-84 years. Level of education was expressed
in two categories: low (primary education or less) versus high (partial
secondary education to completed university education). At follow-up, a
subsample of elderly people was selected for further diagnostic evaluation,
using a memory test in addition to the MMSE. Clinical diagnoses of AD were
made according to DSM-IV (Diagnostic and Statistical Manual of Mental
Disorders) criteria. To examine the extent to which loss to follow-up may
have affected the results, a sensitivity analysis was performed comparing
two extreme possibilities. Furthermore, to examine to what extent use of
the MMSE only may have affected the results, the observed odds ratio (OR)
was compared with the OR based on only those AD patients who were selected
for diagnostics with the MMSE alone. RESULTS: After an average of 3.2
years, 77 people had developed AD. Multivariate logistic regression
analyses indicated that a low level of education was associated with
incident AD (OR adjusted for age and sex 2.09; 95% CI: 1.29-3.38). The
results of the sensitivity analysis still indicated that a low level of
education was associated with incident AD. Screening with only the MMSE led
to a higher OR than the one observed. CONCLUSION: Selective attrition and
use of cognitive screening tests that are associated with educational level
may influence the strength of the association between a low level of
education and incident AD; however, it appears that these influences cannot
completely explain this association.
ARTICLES
Education and incident Alzheimer's disease: a biased association due to selective attrition and use of a two-step diagnostic procedure?
Institute for Research in Extramural Medicine (EMGO Institute), Vrije Universiteit, Amsterdam. MI.Geerlings@scw.vu.nl
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