Association of education with reported age of onset and severity of Alzheimer's disease at presentation: implications for the use of clinical samples.

Clinical samples are commonly used to attempt to infer factors that are etiologically important in Alzheimer's disease. Use of clinical samples for these purposes is valid if and only if detection of Alzheimer's disease is unbiased with respect to the factors being studied. The issue of how education influences detection of Alzheimer's disease is controversial. The purpose of the present investigation was to shed light on the role of education in the detection of symptoms of Alzheimer's disease. To do so, we examined the association of education with age of symptom onset, severity of disease at diagnosis, and time from symptom onset until diagnosis in a large clinical sample of Alzheimer's disease patients from throughout California. The diagnoses of the 1,658 cases in the sample were made in 1985-1990. Age of onset was defined as the age at which symptoms first appeared, according to family members. Severity of dementia was measured with Blessed-Roth Dementia Rating Scale scores. The reported age at symptom onset was later in those with less education (p < 0.0001). However, decreasing education was associated with greater severity of disease at presentation (p < 0.008), suggesting that a lower educational level may lead to later detection of Alzheimer's disease and referral to clinical centers at a later stage of disease. This implies that using clinical samples of Alzheimer's disease patients to study factors correlated with age at onset may lead to biased conclusions, if these factors are also associated with education.