Amyloid-beta42 plasma levels are elevated in amnestic mild cognitive impairment.

Amnestic mild cognitive impairment (aMCI) is considered a prodromal stage of Alzheimer's disease (AD). We measured plasma levels of amyloid-beta40 (Abeta40) and Abeta42 in 191 subjects with aMCI. Seventy-nine of them were clinically followed for two years. In the total cohort of aMCI cases, the average level of Abeta42, as well as the Abeta42/Abeta40 ratio, was significantly higher than those of the 102 cognitively normal age-matched subjects. The aMCI cases that converted to probable AD within 2 years had higher levels of Abeta42 and, to a lesser extent, Abeta40 than the stable cases. However the large variability of measured values indicates that plasma Abeta is not a suitable marker of incipient AD.