Amyloid-β42 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-β40 (Aβ40) and Aβ42 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 Aβ42, as well as the Aβ42/Aβ40 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 Aβ42 and, to a lesser extent, Aβ40 than the stable cases. However the large variability of measured values indicates that plasma Aβ is not a suitable marker of incipient AD.