Recent advances in
biomarkers provide the possibility of early or preclinical diagnosis of Alzheimer's pathology. Currently, decreased levels of Aβ-42 and increased levels of
tau proteins in cerebral spinal fluid are considered reliable
biomarkers of
Alzheimer's disease (AD); however, little evidence exists for the use of
amyloid and
tau protein levels in the plasma as useful
biomarkers. We investigated the potential use of plasma
biomarkers to diagnose AD and explored their relationships with brain Aβ deposition in
amyloid imaging. We used an immunomagnetic reduction assay to measure the plasma levels of Aβ40, Aβ42, and
tau proteins in 20 older control participants and 25 participants who had either
mild cognitive impairment due to AD or early AD
dementia. All participants received (11)C-labeled
Pittsburgh compound B PET scans. The sensitivity of the plasma tau level at the cutoff value of 28.27 pg/mL was 92%, and the specificity was 100%; the sensitivity of the Aβ42/40 ratio at the cutoff value of 0.3693 was 84%, and the specificity was 100%. Regression analyses of the effects of
plasma protein levels on brain
amyloid retention, as determined by standard uptake value ratios in either side of the frontal, parietal, and temporal lobes and the precuneus, are predicted only by ratios of plasma Aβ42/40 (R(2) 0.326-0.449, all p < 0.001) but not by plasma tau levels. Plasma Aβ in terms of Aβ42/40 might provide an indirect estimation of Aβ deposition in the brain.