Plasma A beta but Not Tau is Related to Brain PiB Retention in Early Alzheimer's Disease

Recent advances in biomarkers provide the possibility of early or preclinical diagnosis of Alzheimer's pathology. Currently, decreased levels of A beta-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 A beta deposition in amyloid imaging. We used an immunomagnetic reduction assay to measure the plasma levels of A beta 40, A beta 42, 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 C-11-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 A beta 42/40 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 A beta 42/40 (R-2 0.326-0.449, all p < 0.001) but not by plasma tau levels. Plasma A beta in terms of A beta 42/40 might provide an indirect estimation of A beta deposition in the brain