The influence of diagnosis, intra- and inter-person variability on serum and plasma Aβ levels

Evidence suggests that high peripheral beta-amyloid (A beta)(1-40) levels and low ratios of A beta(1-42)/A beta(1-40) are associated with increased risk for Alzheimer's disease (AD). In this cross-sectional design, serum and plasma samples from 67 AD patients and 146 controls (similar in age and gender) were evaluated using A beta(1-40) and A beta(1-42) ELISA. Coefficient of variance was calculated for intra- and inter-person variability of A beta(1-40) and A beta(1-42). A beta(1-40) correlated with age, MMSE and their A beta(1-42)/A beta(1-40) ratios (p < 0.05). Significantly higher A beta(1-40) levels were observed in AD patients than controls (p < 0.05) but no difference was observed for A beta(1-42) (p > 0.05). Serum A beta(1-42)/A beta(1-40) ratios were also significantly lower in AD patients than controls (p < 0.05). Lower intra-person than inter-person variability was observed for serum and plasma A beta(1-40) and A beta(1-42) and these were higher in controls than in AD patients. The intra-person variability of serum A beta(1-40) did not influence the group differences observed between AD patients and controls. Significant interaction was observed between diagnosis and intra-person variability for serum A beta(1-40) levels (p < 0.05) and was supported by our finding of higher intra-person variability for serum A beta(1-40) in controls (26.97%) than in AD patients (18.35%). We confirm the previously observed differences in blood A beta levels between AD and control groups. In addition, we now report the presence of high intra- and inter-person variability possibly due to factors that influence peripheral A beta levels and warrant further investigation before the potential use of A beta as an AD biomarker can be fully exploited. (C) 2007 Elsevier Ireland Ltd. All rights reserved.