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

Evidence suggests that high peripheral beta-amyloid (Aβ)1–40 levels and low ratios of Aβ1–42/Aβ1–40 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 Aβ1–40 and Aβ1–42 ELISA. Coefficient of variance was calculated for intra- and inter-person variability of Aβ1–40 and Aβ1–42. Aβ1–40 correlated with age, MMSE and their Aβ1–42/Aβ1–40 ratios (p<0.05). Significantly higher Aβ1–40 levels were observed in AD patients than controls (p<0.05) but no difference was observed for Aβ1–42 (p>0.05). Serum Aβ1–42/Aβ1–40 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 Aβ1–40 and Aβ1–42 and these were higher in controls than in AD patients. The intra-person variability of serum Aβ1–40 did not influence the group differences observed between AD patients and controls. Significant interaction was observed between diagnosis and intra-person variability for serum Aβ1–40 levels (p<0.05) and was supported by our finding of higher intra-person variability for serum Aβ1–40 in controls (26.97%) than in AD patients (18.35%). We confirm the previously observed differences in blood Aβ 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 Aβ levels and warrant further investigation before the potential use of Aβ as an AD biomarker can be fully exploited.