Quantification of Amyloid-β in Plasma by Simple and Highly Sensitive Immunoaffinity Enrichment and LC-MS/MS Assay

Background: Numerous immunoassays have been developed to quantify amyloid beta(1-40) (A beta(40)) and amyloid beta(1-42) (A beta(42)). Nevertheless, given the low concentration of A beta and the high levels of interfering factors in plasma, quantification of plasma A beta is still challenging. To overcome the problems related to the specificity of A beta immunoassays, this study aimed to develop an immunoaffinity enrichment and LC-MS/MS (IA-MS) assay. Methods: We developed an IA-MS assay using antibody-labeled magnetic beads for purification and LC-MS/MS for A beta quantification. To avoid the loss of A beta due to aggregation in acidic buffer, we used alkaline elution buffer for immunoaffinity enrichment. The concentrations of the A beta s in plasma samples were measured, and the correlation between the plasma and cerebrospinal fluid (CSF) A beta(42)/A beta(40) ratio was also evaluated. Results: The intensities of the A beta mass peaks were significantly higher with the alkaline elution buffer than with the acidic elution buffer (A beta(40): 3.6-fold, A beta 42: 5.4-fold). This assay exhibited high reproducibility (intra-assay and inter-assay precision, %CV <15), and the working ranges of A beta(40) and A beta(42) were determined to be 21.7 to 692.8 pg/mL and 5.6 to 180.6 pg/mL, respectively. The concentrations of A beta(40) and A beta(42) in plasma were measured by IA-MS, and the plasma A beta(42)/A beta(40) ratio was correlated with the CSF A beta(42)/A beta(40) ratio (r(s) = 0.439, P < 0.01). Conclusions: The IA-MS assay has sufficient analytic performance for measuring endogenous A beta(40) and A beta(42) in plasma. This assay can lead to new lines of clinical discovery related to amyloid pathology.