Optimized Standard Operating Procedures for the Analysis of Cerebrospinal Fluid Aβ(42) and the Ratios of Aβ Isoforms Using Low Protein Binding Tubes

Background: Reduced cerebrospinal fluid (CSF) concentration of amyloid-β(1-42) (Aβ(1-42)) reflects the presence of amyloidopathy in brains of subjects with Alzheimer’s disease (AD). Objective: To qualify the use of Aβ(1-42)/Aβ(1-40) for improvement of standard operating procedures (SOP) for measurement of CSF Aβ with a focus on CSF collection, storage, and analysis. Methods: Euroimmun ELISAs for CSF Aβ isoforms were used to set up a SOP with respect to recipient properties (low binding, polypropylene), volume of tubes, freeze/thaw cycles, addition of detergents (Triton X-100, Tween-20) in collection or storage tubes or during CSF analysis. Data were analyzed with linear repeated measures and mixed effects models. Results: Optimization of CSF analysis included a pre-wash of recipients (e.g., tubes, 96-well plates) before sample analysis. Using the Aβ(1-42)/Aβ(1-40) ratio, in contrast to Aβ(1-42), eliminated effects of tube type, additional freeze/thaw cycles, or effect of CSF volumes for polypropylene storage tubes. ‘Low binding’ tubes reduced the loss of Aβ when aliquoting CSF or in function of additional freeze/thaw cycles. Addition of detergent in CSF collection tubes resulted in an almost complete absence of variation in function of collection procedures, but affected the concentration of Aβ isoforms in the immunoassay. Conclusion: The ratio of Aβ(1-42)/Aβ(1-40) is a more robust biomarker than Aβ(1-42) toward (pre-) analytical interfering factors. Further, ‘low binding’ recipients and addition of detergent in collection tubes are able to remove effects of SOP-related confounding factors. Integration of the Aβ(1-42)/Aβ(1-40) ratio and ‘low-binding tubes’ into guidance criteria may speed up worldwide standardization of CSF biomarker analysis.