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To the Editor:The acceptance of the clinical utility of protein biomarkers for Alzheimer disease diagnosis and their integration into routine clinical testing require extensive standardization at different levels (preanalytical, analytical, postanalytical) (1). The approval of such biomarkers by regulatory authorities is hampered in part by ( a ) uncertainty regarding the accuracy of values produced with different measurement technologies, ( b ) the absence of international reference standards, ( c ) the absence of validated reference methods for measuring absolute analyte concentrations, and ( d ) a lack of well-defined recommendations for immunoassay validation (2). The present letter provides a solution for harmonization of cutoff concentration values for different technology platforms by performing immunoassays that use calibrators consisting of undiluted cerebrospinal fluid (CSF).1For this exploratory study, 197 CSF samples had been analyzed previously at the same time (on 5 different immunoplates) by ELISA [INNOTEST® β-Amyloid(1–42); Innogenetics, single analyte, CE version] and Luminex's xMAP assay system with the INNO-BIA AlzBio3 assay (Innogenetics; multiplexed, research use only assay) (3). Differences in the design of the assays have previously been described (4). The plate layouts for CSF testing for the 2 assay formats were identical. No new test runs were performed for the current study. Kit calibrators consisted of synthetic β-amyloid(1–42) dissolved in a phosphate-buffered solution. After run validation (criteria: out of range, precision, bead count), 31 CSF …

Alzheimer Disease Biomarker Testing in Cerebrospinal Fluid: A Method to Harmonize Assay Platforms in the Absence of an Absolute Reference Standard