Comparative evaluation of two immunoassays for cerebrospinal fluid β-Amyloid 1–42 measurement

Aim of the study: Beta-Amyloid 1-42 peptide (beta A42) is a cerebro-spinal fluid (CSF) biomarker, key element of the NIA Alzheimer's disease diagnostic criteria. The enzyme-linked immunosorbent assay (ELISA) has been the mainstay method for beta A42 measurement on cerebrospinal fluid (CSF). Recently, a new beta A42 measurement method in chemiluminescence enzyme immunoassay (CLEIA) is available on Lumipulse G 600 II automatic platform. The aim of the work was to evaluate the concordance of the ELISA and the new method (CLEIA) in the CSF beta A42 levels measurement.Materials and methods: CSF beta A42 levels were assayed in 49 samples using the ELISA method (Innotest beta- amyloid 1-42, Fujirebio Europe N.V., Gent, Belgium) and CLEIA method on Lumipulse G600II fully automatic platform (Lumipulse G amyloid 1-42, Fujirebio Europe N.V., Gent, Belgium). We compared values of the two methods using acceptability interval based on Inherent Combined Imprecision (ICI), the Passing-Bablok regression analysis, the Pearson correlation coefficient (r) and the Bland-Altman plot.Results: The analysis of the ICI showed that the two methods differ substantially. The regression equation (y = -103.04 + 1.52 x) highlighted the presence of proportional systematic difference, without significant deviation from linearity (p =.42). The Pearson correlation coefficient was 0.826. The Bland-Altman plot analysis showed a significant systematic difference in the two methods: ELISA measurements were in average -27.06% (95% CI -31.89 to -22.23%) lower compared to CLEIA ones.Conclusions: Our study highlighted a difference between the two methods. Therefore, the cut-off for the normal levels of beta A42 should be reviewed in the laboratory report