ABTEST‐IA: A new reliable tool for the quantification of Aβ40 and Aβ42 in CSF.

Background: The ratio Aβ42/Aβ40 in cerebrospinal fluid (CSF) is considered one of the most reliable biomarkers of Alzheimer's Disease. ABtest40‐IA and ABtest42‐IA have been widely used for amyloid‐β (Aβ) quantification in plasma samples. The aim of this study was to validate these tests for the quantification of Aβ40 and Aβ42 in CSF. Method: Analytical validation of ABtest40‐IA and ABtest42‐IA was accomplished following FDA and EMA reference guidelines. Calibration curves, sensitivity, specificity, accuracy, precision, recovery and dilution linearity were evaluated. Correlations between ABtest‐IA and an independent fully automated IVD test, (Lumipulse ® G β‐amyloid 1‐40 and 1‐42, Fujirebio), were estimated using 93 CSF samples of PET‐Aβ positive (75%) and PET‐Aβ negative (25%) individuals. Differences in CSF Aβ42/Aβ40 levels between both PET‐Aβ groups were analyzed (Mann Whitney test). The ability of CSF Aβ42/Aβ40 ratio to discriminate between PET‐Aβ positive and PET‐Aβ negative subjects was evaluated by means of Receiver Operating Characteristic (ROC) curve analysis, considering PET‐Aβ the gold standard. Result: Calibration curves ranges were set at 43.9 to 500 pg/mL for ABtest40‐IA and 5 to 100 pg/mL for ABtest42‐IA. Intra‐assay, inter‐assay and inter‐lot coefficients of variation were below 5.2% and 7.4% for ABtest40‐IA and ABtest42‐IA respectively. Accuracy evaluation provided mean relative errors of 4.6% and 7.5%. Spearman correlation coefficients between ABtest‐IA and Lumipulse® were 0.96, 0.95 and 0.93 for Aβ40, Aβ42 and Aβ42/Aβ40 respectively (p values < 0.0001). As expected, Aβ42/Aβ40 ratio levels were significantly lower in the PET‐Aβ positive group (p < 0.0001). ROC analysis of CSF Aβ42/Aβ40 gave an AUC of 0.85 (95% CI 0.74 to 0.97), very similar to that obtained with Lumipulse®: 0.84 (95% CI 0.72 to 0.98). The overall agreement of PET‐Aβ and ABtest‐IA Aβ42/Aβ40 was 90%, being false positives the majority (78%) of the misclassified subjects. Results are supporting that CSF Aβ42/Aβ40 is an early biomarker of cerebral amyloid deposition. Conclusion: ABtest40‐IA and ABtest42‐IA were successfully validated as new precise and accurate tools for the quantification of Aβ40 and Aβ42 in CSF samples. ABtest‐IA measures of Aβ42/Aβ40 ratio in CSF identified individuals with brain amyloid deposition with equivalent ability to other test commercially available. [ABSTRACT FROM AUTHOR]