Quantitative Measurement of Cerebrospinal Fluid Amyloid-β Species by Mass Spectrometry

BACKGROUND: High sensitivity liquid chromatography mass spectrometry (LC-MS/MS) was recently introduced to measure amyloid-β (Aβ) species, allowing for a simultaneous assay that is superior to ELISA, which requires more assay steps with multiple antibodies. OBJECTIVE: We validated the Aβ(1-38), Aβ(1-40), Aβ(1-42), and Aβ(1-43) assay by LC-MS/MS and compared it with ELISA using cerebrospinal fluid (CSF) samples to investigate its feasibility for clinical application. METHODS: CSF samples from 120 subjects [8 Alzheimer’s disease (AD) with dementia (ADD), 2 mild cognitive dementia due to Alzheimer’s disease (ADMCI), 14 cognitively unimpaired (CU), and 96 neurological disease subjects] were analyzed. Aβ species were separated using the Shimadzu Nexera X2 system and quantitated using a Qtrap 5500 LC-MS/MS system. Aβ(1-40) and Aβ(1-42) levels were validated using ELISA. RESULTS: CSF levels in CU were 666±249 pmol/L in Aβ(1-38), 2199±725 pmol/L in Aβ(1-40), 153.7±79.7 pmol/L in Aβ(1-42), and 9.78±4.58 pmol/L in Aβ(1-43). The ratio of the amounts of Aβ(1-38), Aβ(1-40), Aβ(1-42), and Aβ(1-43) was approximately 68:225:16:1. Linear regression analyses showed correlations among the respective Aβ species. Both Aβ(1-40) and Aβ(1-42) values were strongly correlated with ELISA measurements. No significant differences were observed in Aβ(1-38) or Aβ(1-40) levels between AD and CU. Aβ(1-42) and Aβ(1-43) levels were significantly lower, whereas the Aβ(1-38/1-42), Aβ(1-38/1-43), and Aβ(1-40/)Aβ(1-43) ratios were significantly higher in AD than in CU. The basic assay profiles of the respective Aβ species were adequate for clinical usage. CONCLUSION: A quantitative LC-MS/MS assay of CSF Aβ species is as reliable as specific ELISA for clinical evaluation of CSF biomarkers for AD.