CHI3L1 and Cognitive impairment: This study aims to establish the correlation between YKL40 levels and AD progression through a meta-analysis of YKL40 levels in cerebrospinal fluid (CSF) and peripheral blood.<h4>Methods</h4>Comprehensive searches were conducted in PubMed, Medline, Web of Science, and the Cochrane Library to identify observational studies reporting CSF and peripheral blood YKL40 levels in AD patients, mild cognitive impairment (MCI) patients, preclinical AD (pre-AD) and healthy controls (HCs).