The results of this meta-analysis indicated that overall levels of YKL-40 were found to be higher in AD patients than HCs (SMD = 0.608, 95%CI: [0.272, 0.943]; p = 0.000; Figure 4), and high heterogeneity was observed to exist across studies (I2 = 90.7%, p = 0.000; Figure 4). Here, CHI3L1 is linked to Alzheimer disease.