The subgroup analyses according to the AD criteria suggested that YKL-40 levels were higher in AD patients than in HCs in studies that used NIA-AA as the diagnostic criteria (SMD = 1.180, 95%CI: [0.825, 1.535]; p = 0.000; Table 3, Figure S5), but we could still observe high heterogeneity in the results of the analysis (I2 = 74.9%, p = 0.003; Table 3, Figure S5). Here, CHI3L1 is linked to Alzheimer disease.