NEFL and Alzheimer disease: Fixed effect model analysis showed that NFL chain level was significantly higher in the serum of patients with MCI (0.361 SMD, 95% CI, 0.286–0.435, p = 0.000, I2 = 49.179) and AD (0.808 SMD, 95% CI, 0.727–0.888, p = 0.000, I2 = 39.433) compared with healthy individuals.