In the comparison between DS with and without AD, across 12 studies involving 594 DS with AD and 2210 without AD individuals, the SMD was 1.88 (95% CI [1.73, 2.03], z = 25.09, p < 0.00001), showing a significantly higher plasma NfL concentration in the DS with AD group, with moderate heterogeneity (I2 = 44%, p = 0.05). Here, NEFL is linked to Alzheimer disease.