NEFL and Alzheimer disease: Finally, in the comparison between DS‐AD and DS‐pAD, across 9 studies involving 549 DS‐AD and 379 DS‐pAD individuals, the SMD was 0.62 (95% CI [0.49, 0.76], z = 8.96, p < 0.00001), indicating significantly higher plasma NfL levels in the DS with AD group, with no heterogeneity (I2 = 0%, p = 0.83).