Across all groups, plasma NFL levels were similar between APOE ε4 carriers and non-carriers (all participants, p = 0.22; AD group, p = 0.97, MCI group, p = 0.60; PDND group, p = 0.21; PDD group, p = 0.95; HC group, p = 0.95). Here, APOE is linked to Alzheimer disease.