GFAP and Alzheimer disease: Furthermore, individuals with a positive CSF Aβ1–42/1–40, but with Aβ PET levels below the cut off for being deemed Aβ PET-positive (i.e., individuals in the earliest preclinical stage of AD), were observed to have significantly higher plasma GFAP concentrations than Aβ-negative individuals, despite there being no significant difference in CSF GFAP concentrations between the two groups (Benedet et al., 2021).