GFAP and Alzheimer disease: A 2021 meta-analysis including 2 blood astrocyte biomarkers, GFAP and S100B, found a significant difference between the AD and cognitively unimpaired (CU) groups for S100B, but not for GFAP.4 However, of the small number of eligible studies (5 for each biomarker), only 2 reported the direct blood biomarker level: the other 8 measured autoantibodies to the biomarker or levels in plasma astrocyte-derived exosomes.