Plasma GFAP distinguished ALS-AD subjects more accurately (area under the curve (AUC) 0.932 ± 0.027) than plasma p-tau181 (AUC 0.692 ± 0.058, p < 0.0001) and plasma neurofilament light chain protein (AUC, 0.548 ± 0.088, p < 0.0001). Here, GFAP is linked to Alzheimer disease.