GFAP and Alzheimer disease: While this study focuses on end‐stage disease in LBSD, it will be critical for future studies to explore whether our findings generalize to other neurodegenerative disease and other stages of disease: whether plasma GFAP is more sensitive to intermediate ADNC than p‐tau181 in primary AD cases, and whether plasma GFAP is elevated in early/prodromal stages of disease in AD and LBSD.