Dual-phase [F]AV45 positron emission tomography (PET) is highly promising in the assessment of neurodegenerative diseases, allowing to obtain information on both neurodegeneration (early-phase; eAV45) and amyloid deposition (late-phase; lAV45) which are highly complementary; yet eAV45 needs further evaluation. This study aims at validating eAV45 as an optimal proxy of [F]FDG PET in a large mixed-population of healthy ageing and Alzheimer's clinical syndrome participants (n = 191) who had [F]FDG PET, eAV45 and lAV45 scans. We found early time frame 0-4 min to give maximal correlation with [F]FDG PET and minimal correlation with lAV45. Moreover, maximal overlap of [F]FDG PET versus eAV45 associations with clinical diagnosis and cognition was obtained with pons scaling. Across reference regions, classification performance between clinical subgroups was similar for both eAV45 and [F]FDG PET. These findings highlight the optimal use of eAV45 to assess neurodegeneration as a validated proxy of [F]FDG PET. On top of this purpose, this study showed that combined [F]AV45 PET dual-biomarker even outperformed [F]FDG PET or lAV45 alone.