GFAP and Alzheimer disease: Early in the AD continuum (preAD vs. controls), the multivariate model, including the interaction between Aβ40 and p‐tau181, and ratios Aβ42/GFAP and Aβ42/p‐tau181, produced slightly higher (≈ 5%) AUC values compared to individual markers, while this difference was reduced to 1% at the AD dementia stage compared to controls.