Blood–brain barrier permeability is commonly measured through the ratio of serum to albumin present in cerebrospinal fluid (CSF), with albumin increasing with BBB permeability; however, changes in albumin may not be detectable in prodromal AD and cannot identify specific pathological brain states or localize BBB leakage (Janelidze et al., 2017; Raja et al., 2018; Sweeney et al., 2018, 2019; Erickson and Banks, 2019; Varatharaj et al., 2019; Galea, 2021; Hussain et al., 2021). This evidence concerns the gene ALB and Alzheimer disease.