FAP and fibrosis: This study evaluated a marker of activated mesenchymal fibrogenic cells, circulating fibroblast activation protein (cFAP), in a novel diagnostic algorithm, FAP Index, for patients with MASLD.<h4>Methods</h4>Two retrospective cohorts recruited from tertiary hepatology clinics were studied as training (n = 160) and external validation cohorts (n = 332), with prevalence of histologic advanced fibrosis (F3-F4) of 20% and 11%, respectively.