GFAP and Alzheimer disease: Experiencing one or more episodes of delirium was associated with accelerated decline on the CDR-Sb at 18-months (β: 2.63; 95% CI: 1.55, 3.71; adjusted p < 0.001).<h4>Interpretation</h4>Biomarkers of neuroinflammation (GFAP), neurodegeneration (p-tau217) and incident delirium, rather than systemic inflammatory biomarkers, were associated with clinically-significant decline in mild-moderate AD.<h4>Funding</h4>European Commission (FP7 grant; 279093); Meath Foundation (MFRG 121/2021); Wellcome Trust (227946/Z/23/Z & 203930/B/16/Z); Health Research Board (203930/B/16/Z; ECSA-2024-003).