We systematically searched PubMed, Web of Science, and Embase for studies investigating the association between multimorbidity—defined as the co-occurrence of multiple chronic conditions in the same individual—and levels of cerebrospinal fluid (CSF) or blood biomarkers of AD, focusing on the most established AD biomarkers (amyloid-beta, phosphorylated-tau, total-tau, neurofilament light chain, and glial fibrillary acidic protein). The gene discussed is GFAP; the disease is Alzheimer disease.