Elevated CSF Serotonin in Prodromal Alzheimer’s Disease Patients Developing Psychosis

Introduction: Psychotic symptoms (PS) in Alzheimer’s disease (AD) are associated with unfavorable prognosis, including accelerated functional decline and reduced survival. Multiple neurotransmitter systems have been implicated in the pathophysiology of PS, with the serotonergic system emerging as particularly relevant. Materials and Methods: Between 2010 and 2020, 120 patients with prodromal AD and 26 cognitively healthy controls underwent comprehensive evaluation, including clinical history, neurological and neuropsychological assessment, neuroimaging, and lumbar puncture. All participants underwent longitudinal clinical monitoring for a minimum of five years or until the emergence of PS. In February 2024, baseline cerebrospinal fluid (CSF) serotonin (5-HT) concentrations were quantified using competitive ELISA (FineTest, Wuhan, China). Results: CSF 5-HT levels were significantly elevated (p < 0.003) in patients who subsequently developed psychosis (n = 49) compared with those who remained free of PS during the 8-year follow-up (n = 19). A threshold of 4.89 ng/mL yielded 80% sensitivity for identifying individuals at risk. Baseline Neuropsychiatric Inventory (NPI; p < 0.001) and Unified Parkinson’s Disease Rating Scale part III (UPDRS III; p < 0.009) scores also demonstrated strong discriminative capacity. Conclusions: Measurement of CSF 5-HT and detailed clinical profiling in prodromal AD may provide predictive value for psychosis onset within 8 years of diagnosis. To our knowledge, this is the first study to report CSF 5-HT findings in AD patients.