BACKGROUND: Sepsis is a critical condition associated with high morbidity and mortality, emphasizing the need for reliable biomarkers for its diagnosis and prognosis. This study uses advanced immunological techniques to evaluate monocytic CD39 (mCD39) expression as a potential marker in sepsis. METHODS: This prospective observational cohort study included 206 participants from the First Affiliated Hospital, Zhejiang University School of Medicine between April 2022 and September 2023. Participants were categorized into four groups: healthy donors, patients with mild infections, post-cardiac surgery patients (non-infectious inflammation), and sepsis patients. Peripheral Blood Mononuclear Cells were analyzed using mass cytometry time-of-flight (CyTOF) with a 42-marker immune panel and flow cytometry targeting monocytes. Statistical analyses included ROC curves for diagnostic and prognostic performance and Kaplan-Meier survival analysis for prognostic evaluation. RESULTS: Sepsis patients exhibited significantly lower monocytic CD39 expression than mild infection and post-surgery groups (p <
0.05). The diagnostic performance analysis revealed that mCD39 effectively distinguished sepsis from mild infection (AUC = 0.877) and non-infectious inflammation (AUC = 0.935). Prognostic analysis identified low mCD39 expression as a strong predictor of short-term survival, with a 7-day survival AUC of 0.85 (p = 0.037). Kaplan-Meier analysis showed that sepsis patients with low mCD39 expression had significantly lower 28-day survival rates (56.7% vs. 80.6%, p = 0.016). CONCLUSIONS: Low CD39 expression on monocytes might serve as a potential diagnostic biomarker and a strong predictor of poor prognosis in sepsis patients.