BACKGROUND: The immune system may play an important role in the aetiology of psychotic disorders and there is increasing interest in the relationship between immune markers and clinical outcomes in psychosis. The present study investigated whether subclinical systemic inflammation was associated with length of stay in individuals with psychosis admitted to a psychiatric hospital. We tested the hypothesis that a higher level of subclinical systemic inflammation, as measured by the neutrophil-to-lymphocyte ratio (NLR) would be associated with a longer period in hospital. METHOD: Retrospective cohort study based on electronic health records. We included patients with a psychosis spectrum disorder (ICD10: F20-F29) who had a routine blood test upon being admitted to a psychiatric hospital within the South London and Maudsley NHS Foundation Mental Health Trust, London, UK between 2013 and 2019. Multiple linear regression was used to determine the association between the NLR at the time of admission and the duration of the corresponding hospital stay, adjusting for covariables. RESULTS: Data from 1683 individuals with psychosis were analyzed. The median admission duration was 31 days (interquartile range = 48 days). Higher neutrophil-to-lymphocyte ratio (NLR) was significantly associated with longer admission (B = 0.07, p <
0.003) after adjusting for covariates. CONCLUSION: An association between a NLR and a longer admission, whilst controlling for relevant covariables, was observed highlighting the potential utility of inflammatory markers as prognostic marker in clinical settings.