Abstract
OBJECTIVE: Adolescence is a high-risk period for mental disorders, and early clinical presentations often show uncertainty and pluripotentiality. Longitudinal evidence on transitions from non-specific diagnoses to severe mental illness (SMI-defined as schizophrenia spectrum and bipolar disorders) in non-Western populations remains limited. Using real-world data, we aimed to characterize diagnostic stability, transitions from non-SMI to SMI, and sex- and age-related predictors in Chinese adolescent inpatients. METHODS: This retrospective longitudinal cohort study utilized electronic medical records from a large tertiary psychiatric hospital in Eastern China (2010-2026). We included 884 first-time inpatients aged 12-17 years with ≥3 years of follow-up and at least two complete inpatient records. ICD-10 diagnoses were grouped into SMI and non-SMI categories. Sankey diagrams and transition matrices were used to describe diagnostic trajectories from baseline to the last admission. Among patients with non-SMI at baseline, Kaplan-Meier analyses examined the time to transition to SMI, and multivariable logistic regression tested the independent effects of sex and baseline age on SMI conversion. RESULTS: Over a median follow-up of 4.60 years (IQR 3.63-6.53), SMI diagnoses showed high stability: 81%(243/300)of schizophrenia spectrum disorders and 74%(104/141) of bipolar disorders remained unchanged. Overall, 39.2%(346/884)of patients experienced at least one diagnostic change, primarily within non-SMI categories; depressive disorders were the most frequent antecedent of bipolar disorder (13%(22/171) converted). Sex-stratified analyses suggested that certain externalising and obsessive-compulsive presentations in males, and internalising and stress-related presentations in females, were more frequently followed by SMI; however, several subgroup estimates were based on small numbers and should be considered exploratory. Kaplan-Meier curves indicated that the risk of transition from non-SMI to SMI clustered between 4 and 8 years after the first admission. Each 1-year increase in baseline age was associated with a 38% higher risk of SMI conversion (OR = 1.38, 95% CI 1.19-1.60, P < 0.001), and, after adjusting for age, males had approximately twice the risk of SMI conversion compared with females (OR = 1.90, 95% CI 1.17-3.07, P = 0.009). CONCLUSIONS: Adolescent psychiatric diagnoses show substantial longitudinal evolution, with relatively stable SMI once established but appreciable medium- to long-term progression from non-SMI to SMI. The identified sex-specific pathways and the 4-to-8-year high-risk window support longitudinal, developmentally informed monitoring-particularly for older male adolescents with severe or atypical non-SMI presentations.