Diagnostic conversion from unipolar depression to bipolar disorder, schizophrenia, or schizoaffective disorder: A nationwide prospective 15‐year register study on 43 495 inpatients

Objective To examine temporal patterns and predictors for diagnostic conversion from unipolar depression (UD) to bipolar disorder (BD), schizophrenia, and schizoaffective disorder (SAD). Methods A prospective nationwide register-based cohort (n = 43 495) of all first psychiatric hospitalizations due to UD during 1996-2011 was followed up to 15 years. We used cumulative incidence function (CIF) analyses and the Fine-Gray subdistribution model to define the cumulative incidence of the conversions and subdistribution hazard ratios (SHRs) for predictors. Results The overall 15-year cumulative incidence of conversion was 11.1% (95% CI 10.7-11.6): 7.4% (95% CI 7.0-7.8) for BD, 2.5% (95% CI 2.3-2.7) for schizophrenia, and 1.3% (95% CI 1.1-1.4) for SAD. The highest crude incidence rate emerged during the first year. Psychotic depression predicted higher conversion risk to BD (SHR = 2.0, 95% CI 1.5-2.7), schizophrenia (SHR = 5.3, 95% CI 3.3-8.7), and SAD (SHR = 10.6, 95% CI 4.0-28.4) than mild depression. Female sex, greater overall disturbance, and comorbid personality disorder predicted conversion to BD, whereas young age and male sex to psychotic disorders. Conclusions Among patients with first hospitalization due to UD, approximately one in nine converts to another major psychiatric disorder during 15 years, with the highest risk occurring within the first year. Patients with psychotic depression are particularly vulnerable for conversion to other major psychiatric disorders. Conversion to psychotic disorders occurs earlier than to BD. Males are at higher risk for progression to psychotic disorders, whereas females, patients with recurrent depressive episodes, severe disturbance of overall functioning, and personality disorder are at higher risk for converting to BD.