Determining the cumulative incidence of treated schizophrenia and predictors of transition in an early intervention for psychosis service.

Background: The accurate measurement of the incidence of schizophrenia can inform public policy and ensure equitable allocation of limited resources. First contact cases under-estimates the true incidence because of later transitions to schizophrenia. This study aimed to determine the:(i)24 months cumulative incidence of treated DSM-5 schizophrenia diagnosed during treatment at the Early Psychosis Prevention and Intervention Centre (EPPIC) service by including cases of first episode psychosis (FEP) which subsequently transitioned to a diagnosis of schizophrenia; (ii) extent of the under-estimation by using first contact cases; and (iii) proportion and predictors of transition to schizophrenia or bipolar disorder.

Methods: This observational study included all consecutive cases of FEP for people aged 15-24 who attended the EPPIC service from 2011 to 2016, data was recorded prospectively but collected by researchers retrospectively. Poisson regression and Cox regression were used to determine incidence rate ratios and hazard ratios respectively.

Results: At presentation, of the 1220 young people with a FEP, 17.2 % had a diagnosis of schizophrenia compared to 30.2 % after a median of 93 weeks (I.Q.R.: 62-106). The incidence rates of first contact diagnosis of schizophrenia ranged from 17.6 to 28.2 per 100,000 and the cumulative incidence rate ranged from 28.2 to 42.3. The incidence of schizophrenia was 57 % higher with the cumulative method (IRR = 1.57,95%C.I.: 1.30-1.90,p < .001). At discharge, 30.2 % met criteria for schizophrenia and 16.3 % for bipolar disorder.

Conclusions: Even when considering the limitations of the study, such as the narrow age bracket of the participants, the cumulative incidence of schizophrenia should be used to determine a more accurate incidence.