Methods: A population-based cohort study using the semi-structured Psychosis-like Symptoms interview of psychotic experiences at ages 12, 18, and 24 (N=7900 with any data). Incidence rates were estimated using flexible parametric modelling, and positive predictive values (PPV), sensitivity, specificity, and area under the curve estimated for prediction.
Results: The incidence rate of psychotic experiences increased between ages 13-24 years, peaking during late adolescence. Of 3866 interviewed at age 24, 313 (8.1%, 95%CI 7.2%, 9.0%) had a definite psychotic experience since age 12. 109 individuals (2.8%) met criteria for a psychotic disorder up to age 24, of whom 70% had sought professional help. Prediction of current psychotic disorder at age 24 (N=47, 1.2%) by both self-report and interviewer-rated measures of psychotic experiences at age 18 (PPVs 2.9% and 10.0% respectively) was improved by incorporating information on frequency and distress (PPVs 13.3% and 20.0% respectively), although sensitivities were low. The PPV of an at-risk mental state at age 18 predicting incident disorder ages 18-24 was 21.1% (95%CI 6.1%-45.6%; sensitivity 14.3%, 95%CI 4.0%-32.7%).
Conclusions: Our study shows a peak in incidence of psychotic experience during late adolescence, and an unmet need for care in young people with psychotic disorders. Because of the low sensitivity, targeting individuals in non-help-seeking samples based only on more severe symptom cut-off thresholds will likely have little impact on population-levels of first-episode psychosis.