Lifetime incidence and age of onset of mental disorders, and 12-month service utilization in primary and secondary care: a Finnish nationwide registry study.

Background: Previous studies have estimated the lifetime incidence, age of onset and prevalence of mental disorders, but none have used nationwide data covering both primary and secondary care, even though mental disorders are commonly treated in primary care. We aimed to determine lifetime incidence, age-specific incidence, age of onset and service utilization for diagnosed mental disorders.

Methods: This register-based cohort study followed the entire population of Finland from 2000 to 2020. We estimated the cumulative incidence of diagnosed mental disorders with the Aalen-Johansen estimator, accounting for competing risks such as death and emigration. We also calculated age-specific incidence and 12-month service utilization as of 31 December 2019, providing diagnosis-, age- and gender-specific estimates.

Results: We followed 6.4 million individuals for 98.5 million person-years. By age 100, lifetime incidence of any diagnosed mental disorder was 76.7% (95% CI, 76.6-76.7) in women and 69.7% (69.6-69.8) in men; in psychiatric secondary care, it was 39.7% (39.6-39.8) and 31.5% (31.4-31.6). At age 75, stricter estimates for non-organic disorders (ICD-10: F10-F99) were 65.6% (65.5-65.7) for women and 60.0% (59.9-60.1). Anxiety disorders (F40-F48) had the highest cumulative incidence. Median age of onset of non-organic mental disorders was 24.1 (interquartile range, 14.8-43.3 years) in women and 20.0 (interquartile range, 7.3-42.2 years) in men. Service utilization within 12 months was 9.0% for women and 7.7% for men.

Conclusions: Most, though not all, individuals experience at least one type of mental disorder, often during youth. Capturing the overall occurrence of mental disorders requires including both primary and secondary care data.