Annual incidence rate of schizophrenia and schizophrenia spectrum disorders in a longitudinal population-based cohort study

Longitudinal incidence studies of schizophrenia spectrum disorders (SSD) performed in mental health service organizations are prone to confounding factors not found in research performed in the general population. To estimate the incidence rates (IRs) over a 10-year period of SSD (broadly defined) and schizophrenia (narrowly defined) in the general population and to analyze associated risk factors. A cohort study (1996-2006) in a large general practitioners research database was conducted with longitudinal medical records of 350,524 patients throughout the Netherlands. Cases of SSD were identified and classified by systematic review of medical records. Age- and gender-specific IRs were calculated per calendar year, date of birth, degree of urbanicity and deprivation. Overall IR of SSD in this population was 22/100,000 person years (PY) (95 % CI 19-24). IR of schizophrenia was 12/100,000 PY (95 % CI 10-14). Period prevalence was 3.5 per 1,000 PY. IRs were higher in men compared to women, had a peak at age 15-25 years, decreasing rapidly after 25 years by 40 % per 10 years. IRs of SSD were significantly higher in urban areas, irrespective of deprivation. No association was found between IRs of SSD and living in deprived areas or month of birth. There was no significant time trend of the IR during the period under study. IRs of SSD are higher in urban areas, independent of social deprivation. Age- and gender-specific differences in IR were found. The magnitude of these differences was larger in narrowly defined schizophrenia than in SSD.