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Three prospective population-based cohort studies were conducted using linked de-identified register data from several nationwide Swedish registers. The cohorts included all individuals who were 19–23 years old and lived in Sweden all of the year 1995, 2000, or 2005, respectively. Each cohort was followed up regarding suicide attempt and suicide for five years after the year they were included in respective cohorts (1996–2000; 2001–2005, and 2006–2010) (Figure 1).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0111618.g001 Timeline of the three cohorts.The three cohorts included all individuals 19–23 years of age and living in Sweden in 1995, 2000, and 2005, respectively. Each cohort was followed for five years from the year after inclusion.
The study population was based on linkage of several public national registers. Ethical vetting is always required when using register data in Sweden. The ethical vetting is performed by regional ethical review boards and the risk appraisal associated with the Law on Public Disclosure and Secrecy is done by data owners. The ethical review boards can however waive the requirement to consult the data subjects (or in case of minors/children the next of kin, careers or guardians) directly to obtain their informed consent, and will often do so if the research is supported by the ethical review board and the data has already been collected in some other context. This means that for this specific study no written informed consent was given by participants (or next of kin/caregiver in the case of children) for their clinical records to be used. Patient records/information was anonymized and de-identified prior to analysis. This was done by Statistics Sweden, the authority responsible for data linkage. Researchers received de-identified data. This project has been evaluated and approved by the Regional Ethical Review Board of Karolinska Institutet, Stockholm, Sweden (protocol nr 2007/762-31).
Data on the main DP diagnosis was obtained from the National Social Insurance Agency. The diagnoses were classified according to the International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) [6] and categorized into psychiatric diagnoses (F00–F99) and somatic diagnoses (all other diagnostic codes), and missing information, respectively. The psychiatric DP diagnoses were categorized into the following subgroups: “Schizophrenia” including schizophrenia, schizotypal and delusional disorders (F20–F29); “Depression/anxiety” including unipolar depression/neurotic, stress-related and somatoform disorders (F32–F33, F40–F49); “Mental retardation” (F70–F79); “Pervasive developmental disorder” (F84); “Hyperkinetic disorder” (F90); and “Other mental disorders” (F00–F19, F30–F31, F34–F39, F50–F59, F60–F69, F80–F83, F85–F99).
Information about sex, country of birth, and parental education was obtained from the Longitudinal Integrated Population-based Database for Labour-market Research (LISA) held by Statistic Sweden, and then linked with the Multi-Generational Register. Country of birth was classified as either Sweden or other. Parental level of education registered five years before inclusion in the cohort (1990, 1995, and 2000 respectively) was classified as low (compulsory school or less, ≤9 years), medium (upper secondary school, 10–12 years), or high (college or university, >12 years). We used either maternal or paternal education, whichever was the highest, in accordance with the dominance principle [14].
Suicidal behaviour and psychiatric inpatient care: Information on suicide attempts and suicides among the participants was obtained from the National Patient Register and the Cause of Death Register, held by the National Board of Health and Welfare. Suicide attempts and completed suicides were classified according to the ICD-10, [7] defined by the codes X60–X84 and Y10–Y34. Codes with uncertainty about intention (Y10–Y34) were included, primarily in order to limit temporal and regional variation in ascertainment routines. [15], [16] Suicide attempt was measured in two separate periods for each cohort: for the five years following the year the cohort was defined (e.g., from 1 January 2006 to 31 December 2010 for the cohort of 2005), and anytime before the baseline. Information about previous inpatient care with a psychiatric diagnosis was retrieved for the 6 years preceding baseline (e.g., from 1 January 2000 to 31 December 2005 for the cohort of 2005). Psychiatric diagnosis was defined as any mental and behavioural disorder in ICD-10 (F00–F99).
Information on parental suicide attempts between 1964 and 31 December of the inclusion year (1995, 2000 and 2005 respectively) was obtained from the National Patient Register and defined by the ICD-9 codes E950–E959 and E980–E989 or the ICD-10 codes X60–X84 and Y10–Y34. Information on parental suicide from the year when the child was born to 31 December the year the cohort was defined was obtained from the Causes of Death Register. Events of parental suicide attempts or suicide were combined to one dichotomized measure of suicidal behaviour.
Differences between the cohorts regarding DP diagnoses, socio-demographic variables, inpatient care and suicidal behaviour were tested with the χ2-test. We calculated the Hazard Ratios (HR) with 95% confidence intervals (CI) for suicide attempt and suicide during the five years following inclusion in the cohort. Cox proportional hazard regression models were used, with person-time during follow-up as the underlying time scale. The data conformed to the proportional hazards assumption. Individuals were followed from 1 January the year following inclusion in the cohort to the event, death, emigration, and end of follow-up, whichever came first. We performed both univariate and multivariate analyses. In a first set of analyses, the categories of psychiatric DP diagnoses, DP with somatic diagnoses, and missing information on DP diagnoses were contrasted to no DP (reference category). In a second set of analyses, DP due to all the subgroups of psychiatric diagnoses and the category of other diagnoses were contrasted to no DP (reference category). In the analyses of suicide, division into different diagnostic subgroups was not possible due to lack of power. The statistical analyses were performed with IBM SPSS 20.
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