The study was designed as a prospective cohort study by linking Swedish register data. All people 16 to 64 years-old on 31.12.2004 and registered in Sweden who initiated at least one sickness absence spell due to a stress-related mental disorder (SRMD) in 2005 (N= 38,870) comprised the study base. We excluded 2059 and 265 individuals who were on disability and old-age pension before 2005, respectively, and two further individuals who were erroneously coded as dead before 2005. Also, individuals with missing information on educational status (N=41), or on the length of the first sickness absence spell due to SRMD initiated in 2005 (N=201), were excluded. This left 36,304 individuals (26,315 women and 9,989 men) to be included in the study.

The following register data sources were used for all the individuals in the study: 1) Statistics Sweden: Longitudinal Integration Database for Health Insurance and Labour Market Studies (LISA), which includes socio-demographic information on age, sex, country of birth, place of residence, family situation and educational status; 2) the National Board of Health and Welfare: (i) the National Patient Register, which includes information on dates and diagnoses for inpatient and outpatient care, (ii) the Cause of Death Register, with data on dates and causes of death; 3) the Social Insurance Agency (SIA): Micro-Data for Analyses of Social Insurance (MiDAS), with information on date, grade and diagnosis of sickness absence and disability pension. The linkages were based on the unique personal numbers (de-identified) of all residents in Sweden.

The Swedish social insurance system

All people above the age of 16, living in Sweden, with an income from work or unemployment benefit, who – due to disease or injury have a reduced work capacity – are covered by national sickness insurance and can receive sickness benefit [12]. After a first qualifying day, the employer is responsible for sick pay for the first 14 days of a sick leave spell; thereafter, sickness benefit is paid by the Social Insurance Agency. Self-employed people have more qualifying days, and they, as well as the unemployed, receive all sickness benefit from the Social Insurance Agency. A physician’s certificate is required after seven days of self-certification. For this study, data on sick leave with benefit from the Social Insurance Agency were employed.

Diagnostic criteria

All diagnoses related to risk factors and outcomes were based on the corresponding codes of the International Classification of Diseases (ICD) in ICD 10. The definition of stress-related mental disorders in sickness absence spells was based on the corresponding ICD 10 Code F43.

Measures of risk factors

Baseline socio-demographic characteristics from the LISA database were measured at the end of 2004 and categorized as shown in Table 1. Variables on sickness absence relate to spells initiated in 2005. Number of sickness absence spells regardless of diagnosis was categorised as follows: 1, 2 and > 2 spells. The variable on diagnosis-specific sickness absence spells was categorised as follows: only spells due to mental disorders, one additional spell due to a somatic disorder, and multiple additional spells due to somatic disorders. Duration of the first sickness absence spell due to SRMD initiated in 2005 was grouped in five categories: 1–14, 15–90, 91–180, 181–365, and more than 365 days. The following covariates were dichotomized: having at least one full-time period (at least one day with 100% sickness absence) in the first sickness absence spell due to SRMD; having at least one part-time period in the first sickness absence spell due to SRMD, and whether or not individuals had any repeated sickness absence spells due to SRMD.

Table 1 Descriptive statistics for individuals (N=36,304) sickness absent due to stress-related mental disorders Full size table

Categorisation of health care variables was based on the diagnosis-specific median number of days and number of visits in inpatient (2000–2005) and outpatient care (2001–2005) (four variables). The median length of hospital stays due to mental and somatic diagnoses were 8 and 4 days, respectively, while the median numbers of visits in outpatient care due to mental and somatic diagnoses were one and four, respectively.

Measurement of outcome

Individuals with inpatient care due to ICD-10 codes X60-X84 were regarded as cases of suicide attempt. We considered only the first suicide attempt during the follow-up period. People with similar ICD codes in the Cause of Death register were considered as cases of suicide.

Statistical analysis

Cox proportional hazard regression models were applied after ensuring that the proportional hazard assumption was met. With regard to suicide attempt, individuals were followed from 01.01. 2006 to the date of inpatient care due to suicide attempt, death, emigration, or 31.12. 2009, whichever came first. With regard to suicide, follow-up was from 01.01. 2006 to the date of suicide, death by other causes than suicide, emigration, or 31.12. 2008, whichever came first. Hazard ratios for suicide attempt and suicide were estimated with 95% confidence intervals. The multivariate models included both significant predictors and significant confounders.

The partial likelihood ratio test was used to test for possible interactions between risk factors and to test the predictive value of variables in case of collinearity. Attributable proportions (APs) were estimated as follows: AP= (HR-1)/HR*f, where f denotes the proportion exposed to the various risk factors in all who attempted or committed suicide [13]. All statistical analyses were performed using SPSS version 20.

Ethical considerations

The study population was based on several linked public national registers, derived from three different authorities. Ethical vetting is always required when using register data in Sweden. The vetting is performed by regional ethical review boards, and a risk appraisal associated with the Law on Public Disclosure and Secrecy is performed by data owners. The ethical review boards can, however, waive the requirement to consult the data subjects (or, in case of minors/children, next of kin, carers or guardians) directly to obtain their informed consent, and will often do so if the research is supported by an ethical review board and the data have already been collected in some other context. This project was evaluated and approved, according to these standards, by the Regional Ethical Review Board of Karolinska Institutet, Stockholm, Sweden.