In a study of employees listed as sick with low back pain disorders, Denmark has shown poor rates of return to work compared with other countries,1 suggesting a relatively high risk of permanent labour market expulsion among those experiencing sickness absence. As in most other OECD countries, the costs for society associated with work disability have risen during the past decade: currently, approximately 8% of the Danish population aged 20–64 years receive permanent disability benefits.2 All citizens below the official retirement age are eligible for these state financed disability benefits, based on a workability assessment performed by the municipal authority granting the benefits. Several reforms of the Danish disability pension schemes have been performed during the study period in order to restrict access to permanent disability pension. However, according to Statistics Denmark, costs for disability pension and rehabilitation in Denmark have risen from €4.5 billion in 1995 to €8.1 billion in 2004 (source: http://www.statistikbanken.dk). Furthermore, work disability costs in terms of worsening of individual well-being as a result of exclusion from working life have also been proven to be substantial in previous studies: work disabled are more prone to experience various future consequences in terms of social inactivity and isolation, suicide and poor financial circumstances.3

There seems to be increasing recognition of the abilities of certain measures of sickness absence to measure physical, psychological, and social functioning as well as to predict hard end points such as mortality in working populations.4^–6 In contrast, only a few studies have assessed predictive abilities of sickness absence in terms of future disability pension. In the Finnish 10-town study among 46 589 municipal employees, sickness absence periods longer than three days were a stronger predictor of later disability pension than were shorter sickness absence periods.7 Among 10 077 long-term sickness absentees from a random sample of the Norwegian population, disability pension was predicted by sickness absence periods exceeding 28 weeks.8 In addition, there are a few small-scale studies with varying definitions of sickness absence and these studies have also reported a link between increased sickness absence and increased risk of future disability pension.9^–11 As long sickness absences and incident disability pensions are rare events, the sample size in all previous studies may have remained too small for a detailed analysis of the association between absence duration and pension risk. Moreover, most studies were based on selected samples and no data are available specifically for private sector employees.

The aim of this study was to examine the associations between duration of compensated sickness absence and future disability pension among the total population of private sector employees receiving sickness absence compensation in Denmark in 1998. To determine specifically whether sickness absence duration represents a risk marker sufficiently distant to provide time to intervene and potentially prevent early disability, we performed age and gender-stratified analysis for 225 056 private sector employees between 18 and 65 years of age at study entry.

METHODS

Data and population

The study is based on the DREAM register.12 DREAM (a Danish acronym for The Register-based Evaluation of Marginalization) contains weekly information on granted sickness absence compensation (SAC) in 1998 for all private sector employees and granted disability pension for all citizens in Denmark from 1999–2004. A subject is only included in DREAM if he or she is a recipient of a sickness absence or disability benefit. SAC is given to the employer, who can apply for a refund from the State for employees after two weeks of sickness absence. The employer covers SAC for the first two weeks if the employee has been employed for eight consecutive weeks prior to sickness absence and has worked for at least 74 h during this period. The employee is eligible for SAC from the municipality if he or she has been employed for at least 13 consecutive weeks prior to sickness absence and has worked for at least 120 h during this period. Exception from this rule can be made if the employee is a member of an unemployment insurance fund, has finished vocational training of at least 18 months duration within the last month, or is a trainee. Duration of sickness absence was assessed for all new cases occurring between 1 January and 31 December 1998, among those who had received no social transfer payments during the last week of 1997, and with no social transfer payment in the week preceding onset of SAC. People not receiving SAC are not included in the study, as they are not recorded in DREAM. Maternity related absence is excluded. This study assesses SAC and risk of future disability pension among all employed in the private sector (1 495 952 persons of whom 36.6% were women and 63.4% were men) in Denmark in 1998.

Outcome

In this study, sickness absence is defined according to the length of the first occurring event of SAC. A person can normally receive SAC for no longer than 12 months. After this, he or she will either have to return to work, receive unemployment benefits or disability pension. The municipal authorities grant disability pension. By law, it is possible for the recipient to work part-time while receiving disability pension, and also to return to work. However, these options are rarely used, and in reality disability pension is a permanent way of exit from the labour market in Denmark.13 To eliminate confounding attributable to sickness absence periods immediately prior to disability pension, the follow-up period for disability pension started 24 months after the assessment period of sickness absence. Hence disability pension cases were identified from 1 January 2001 to 31 December 2004. The study design is shown in figure 1.

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Figure 1 Study design.

Assessment of sickness absence

Duration of sickness absence is based on the weekly records of SAC in DREAM. The range of duration varies from 1–53 weeks. A total of 240 815 persons above the age of 18 years receiving SAC during 1998 were identified. They had not received SAC or any other social transfer payment in the final week of 1997. During the two-year period from 1999 through 2000, a total of 15 759 persons were either disability pensioned, old-age pensioned, emigrated or died. They were excluded from the study, as they were no longer under risk of disability pension in the follow-up period from 2001 through 2004. This left a total of 225 056 persons receiving SAC in 1998 to be under risk for disability pension from 2001 to 2004. These 225 056 (39.2% women, 61.8% men) constitute the basis of analysis in this study.

Other variables

The study includes information of sex, age at baseline, and immigration, retirement and death during the study period.

Analysis

We fitted Cox proportional hazards models to study the associations between duration of sickness absence and incidence of disability pension, separately for men and women. Employees were censored at the time of death, immigration or retirement due to age.

Adding a time-dependent interaction term between duration of sickness absence and the logarithm of follow-up time confirmed that the proportional hazards assumption was justified. Hazard ratios and 95% confidence intervals were adjusted for age. Stratified analyses were performed for employees younger than 40 and those 40 or older at study entry. We tested the effect of an interaction between age group and sickness absence duration on future disability pension by including the corresponding interaction term in regression models that already included the main effects.

RESULTS

A total of 5694 persons (2.5%) received disability pension during follow-up. Of these, 2652 (46.6%) were women and 3042 (53.4%) were men. The incidence of disability pension events was higher among women (3.1%) than among men (2.2%). The distribution of disability pension events according to duration of absence is shown in table 1.

There was a significant stepwise increase in risk for disability pension according to duration of SAC, peaking at 26 or more weeks, where the hazard ratio was 6.65 for women (95% CI 5.77 to 7.66) and 7.57 for men (95% CI 6.56 to 8.74) compared to those with one week of SAC (not shown).

Further analysis showed differences in excess risk of disability pension associated with specific SAC duration between different age strata. Tests for interaction were highly significant for women (Wald test statistic = 35.4, df = 8, p<0.001) and for men (Wald test statistic = 76.0, df = 8, p<0.001). For those 40 years or older, there was a significant risk of disability pension after three weeks of SAC for both women and men. There was a hazard ratio of 6.65 for women and 7.17 for men with 26 or more weeks of SAC (fig 2).

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Figure 2 Risk for disability pension for women and men 40 years or older. Age adjusted hazard ratios and 95% CI; n = 92 361 (36 435 women, 55 926 men).

For those younger than 40 years, there was a similar excess risk for absence periods below eight weeks. For periods exceeding eight weeks, men in particular experienced a profound increase in risk, peaking with a hazard ratio of 16.07 for 26 or more weeks of SAC. The corresponding hazard ratio for women was 12.59 (fig 3).

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Figure 3 Risk for disability pension for women and men below 40 years of age. Age adjusted hazard ratios and 95% CI; n = 132 695 (49 451 women, 83 244 men).

DISCUSSION

In the population of employees working in the private sector in Denmark in 1998, 225 056 people experienced periods of SAC. Of these, 5694 (2.5%) received disability pension during follow-up, more men (53.4%) than women (46.6%). There was a clear upward trend in risk with prolonged SAC. A profound difference was found between the younger (<40 years) and older (⩾40 years) age strata—for example, men under 40 experiencing more than 26 weeks of SAC had a 16-fold risk of disability pension. The corresponding figure for men 40 years or older was approximately 7. For women, the corresponding figures were 12.6 and 6.7 respectively. To our knowledge, this is the largest investigation of associations between sickness absence and disability pension, and also the first to be performed on a cohort covering all subjects within a specific country.

Methodological considerations

The present study was based on register data, which allowed us to avoid sample attrition. Furthermore, the study is based on the total population of private sector employees receiving SAC in Denmark, eliminating issues such as selection bias into the study. The design uses a 24 month washout period in accordance with maximum possible duration of SAC, thereby avoiding that the disability pension period began immediately after the SAC assessment period. Therefore, sickness absence does not run directly into a disability pension, which could otherwise seriously inflate observed associations. The washout period can vary between subjects according to time for onset of SAC. This could influence results if there was some systematic difference between sickness absence periods occurring early in the year and sickness absence periods occurring late in the year, which we find unlikely. Anyhow, all subjects have more than 12 months of washout, which is more than the maximum time one can receive SAC. The DREAM register consists solely of recipients of social transfer payments. Therefore, it does not allow the study of risk of disability pension for employees with no sickness absence, but merely comparison of risk associated with duration of received sickness absence compensation. Hazard ratios for disability pension would likely have been even larger for long periods of sickness absence, if compared to people with no sickness absence. SAC is registered in DREAM if a person has more than one day of SAC in a given week. This could lead to an overestimation of a single day on SAC counting as an entire week. In contrast, there could be an underestimation of sickness absence as a whole, because employers in some cases may not report all sickness absence, most often as a result of neglect. This could lead to underestimation of the “true” sickness absence level. The study could therefore potentially have fewer cases included than are actually sickness absent. However, this is unlikely to affect the analyses of associations in this study, as there are no such concerns with regards to the reporting of disability pension from the municipalities. Nor does it affect the implications of using the register information on granted SAC as an indicator of future disability pension.

Related studies

Previous studies have included diagnose-specific disability pension7 and also diagnose-specific sickness absence.8 The present study does not include underlying diagnoses on either disability pension or SAC. However, this is not likely to dilute the results, as the studies including diagnosis have proven duration of sickness absence and age to be the stronger predictors even when taking into account the underlying diagnosis.8 The present investigation found a stronger association between SAC and disability pension for younger employees than for older. Whether this is because older employees with health issues have alternative exit options compared with younger employees is unknown. An alternative explanation could be that in particular long-term SAC among the younger could be the result of different kinds of problems (diagnoses) than among older employees. SAC is less prevalent among younger than older employees, and this type of selection can affect the strength of the association between SAC and disability pension across age strata.

As the study is based on all private sector employees in Denmark, the study is representative by definition. However, in order to clarify whether the findings of risk across age strata accounts for the working population in general, it seems relevant to conduct a study including public sector employees as well. Furthermore, replication in other countries would be important to determine whether the associations are generalisable across different pension schemes and population characteristics in contemporary settings.

CONCLUSION

Costs of disability pensions are steadily growing in many European and Scandinavian countries and in the USA.2 14 15 Growing numbers of younger disability pension recipients are a particular problem as they may be beneficiaries for decades. The findings of excess risk for those younger than 40 years old indicate that administratively collected data on sickness absence compensation are an important predictor of future disability pension. The use of information on sick leave may improve the effectiveness of early interventions by policy makers, case managing authorities, employers and physicians.