Long-term sick leave is a burden for individuals and society at large, yet very little is known about the underlying reasons for it. Researchers at the Norwegian Institute of Public Health, in collaboration with Australian and British institutes, have identified anxiety as a more important risk factor than previously thought.

Common mental disorders such as anxiety and depression will affect 1 of 3 of us at some point in our lives. The core symptoms of mental disorders affect a person's emotional, cognitive and social functioning, which can impact on working ability. Previous studies have found a link between mental disorders and sick leave, though they have been uncertain as to whether mental disorder increases the risk of sick leave, or the other way around. Prolonged absence from the workplace can contribute to avoidance behaviour, especially in those with anxiety, which can make it even harder for these individuals to get fully back to work. It is therefore important to examine the long-term associations between common mental disorders and sick leave in order to help plan more effective interventions aimed to prevent and reduce sick leave among individuals with common mental disorders.

This study examined anxiety and depression levels among 13 436 participants in the Hordaland Health Study. Common mental disorders were assessed at the start of the study with the Hospital Anxiety and Depression Scale. Participants were then followed for up to 6 years, retrieving information on sick leave of 16 days or more from the official Norwegian registry over state paid sick leave benefits. Information on other possible causal factors such as socioeconomic status and physical health was also obtained from the health study.

Anxiety is most important

The study has several findings that have not been previously shown in similar studies. Firstly, it shows that common mental disorders increase the risk of very prolonged absence (over 90 days) and repeated episodes of sick leave. Secondly, it shows that the risk of these outcomes is highest among those with both anxiety and depression simultaneously. Thirdly, the results indicate that anxiety may be more important than depression:

"Surprisingly, we found that anxiety alone is a stronger risk factor for prolonged and frequent sick leave than depression alone. Further, anxiety seems to be a relatively stable risk factor for sick leave, as we found an increased risk of sickness absence up to six years after the anxiety level was assessed," says Ann Kristin Knudsen, who is lead author of the study and PhD student at the University of Bergen and the Division of Mental Health at the Norwegian Institute of Public Health.

The effects of pain

A number of risk factors can simultaneously influence long-term sick leave. In particular, pain was found to have a considerable impact on the association between common mental disorders and sick leave: adjusting for pain ("removing" its effect in the statistical model) reduced the association.

"Adjusting for pain may have given us artificially low effect sizes, since pain, anxiety and depression are closely related and may reflect the same underlying health condition," explains Knudsen.

In other words, the association between common mental disorders and sick leave may actually be stronger.

Benefits of prospective studies

This is the first study with a long enough follow-up period (6 years) to show that the effect of mental disorder on sick leave remains over time.

"Previous research has largely been based on patient data, organizational data or diagnoses of sick leave certificates, or in studies where the prevalence of mental disorder was measured during sick leave. The latter is problematic because we do not know what comes first, sick leave or mental health problems," explains Knudsen.

Since this was a prospective study, following individuals both with and without common mental disorders over time, it provides evidence that common mental disorders increases risk of sick leave and not the other way around. On the other hand, people with common mental disorders also suffered several episodes of sick leave during the follow-up time, which may indicate that both sick leave and the mental health problems affect each other and thus result in a "vicious circle" with repeated sick leaves.

Implications for clinicians

This study is one of the few to show the importance of anxiety, which has largely been ignored in previous studies in this area. Previous studies not accounting for the impact of anxiety may thus have overestimated the effect of depression on sick leave. Without awareness of anxiety, physicians and clinicians may not detect anxiety sufficiently, which can hamper rehabilitation efforts. Future research should focus on how health professionals dealing with sick leave can best identify anxiety and other mental disorders to help prevent the prolongation of sick leave or further sick leave episodes.

The study was a collaboration between the Norwegian Institute of Public Health, the University of Bergen, the University of New South Wales and the Black Dog Institute in Australia, and King's College London UK.

About the Hordaland Health Study

The Hordaland Health Study was conducted between 1997 and 1999 with people aged 40 to 46 living in Hordaland county in Norway. Both physical and mental health, and social and economic factors that affect health, were surveyed, and this information has been linked with public health records, such as sick leave and disability pension registries. The link between individual data from the survey and information in the official health records is relatively unique in an international context, and enables complete studies with long follow-up.