Sickness absence among patients with chronic pain in Swedish specialist healthcare

Sammanfattning: Background: Chronic pain beyond three months is a global public health problem. Every third adult suffers from a chronic pain condition, resulting in a socioeconomic burden that corresponds to 3-10% of gross domestic product in western economies. This burden can be largely attributed to absenteeism-related productivity loss where a few highly impaired individuals are the most resource-intensive. Simultaneously, a detailed overview of sickness absence (SA) associated with chronic pain is complicated by incongruent classification due to conflicting perspectives on the condition as either a symptom or a disease in its own right. Aim: Based on a well-defined chronic pain population in the Swedish specialist healthcare, this thesis primarily aims to provide a SA overview, to explore the possibility of SA prevention, and to evaluate interdisciplinary treatment (IDT) as a SA intervention. A secondary objective was to assess the psychometric properties of three questionnaires that measure the core domains of the chronic pain experience. Methods: The aims were addressed in three register-based studies using microdata from five Swedish national registers. Study I used sequence analysis to describe SA in 44,241 patients over a 7-year period and subsequently developed a machine learning-based model to predict chronic pain-related SA in the final two years. Study II emulated a target trial to compare the total SA duration over a 5-year period for 25,613 patients that were either included in an IDT program or in other/no interventions. Study III analyzed the properties of the Short Form-36 Health Survey (SF-36), the EuroQol 5-Dimensions instrument (EQ-5D), and the Hospital Anxiety and Depression Scale (HADS) within the item response theory-framework. Results: SA increased from 17% to 48% over the five years before specialist healthcare entry to then decrease to 38% over the final two years. With information on eight predictors, it was possible to discriminate between patients that would have low or high SA in the coming two years with 80% accuracy. SA trends were similar for patients in IDT programs and other/no interventions, albeit the IDT patients had 67 (95% CI: 48, 87) more SA days over the complete 5-year period. Finally, the psychometric evaluation revealed that SF-36 adequately captured physical and mental health, while HADS was suitable as a measure of overall emotional distress, and EQ-5D had insufficient precision for any meaningful application. Conclusion: Our findings are most useful to guide policy and research. SA in the studied patients remained high over the entire observation period. Decision support tools could prove valuable in identifying patients at risk of high SA earlier in the healthcare chain in order to direct preventative measures. We found no support for IDT decreasing SA more than other/no interventions, but it is possible that this was a consequence of our methodology. Further studies of the IDT effects are needed, but uncontrolled designs that attribute SA change over time to IDT are inappropriate for this purpose, as the SA peak observed around specialist healthcare entry is likely to be driven by the referral procedure. Finally, SF-36 and HADS are psychometrically sound measures of the chronic pain experience core domains.