Within-individual analysis of pain and sickness absence among employees from low and high occupational classes: a record linkage study

Objectives Pain is linked to an increased risk of sickness absence (SA); however, the extent to which unmeasured time-invariant differences explain this association is yet unknown. Therefore, we determined the within-individual associations between pain and short-term (in the survey year) and long-term (2 years following the survey years) SA risk in high and low occupational classes while controlling for the potential bias due to unobservable time-invariant characteristics. Methods The Helsinki Health Study data consisting of midlife public sector employees with mailed surveys from up to four time points, and SA record linkage were used (3983 persons). The within-individual estimates were calculated using hybrid negative binomial regression models. Results Acute/subacute pain was associated with a 13% increase in the rate of short-term SA days (incidence rate ratio 1.13 [95% CI 1.01 to 1.27]), while the association was somewhat stronger for chronic pain (1.32 [1.19–1.47]). For the employees in the low occupational class, these associations were robust (1.29 [1.10–1.50] for acute/subacute and 1.43 [1.23–1.66] for chronic pain), whereas only chronic pain was associated with SA among those in the high occupational class (1.25 [1.08–1.46]). Chronic pain was also associated with SA days in the long term without occupational class differences. Similar results were obtained for multisite pain (pain in several locations). Conclusions These results indicate that particularly chronic and multisite pain have a within-individual link to SA but ignoring unobservable differences between those reporting pain and those not might yield overstated effect sizes. Pain might have a different relation to SA in low and high occupational classes.