Reference intervals of work ability and productivity loss and their use in patients with inflammatory rheumatic and musculoskeletal diseases.

Objectives: To establish reference intervals (RIs) for work ability, at-work productivity loss and overall productivity loss in the general working population and to compare work ability and at-work productivity loss of patients with inflammatory rheumatic and musculoskeletal disease (iRMD) with this population.

Methods: Cross-sectional analysis among patients with iRMDs and population controls without iRMDs having paid work and participating in a Dutch cohort study. They reported on three work outcomes: work ability (0-10), at-work productivity loss and overall productivity loss (0%-100%). A generalised additive model for location, shape and scale parameters was used to establish age-specific RIs and percentile curves for controls. The proportion of patients and controls below each percentile curve was compared.

Results: 413 controls were included; 73% female, with mean age 53 (SD 10) years, 60% had high education, mean work ability was 8.7 (1.6), at-work productivity loss 6.3% (7.2) and overall work productivity loss 11% (25.6).Percentile curves illustrated that work ability and at-work/overall work productivity loss were worse with increasing age. For instance, for work ability, the 95% RI for 22 to 29-year individuals was 5.9-10, while for individuals between 50 and 59 years, it was 4.9-9.1. Patients compared with controls had worse work outcomes, especially for at-work and overall productivity loss.

Conclusion: Work ability and productivity are not perfect in the general population, based on the newly developed RIs for the three work outcomes. This calls for caution to not overestimate the iRMD impact on work outcomes. Nevertheless, iRMD patients have worse work ability and higher work productivity loss, compared with controls.