• Ivan A Steenstra, Dirk L Knol, Paulien M Bongers, Johannes R Anema, Willem van Mechelen, and Henrica C W de Vet.
• Institute for Work and Health, Toronto, Canada.
• Spine. 2009 May 20;34(12):1243-9.

Study DesignExploratory subgroup analysis in a randomized controlled trial (RCT).ObjectiveTo detect possible moderators in the effectiveness of a workplace intervention in a population of workers with sick leave due to sub acute nonspecific low back pain.Summary Of Background DataIn a recently published RCT, a workplace intervention was effective on return to work, compared to usual care. Examining the heterogeneity of effect sizes within the population in this RCT (n = 196) can lead to information on the effectiveness of the intervention in subgroups of patients.MethodsA subgroup analysis was performed by adding interaction terms to the statistical model. Before analysis the following possible moderators for treatment were identified: age, gender, pain, functional status, heavy work, and sick leave in the previous 12 months. Cox regression analyses were performed and survival curves were plotted.ResultsThe interaction (P = 0.02) between age (dichotomized at the median value) and the workplace intervention indicates a modifying effect. The workplace intervention is more effective for workers > or =44 years (HR, 95% CI = 2.5, [1.6, 4.1] vs. 1.2 [0.8, 1.8] for workers <44 years old). The interaction between sick leave in the previous 12 months and the workplace intervention is significant (P = 0.02). The intervention is more effective for workers with previous sick leave (HR, 95% CI = 2.8 [1.7, 4.9] vs. 1.3 [0.8, 2.0]). A modifying effect of gender, heavy work, and pain score and functional status on the effectiveness of this intervention was not found.ConclusionThe findings from these exploratory analyses should be tested in future RCTs. This workplace intervention seems very suitable for return to work of older workers and workers with previous sick leave. Gender, perceived heavy work, and baseline scores in pain and functional status should not be a basis for assignment to this intervention.

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