Estimating time to reinjury among Washington State injured workers by degree of permanent impairment: using state wage data to adjust for time at risk

Background: Many injured workers are reinjured, but reinjury risk is challenging to quantify. Because many injured workers face delayed return-to-work, or return to part-time or intermittent jobs, a calendar timescale may overestimate actual work-time at risk, yielding underestimated reinjury rates. Objectives included determining: (1) reinjury risk by degree of permanent impairment and other factors, and (2) how choice of timescale affects reinjury estimates. Methods: This retrospective cohort study included Washington State workers' compensation (WC) claims for 43,114 injured workers, linked to state wage files (2003-2018). Three timescales were used to define at-risk denominators: (1) calendar quarters; (2) quarters with any wages; and (3) full-time equivalent (FTE) quarters, defined as cumulative work hours ÷ 520. Associations between reinjury outcomes and worker, injury, job, and WC vocational rehabilitation program participation characteristics were assessed using Cox proportional hazards regression. Results: Overall reinjury rates were 5.9 per 100 worker-years using a calendar timescale (95% confidence interval [CI]: 5.8-6.0), 10.0 using any-wage quarters (95% CI: 9.9-10.2), and 12.5 using FTE quarters (95% CI: 12.3-12.7). Reinjury rates were highest in the first two quarters after initial injury, remaining elevated for about 4 years. Using FTE quarters, workers with =10% whole body impairment had a 34% higher risk of reinjury relative to workers with no permanent partial disability award (95% CI: 1.25-1.44); no difference was detected using calendar time. Conclusions: Timescale substantially affects reinjury estimates and comparisons between groups with differential return-to-work patterns. Linking wage data to WC claims facilitates measurement of long-term employment, yielding more accurate reinjury estimates.