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. [ABSTRACT FROM AUTHOR]