Panel Study of Workers’ Compensation Insurance (PSWCI) data from 2018 to 2021 by the Korea Workers’ Compensation & Welfare Service (KCOMWEL) were retrospectively analyzed. The PSWCI is a nationwide annual panel study of industrial injuries that collects data on industrial injuries insurance coverage and socioeconomic characteristics after the termination of medical treatment. The PSWCI is administered by tablet-assisted personal interviews (TAPI) with a visitor interview. In total, 81,252 IIW terminated medical treatment between January and December 2017. In the PSWCI, 3,294 participants among 81,252 IIW were included through proportional stratified random sampling based on disability grade (six categories), sex (two categories), age group (four categories: age below 30s, 40s, 50s, and above 60s), and systematic sampling based on area of residence (six categories) and rehabilitation service usage (two categories). In 2018, the experience of on-site first aid, disability level, and the duration of medical treatment for industrial injuries were evaluated. The number of outpatient visits and hospitalizations was assessed annually from 2018 to 2021. At the initial stage of analysis, 202 participants were excluded because they did not answer several questions regarding the variables. As a result, 3,092 participants (2,562 males and 530 females) were analyzed and contributed 11,167 observations during the 4-year study period. As of 2021, 2,797 participants remained in the PSWCI (retention rate: 84.9%).

The participants were asked whether they experienced on-site first aid by co-workers at the time of the industrial injuries and the response items were “yes” or “no.” According to the Occupational Safety and Health Act, business owners are obliged to prevent industrial injuries and provide employees with information on safety and health at relevant places of business, including tips for first aid [1]. Therefore, the KOSHA provides professional tips for first aid according to the type of industrial injuries [12]. For example, “Do not move the patient recklessly, and keep him/her in a comfortable position” is recommended as a general tip for fractured patients. In this study, industrial injuries involved accidents on duty, but not occupational diseases or commuting accidents.

The participants were asked, “What is your history of visiting medical institutions in the past year?” The evaluation period for healthcare utilization extended from one year before the survey date to the survey date. Regardless of the reason for healthcare utilization, the number of outpatient visits, hospitalizations, and duration of hospitalization were evaluated. Pharmacies and nursing homes were excluded from the utilization count, and health checkups were excluded from the number of outpatient visits. Circuit outpatient visits were also excluded. For example, receiving treatment in more than two different departments at the same medical institution was only considered as one visit. If participants were admitted to the hospital for 365 days, and if they were admitted or discharged on the day of the emergency room visit, they were marked as hospitalized once. The total number of outpatient visits and hospitalization were counted because of study data limitations. According to Article 77 of the Industrial Accident Compensation Insurance Act in Korea, there is an additional medical care service provided for complications arising from occupational injuries [13]. However, only 43.14% of survey participants were aware of the existence of this service, as the beneficiaries are required to apply for it themselves. Furthermore, not all types and severities of complications are covered by this service (For example, the service exclusively covers facial nerve injuries that are classified as more severe than grade [12]. There is a specific list of 14 covered complications, and prior approval of medical necessity is required [14]. Therefore, to assess prognosis of industrial injuries after medical treatment, we considered the total outpatient visit and hospitalization count.

The participants were asked, “How long did it take for you to be transferred to the medical institution?” The transfer time to the medical institution was classified into four groups according to the PSWCI: less than 0.5 h, 0.5 ~ 1 h, 1–2 h, and more than 2 h.

Several time-varying socioeconomic and health-related characteristics were adjusted. Patients were classified as either male or female. The age group was divided according to the PSWCI: age below 30s, 40s, 50s, and above 60s. Education level was divided into university or higher, and high school graduation or lower. Current economic activity was divided into employed, unemployed, and economically inactive. Current household income was divided according to the quintile of household income. Household income is the sum of earned, financial, real estate, and other income. Statistics Korea data for the entire population were used as the income quintile cutoff for each year [15]. Area of residence was divided into metropolitan and provincial (rural). Medical histories of chronic diseases such as cancer, hypertension, and diabetes before industrial injuries were divided into without chronic disease and with chronic disease. The types of injuries due to industrial injuries were divided into fracture, sprain, back pain/musculoskeletal disease, amputation, cuts, bruising/concussion, rupture/laceration, burns, and other (abrasions, stab wounds, frostbite, contagion/addiction, and internal organ damage). Disability grade was based on 14 levels of physical/mental aftereffects of industrial injuries designated by law after the termination of primary medical care and symptom fixation [1]. The higher the disability grade, the more severe the disability level. In the analysis, disability grade was divided into the following groups as given by the PSWCI: grades 1–3 (most severe), 4–7, 8–9, 10–12, 13–14, and no disability. The period of primary medical care, which refers to the initial acute management at the designated hospital for IIW, was divided into the following as given by the PSWCI: < 3 months, 3–12 months, and > 1 year. Variables including sex, age, area of residence, type of injury, disability grade, and period of medical care were administrative data from the KCOMWEL, while variables including education level, current economic activity, current household income, and past medical history before accidents were self-reported.

A generalized estimating equation (GEE) Poisson regression was applied for the analysis. Poisson regression with a log link function and an unstructured (UN) working correlation matrix, which had the lowest Quasi-likelihood under Independence Model Criterion (QIC) statistics, was used for the longitudinal data (from 2018 to 2021). The results are presented as adjusted relative risk (aRR) with 95% confidence intervals (CI). Subgroup analyses were conducted to determine the detailed effects based on transfer time to the medical institutions and the covariates. As the variance inflation factors (VIF) for all variables were less than 1.6, there was no evidence of multicollinearity. Version 9.4 SAS software (SAS Institute, Cary, North Carolina, USA) was used. Statistical significance was set at P ≤ 0.05.