INTRODUCTION
Following the National Medical Insurance Act legislation in 1963, the current single insurer, the National Health Insurance Service (NHIS), originated from medical insurance societies for companies with more than 500 employees and insurance societies for public officials and private school employees in 1977. Gradually, it expanded to cover the smaller companies and universal coverage of the self-employed by medical insurance societies in rural and urban areas in 1989. Finally, it was integrated into a single insurer, the National Health Insurance Corporation (NHIC), in 2000 to achieve equity among insurance funds in relation to the financial burden and other managerial issues. In the same year, the separation of prescribing and dispensing drugs was also implemented. The NHIC then changed its name to the NHIS in 2013 [
1,
2]. In 2008, the longterm care insurance (LTCI) system was introduced to alleviate the financial burden on unpaid family caregivers, helping elderly Koreans with difficulties performing activities of daily living or housework due to geriatric diseases [
3].
In Korea, 97% of the population is obliged to enroll in the National Health Insurance (NHI) program. Patients pay approximately 5% to 30% of the total medical costs to clinics or hospitals, although some services are not covered by insurance, such as cosmetic surgery and some unproven therapies. Clinics and hospitals then submit claims to the Health Insurance Review & Assessment (HIRA) service for inpatient and outpatient care, including data on diagnoses as determined by the International Classification of Diseases, 10th revision (ICD-10), procedures, prescription records, demographic information, and direct medical costs to obtain reimbursement for the total medical costs (ranged from 70% to 95%). The remaining 3% of the population not insured by the NHI program are either covered by another medical aid (MA) program or are temporary or illegal residents [
4].
In this study, we reviewed the structure, content, and means of using data procured from the NHI system and HIRA service for the benefit of Korean researchers and presented the latest publication trends for Korean healthcare data procured from the NHI and HIRA databases.
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DISCUSSION
Big data studies on Korean public healthcare using the NHI and HIRA databases are becoming increasingly popular. This phenomenon could be attributed to the strengths of the compulsory nature of the NHI system providing universal coverage for the Korean population and consequently the serial population data that are readily available. In this study, we reviewed the structure, content, and means of using data procured from the Korean NHI system by Korean researchers, especially endocrinologists, and we present the latest publication trends on Korean healthcare.
Fig. 2 illustrates the operational structure of the NHI system. Three key regulators of the healthcare system include the Ministry of Health and Welfare (MOHW), NHIS, and HIRA. The MOHW supervises the NHI program through the formulation and implementation of policies. The NHIS is a nonprofit organization and the single insurer that manages the NHI program. It is responsible for managing enrolled and insured individuals and their dependents (spouse, direct lineal ascendants or descendants, and unmarried brothers or sisters) and collecting contributions and setting medical fee schedules. The HIRA has a quality control role, evaluates healthcare performance, and reviews medical billing and claims. The HIRA also determines whether health care services are medically necessary and ensures that the services are delivered to beneficiaries at an appropriate level and cost [
5]. The NHI, MA, and LTCI are the three main health care programs for universal coverage in Korea. The NHI program classified covers the whole population as either employee or self-employed as a social insurance benefits scheme with the following features: short-term insurance and compulsory contributions made according to ability to pay. The contribution to NHI is calculated based on an employee’s wage and is paid by the company’s employer. In the case of the self-employed, the cost is calculated based on the household income, property, income, vehicle(s) owned, age, and gender. The MA program is managed by the Korean government. It is a public assistance scheme that secures the minimum livelihood of low-income households and assists with self-help by providing medical services. The LTCI program is based on the principle of social solidarity. The program provides benefits for at least 6 months. Although NHI covers the whole population as a compulsory scheme, not all items of healthcare are covered by the program. The aim is to cover the prevention and treatment of sickness and injury that result from daily life and childbirth. The program also covers health promotion and rehabilitation. The NHI provides the same benefits package regardless of the contributions made that are determined by the individual’s ability to pay.
 | Fig. 2.How the National Health Insurance system works. NHIS, National Health Insurance Service; HIRA, Health Insurance Review & Assessment. 
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NHI and HIRA databases were used in various research areas including healthcare and public health policies, medical adherence, prescribing patterns, adverse events, cost-effectiveness, burden of disease, healthcare service utilization, disease incidence and prevalence, and outcomes. As availability and accessibility of NHI and HIRA data have grown in recent years, the data provision review committee has granted use to over 3,000 studies from 2014 to 2019 [
9]. In this study, 83.1% of endocrinology-related big data studies used the NHI database. Despite the similarity of the two databases, their content is different, where the NHI database’s main sections include healthcare use, sociodemographic variables, health screening, and mortality, whereas the HIRA research database’s main sections include general demographic characteristics, healthcare use, diagnoses, and outpatient prescriptions [
10]. Additionally, NHI sample databases include longitudinal cohorts, enabling researchers to perform long-term follow-up studies, whereas the HIRA sample databases include separate cohorts for each year, only suitable for conducting a cross-sectional study or short-term follow-up (less than 1 year) studies. This is because patients in the HIRA sample databases are stratified and re-sampled annually, and patient information cannot be linked across years within the HIRA sample database [
11].
Concerning the strengths and limitations of the NHI and HIRA databases, the NHI database covers detailed information regarding medical examinations, therapeutics including prescription and long-term follow-up of the insured. This allows the researchers to perform longitudinal studies whereas the HIRA database cannot follow individuals. As we described earlier, HIRA data covers healthcare service records from neonates to the elderly in a full range of healthcare settings. This representative and comprehensive dataset broadens Korean research to fields that might not be easily measured using randomized controlled trials by providing demographic information. Using longitudinal data collected from NHI and MA beneficiaries, researchers can conduct cohort studies and investigate long-term outcomes. Furthermore, healthcare agencies provide information to the HIRA, which ensures the reliability of the data, and the vast sample size secures statistical power for complicated analyses [
8]. However, as in the HIRA database, the discrepancy between real-world diagnosis of the insured and data collected from the NHI database, the validity of the research might be compromised. Therefore, the researchers should properly extract samples for the studies by using appropriate definitions and carefully designed inclusion criteria to avoid misleading results. Despite the strengths of the HIRA database, it lacks information such as the severity of diseases and personal health history such as smoking or alcohol consumption. This personal information might interfere with the main outcomes of the research [
10]. Also, uncovered healthcare services such as cosmetic surgeries are not included, and discrepancies between diagnosis information and actual health conditions may impose bias to compromise a study’s validity [
10]. Nonetheless, the benefits overcome the limitations of the NHI and HIRA databases, and these limitations could be further addressed by strategies for the optimal use of the NIH and HIRA databases.
In conclusion, the NHI and HIRA databases represent the entire Korean population and can be used as a population database. The NHIS and HIRA databases have been important resources for endocrinology research and have provided guidelines for Korean endocrinologists to conduct world-leading population-based epidemiology and disease research.
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