Ⅰ. Introduction
Chronic kidney disease (CKD) is considered one of the significant public health challenges[1] because it is a severe condition that reduces life expectancy and typically progresses to end-stage renal disease (ESRD), requiring renal replacement therapy. The progressive nature of CKD, ensuing ESRD, and associated cardiovascular morbidity and mortality considerably burden global healthcare resources[1]. Moreover, disadvantaged populations worldwide exhibit a disproportionate burden of CKD because of differences in CKD occurrence and outcomes[2]. Although many CKD risk factors can be managed and modified to optimize clinical outcomes, socioeconomic and cultural factors in disadvantaged populations militate against optimal clinical results[2]. In addition, underprivileged people exhibit a broader spectrum of CKD risk factors and may be genetically predisposed to an earlier onset and more rapid progression of chronic kidney disease[2].
Socioeconomic health disparities are well-recognized for many diseases[3]. CKD is fascinating to study health inequalities, as health outcomes have a marked social gradient[4]. Social disadvantages, including low socioeconomic status[5] and low education[6], are strongly associated with high rates of CKD. Moreover, individuals with a low socioeconomic status have limited access to healthcare resources[7], higher progression of CKD[8], higher incidence of cardiovascular events[9], and poorer quality of life[10] than individuals with a high socioeconomic status. There is also evidence that individuals with CKD and low socioeconomic status have limited access to quality treatments[11].
Interestingly, most inequality studies on survival after CKD treatment have been performed in Western societies[12][13], despite the high burden of this disease and its contribution to mortality in South Korea. It is not known whether a social gradient exists in health outcomes in South Korea. The implementation of universal healthcare systems in South Korea has resulted in a collapse of barriers to healthcare use, improved access to medical care, high-quality care, and low mortality[14]; however, debates on disparities between different socioeconomic status groups in health outcomes remain[15]. In other words, people with low socioeconomic status have higher mortality rates from various diseases than those with high socioeconomic status. Therefore, this study examined the association between income level and mortality in patients with CKD, using data from the Korean National Health Insurance claims database(KNHICD). Our rationale for investigating survival in patients with CKD was to identify socioeconomic inequity that could be addressed through targeted policies and priority health services for low-income patients.
Ⅱ. Methods
1. Data and Participants
This study used data from the KNHICD, which included information on approximately 1 million patients. Stratified random sample data had age, sex, region, health insurance type, income 10th quintiles, and individual total medical costs based on 2002 and follow-up to 2010. The National Health Security System is a mandatory social insurance program in Korea, and all citizens must join the system. The system involves National Health Insurance (NHI) and Medical Aid programs and is overseen by the Ministry of Health and Welfare. Under this system, all medical care expenses under the NHI or Medical Aid program are reimbursed, and these data are stored and managed by the KNHICD. Therefore, the KNHICD contains all information on reimbursement under the fee-for-service system (NHI) or per diem system (Medical Aid). The cohort data included information such as the unique de-identified number for each patient, age, sex, type of insurance, list of diagnoses according to the International Classification of Diseases (ICD-10), medical costs claimed, prescribed drugs including peritoneal dialysis (PD) solutions, and surgical history, such as hemodialysis (HD), PD catheter insertion, or kidney transplantation (KT). In addition, the unique de-identified number was linked to information on mortality obtained from the Korean National Statistical Office (NSO).
We conducted a cohort study of newly diagnosed CKD (N18) patients to investigate the association between income levels and mortality rates. We included a 2.5% stratified random sample (n = 1,025,340) of KNHICD enrollees as of December 31, 2002. From this pool, we selected 3,994 patients with a primary diagnosis of CKD (N18) between 2002 and 2009. Of these, 822 patients diagnosed with CKD in 2002 were excluded because this was considered a wash-out period. After eliminating patients who submitted claims for CKD in 2002, we selected 3,172 free of CKD at the end of 2002.
2. Follow-up and end points
The outcome variable of this study was all-cause mortality. The duration of mortality follow-up was eight years, from January 2003 to December 2010. Patients were observed for at least 12 months from the date of CKD diagnosis through June 30, 2010, or until death. The time of the kidney transplantation was censored. This unique de-identified number was linked to information on mortality obtained from the Korean NSO. By law, all deaths in Korea must be reported to the NSO within a month of their occurrence.
3. Socioeconomic status
Individual income can be estimated from the NHI premium. The health insurance premium is proportion to monthly income, including earnings and property income. The patients with CKD were classified into the following three groups according to income: (1) low income group (Medical aid and below 30% of the income rank), (2) middle income group (30–70% of the income rank), and (3) high income group (above 70% of the income rank).
Covariates
The demographic and classical risk factors for CKD were included in this study. Demographic characteristics had sex, age (-39, 40-49, 50-59, 60-69, and ≥70 years), type of insurance (health insurance or medical aid), and region(urban or rural). The patient’s comorbidities were identified by reviewing their medical history during the last year before the CKD diagnosis. Diagnoses of hypertension, diabetes mellitus, cerebrovascular accident, congestive heart failure, ischemic heart disease, anemia, cancer, and glomerulonephritis were identified based on the previous literature [16].
4. Statistical analysis
We determined the distribution of the general characteristics of the patients with CKD at baseline. We performed a two-step analysis to investigate the association between socioeconomic status and all-cause mortality in patients with CKD. First, we investigated the association between socioeconomic status and mortality in all the patients with CKD. Second, we analyzed the association between socioeconomic status and all-cause mortality according to dialysis initiation at baseline. We performed Cox proportional hazards analysis to delineate the predictors of mortality. We set the level of significance at P <0.05. SAS 9.4(SAS Institute, Cary, NC) was used for the analysis.
Ⅲ. Result
<Table 1> presents the baseline sociodemographic characteristics of the study participants. Of the 3,172 participants, 843(26.2%) died during the follow-up period. Of the 843 deaths, 250(25.2%), 248(25.6%), and 345(28.4%) occurred in the low-, middle-, and high-income groups, respectively.
<Table 2> shows the Cox proportional hazards analysis results, which assessed the association between income levels and mortality rates. After adjusting for covariates, including risk factors, we found that low income was significantly associated with a high mortality rate (HR 1.315 [1.096–1.577], p = 0.003). CKD patients with diabetes mellitus, cerebrovascular disease, congestive heart failure, ischemic heart failure, ischemic heart disease, and cancer had a higher mortality rate than those without these diseases one year before the diagnosis of CKD.
<Figure 1> presents the results of the subgroup analysis of mortality according to the initiation of dialysis (both peritoneal dialysis and hemodialysis) within one year after the diagnosis of CKD. Patients in the low-income group who started dialysis had a higher mortality rate (HR 1.561 [1.123–2.170]; p = 0.008).
<Figure 1>
Adjusted hazard ratio for mortality by dialysis initiation at baseline (reference: low income)
Ⅳ. Discussion
This population-based study examined the association between income levels and mortality rates in patients with CKD using data from the KNHICD database. After adjusting for comorbidities, CKD patients with low income and those with low income initiating dialysis had significantly higher all-cause mortality rates. Our findings are consistent with other studies investigating the relationship between socioeconomic status and post-CKD survival in different populations[17]. Mortality was substantially higher in the low-SES group than in the high-SES group in Western countries, including the US, UK, and European countries[5][8][17], and similar results have been obtained in Asian countries[6][18]. The type of health insurance (i.e., medical aid) and comorbidities in Korean patients undergoing dialysis are independent predictors of mortality[19].
Studies from other countries have demonstrated that patients with CKD of low socioeconomic status may have high all-cause mortality rates after controlling for risk factors of CKD, such as diabetes mellitus, hypertension, cerebrovascular accident, and congestive heart failure[8][20]. However, a more comprehensive analysis is needed in South Korea, and nationally representative studies on this topic are limited, although the burden of CKD in Korea has gradually increased. The present study may be significant in South Korea, where debates on inequality in health outcomes continue despite universal health coverage[15]. The NHI program in Korea aims to eliminate primary health inequalities, which may be necessary for closing the medical accessibility gap between patients based on their socioeconomic status.
Over the last few decades, studies have suggested multiple reasons for socioeconomic inequality in health or mortality[3][21][22]: poor health behaviors, material deprivation, psychosocial attributes, early life exposure, biological risk factors, poor health status, and late disease recognition. Several plausible mechanisms may explain why patients with CKD with low socioeconomic status have high all-cause mortality rates. A Korean study suggested that individuals with low socioeconomic status have high morbidity rates, low health status, and negative behaviors such as smoking, drinking alcohol, and irregular exercise[23]. These findings indicate that individuals with low socioeconomic status are more susceptible to high CKD mortality rates. In addition, low awareness of CKD may contribute to high mortality rates in individuals with a low socioeconomic status. The understanding of early and advanced CKD is insufficient, even in industrialized nations, and has been reported to be <20%[24]. Some studies have suggested that low awareness of CKD or low health literacy is associated with high mortality and all-cause mortality rates attributable to CKD in individuals with low socioeconomic status[17].
Early recognition of CKD should be promoted to address mortality inequality. The World Kidney Day campaign positively impacted increasing awareness and managing the risk factors for CKD[25]. Moreover, adequate prevention strategies and treatments must be provided to low-income patients. Given that congestive heart failure, coronary artery disease, diabetes, and anemia are prevalent in dying patients[20], efforts to appropriately manage CKD and its risk factors should focus on reducing mortality in individuals of low socioeconomic status. Based on the results of the Korea National Health and Nutrition Examination Survey, as increased diabetes and improved diabetic control neutralize their impact on CKD, improved blood pressure was the fundamental reason for this decrease[26]. This study suggests that various health-related behaviors may have indirectly affected the reduction in CKD by controlling blood pressure and diabetes. Therefore, more screening and prevention programs for CKD are needed for individuals with low socioeconomic status. Establishing prevention and early detection programs for individuals with low socioeconomic status will help reduce inequality in mortality. Moreover, governments should encourage individuals with low socioeconomic status to participate actively in these programs. Furthermore, additional benefits can be introduced to relieve CKD patients’ financial.
The strengths of this study include its population-based design and data collection from the KNHICD, a nationally representative long-term follow-up. The study participants' follow-up was completed by linking each Korean resident’s unique personal identification number to the national mortality data. While a previous study measured socioeconomic status by health insurance type (medical benefit, health insurance), this study measured socioeconomic status as a more subdivided income level using insurance premiums[19]. These findings prove the association between income level and mortality among chronic kidney disease patients with CKD.
Despite these strengths, this study had several limitations. First, potential confounding factors for mortality, such as data on residual renal function, biomarkers of inflammation or nutrition, and dialysis doses, were unavailable. Second, the database did not include information on tobacco use, dietary habits, or other behavioral factors, which may be a risk and prognostic factors for patients with CKD. Third, this study uses individual income estimated from health insurance premiums instead of real income.
Ⅴ. Conclusion
The relationship between socioeconomic status and mortality rates in patients with CKD is well-established worldwide. These findings indicate that low-income CKD patients have high mortality rates in Korea. Promoting targeted policies and priority health services for low-income patients may help reduce the mortality rate in this vulnerable group.
[국문초록]
연구목적: 이 연구는 만성신장질환자의 사회경 제적 지위와 사망간의 연관성을 확인하기 위함이 다. 연구방법: 이 연구는 국민건강보험청구자료를 활용하여 만성신장질환자 3,172명을 대상으로 연구 를 수행하였다. 개인의 소득은 국민건강보험료를 이용하여 저소득층(소득수준의 30%미만), 중소득층 (30-70%), 고소득층(70%)이상으로 구분하였다. 만성 신장질환자의 소득수준과 사망의 연관성을 확인하 기 위하여 콕스비례위험모형을 이용하였다. 연구결 과: 소득수준과 사망간 가능한 교란요인을 보정한 후, 저소득층의 만성신장질환자는 고소득층의 만성 신장질환자 보다 1.314배 사망의 위험이 높았다 (HR 1.315[CI 1.096–1.577], p-value 0.003). 특히, 투석을 받는 저소득층 만성신장질환자들은 고소득 층 투석환자보다 사망의 위험이 1.561배 높았다 (HR 1.561[1.123~2.170], p-value 0.008). 결론: 이 연구는 저소득 만성신장질환자가 사망 위험이 가 장 높다는 것을 발견하였다. 저소득층 환자를 대상 으로 정책 및 의료 서비스가 우선이 된다면 취약 한 저소득층의 사망률을 줄이는 데 도움이 될 수 있을 것이라 제언한다.
