Income Level and Risks of Kidney Function Decline and Mortality Among Older Korean Adults: A Nationwide Cohort Study

Technical Summary: Income Level and Risks of Kidney Function Decline and Mortality Among Older Korean Adults

Problem Statement
Chronic kidney disease (CKD) imposes a significant burden on older adults, with prevalence exceeding 40% in those aged ≥70 years. While socioeconomic status (SES) is recognized as a determinant of CKD, evidence regarding income-related disparities in kidney outcomes specifically within older Asian populations remains limited. Furthermore, existing studies often focus on general populations or incident CKD, lacking analysis on clinically meaningful kidney function decline and mortality in the context of universal health coverage. This study addresses the gap in understanding how income levels influence kidney function decline and all-cause mortality among older Korean adults, accounting for the competing risk of death.

Methodology

• Study Design and Population: A retrospective nationwide cohort study utilizing the Korean National Health Insurance Service–Senior Cohort (NHIS-Senior). The study included 625,448 adults aged ≥60 years who underwent national health screening between 2012 and 2021.
• Exposure: Income level was categorized into five groups based on health insurance premium deciles: Lowest (0–2), Lower-middle (3–4), Middle (5–6), Upper-middle (7–8), and Highest (9–10).
• Outcomes:
  • Primary Kidney Outcome: A composite of a ≥40% decline in estimated glomerular filtration rate (eGFR) or incident end-stage kidney disease (ESKD).
  • Secondary Kidney Outcome: A composite of a ≥30% decline in eGFR or incident ESKD.
  • Mortality Outcome: All-cause mortality.
  • ESKD Definition: Initiation of maintenance hemodialysis, peritoneal dialysis, or kidney transplantation, verified via Rare and Intractable Disease (RID) codes and procedure codes.
• Statistical Analysis:
  • Fine–Gray subdistribution hazard models were employed to analyze kidney outcomes, treating death as a competing risk.
  • Cox proportional hazards models were used for all-cause mortality.
  • Models were adjusted for age, sex, insurance type, geographic region, baseline eGFR, dipstick proteinuria, hypertension, and the Charlson Comorbidity Index (CCI).
  • Sensitivity analyses were conducted using sustained eGFR decline (confirmed on two consecutive examinations).
  • Subgroup analyses examined effect modification by age, sex, CCI score, index year, and baseline eGFR.

Key Results

• Primary Kidney Outcome: Compared to the lowest-income group, the highest-income group demonstrated a significantly lower risk of the primary composite outcome (adjusted Hazard Ratio [HR] 0.83; 95% CI 0.80–0.86). A graded inverse association was observed across income groups.
• Secondary Kidney Outcome: Similar associations were found for the ≥30% eGFR decline or ESKD outcome, with the highest-income group showing an adjusted HR of 0.84 (95% CI 0.83–0.86) compared to the lowest.
• Mortality: Higher income was associated with a lower risk of all-cause mortality. The adjusted HR for death in the highest-income group was 0.78 (95% CI 0.75–0.80) relative to the lowest-income group.
• Subgroup Findings: The protective association of higher income against kidney function decline was consistent regardless of baseline kidney function (eGFR ≥60 vs. <60 mL/min/1.73 m²). However, the association was stronger in specific subgroups: participants aged <70 years, men, and those enrolled in later index years.
• Sensitivity Analysis: Results remained consistent when requiring sustained eGFR decline confirmed on two consecutive examinations.

Key Contributions

1. Population Specificity: The study provides robust evidence from a large, nationwide cohort of over 600,000 older Korean adults, addressing the scarcity of data on income-related kidney disparities in Asian elderly populations.
2. Methodological Rigor: By utilizing Fine–Gray competing-risk models, the study appropriately accounts for the high mortality rate in older adults, which often precedes CKD progression, thereby providing a more accurate estimation of kidney risk than standard Cox models.
3. Clinical Relevance: The use of clinically meaningful endpoints (≥40% and ≥30% eGFR decline or ESKD) rather than just incident CKD diagnosis offers a more nuanced view of kidney health deterioration.
4. Universal Coverage Context: The findings demonstrate that income disparities in kidney health persist even within a universal health coverage system, highlighting that economic factors influence outcomes beyond mere access to healthcare.

Significance and Claims
The paper concludes that lower income is independently associated with higher risks of kidney function decline and all-cause mortality among older Korean adults, despite the presence of universal health coverage. The authors assert that economic disadvantage remains a critical determinant of kidney health in later life. Consequently, they suggest that income should be considered a factor in risk stratification and prevention strategies for older adults. The study highlights that socioeconomic vulnerability, potentially driven by factors such as delayed nephrology referral, health behaviors, and comorbidities, continues to impact kidney outcomes even when healthcare access is theoretically universal. The authors note that these findings support the need for targeted interventions and equitable resource allocation for low-income older adults.