Supplementary Material for: Association between Rheumatoid Arthritis and the Incidence of IgA Nephropathy

Introduction: IgA nephropathy and rheumatoid arthritis are both common autoimmune diseases, but epidemiological data are limited on the relationship between these two diseases. We aimed to assess the association between rheumatoid arthritis and the risk of developing IgA nephropathy.
Methods: In this study, we analyzed 4,311,393 adults using a nationwide epidemiological database in Japan. The definitions of rheumatoid arthritis and IgA nephropathy were based on ICD-10 codes. After excluding individuals with a prior history of IgA nephropathy, study participants were categorized into two groups according to the presence of rheumatoid arthritis. The primary outcome was the incidence of IgA nephropathy between January 2005 and May 2022.
Results: Median (interquartile range) age was 44 (36–53) years, and 2,497,313 (58%) were men. Rheumatoid arthritis was observed in 41,828 individuals (1.0%). During a median follow-up of 1,089 (532–1,797) days, there were 2,610 and 43 incidences of IgA nephropathy in individuals without and with rheumatoid arthritis, yielding incidence rates with 95% confidence intervals of 1.74 (1.67–1.81) and 2.99 (2.22–4.04) per 10,000 person-years, respectively, indicating a higher cumulative incidence in individuals with rheumatoid arthritis (log-rank p= 0.0002). Multivariable Cox regression analysis demonstrated that comorbid rheumatoid arthritis had a higher risk for developing IgA nephropathy (Hazard ratio 1.50, 95% confidence interval 1.10–2.02). Results were consistent even when IgA nephropathy was defined as both the ICD-10 code with the confirmation of performance of kidney biopsy (Hazard ratio 1.70, 95% confidence interval 1.02–2.83).
Conclusion: Our analysis utilizing a largescale population-based cohort concluded that rheumatoid arthritis may increase risk of developing IgA nephropathy.