Association of Inflammatory Bowel Disease with Incident IgA Nephropathy

Key Points We analyzed a nationwide epidemiologic cohort including approximately 4,000,000 individuals. We found a potential association of inflammatory bowel disease with a greater risk of developing IgA nephropathy. Background There have been scarce epidemiologic data on the relationship between inflammatory bowel disease and the incidence of IgA nephropathy. In this study, we assessed whether inflammatory bowel disease was associated with a higher risk of developing IgA nephropathy using a large-scale epidemiologic cohort. Methods We retrospectively analyzed 4,311,393 adults enrolled in the JMDC Claims Database (previously known as the Japan Medical Data Center database), a nationwide epidemiologic database in Japan. The definitions of IgA nephropathy and inflammatory bowel disease (ulcerative colitis or Crohn disease) were based on International Classification of Diseases, 10th Revision codes. Individuals who had a history of IgA nephropathy were excluded. Study participants were categorized into two groups according to the presence of inflammatory bowel disease. Clinical outcomes were collected between January 2005 and May 2022. The primary outcome was incident IgA nephropathy. Results Median (interquartile range) age was 44 (36–53) years, and 2,497,313 (58%) were men. Inflammatory bowel disease was observed in 18,623 individuals (0.4%). Over a median follow-up of 1089 (532–1797) days, there were 2631 incidences of IgA nephropathy and 22 incidences in individuals without and with inflammatory bowel disease, yielding incident ratios with 95% confidence intervals of 1.74 (1.68–1.81) and 3.43 (2.26–5.21), respectively. Kaplan–Meier curves and the log-rank test showed that a cumulative incidence of IgA was higher in individuals with inflammatory bowel disease compared with those without (log-rank P = 0.0028). Multivariable Cox regression analysis demonstrated that individuals with inflammatory bowel disease were at higher risk of incident IgA nephropathy (hazard ratio, 1.96; 95% confidence interval, 1.29 to 2.99). Conclusions We demonstrated the potential association of inflammatory bowel disease with higher risk of developing IgA nephropathy in a general population.