The prevalence of IgA nephropathy in Japan: results from a nationwide survey.

BACKGROUND: Precise epidemiological statistics regarding IgA nephropathy (IgAN) are essential for healthcare resource allocation and optimal therapeutic management. However, a nationwide epidemiological study has not been conducted in Japan over the past twenty years. METHODS: We conducted a mail-based survey targeting departments at hospitals to estimate the annual numbers of patients with IgAN in 2022. Departments were selected using stratified random sampling based on specialty and hospital size, and a total of 2148 departments (nephrology, pediatrics, and urology) were included in the analysis. The overall and sex-specific annual numbers of patients with IgAN were estimated, along with the number of patients with high disease activity. The corresponding prevalence was calculated using these estimates, divided by the midyear population of Japan in 2022. RESULTS: A total of 702 departments responded to the survey representing an overall response rate of 32.7%. The estimated numbers of patients with IgAN were 103,800 (95% confidence interval [CI] 83,431-124,170), including 50,160 men (95% CI 40,034-60,287) and 53,640 women (95% CI 42,799-64,481). The annual prevalence of IgAN was 83.1 per 100,000 population (men: 82.6 women: 83.6), with a male-to-female ratio of 0.99. The estimated numbers of IgAN patients with high disease activity were 15,646 (95% CI 12,464-18,828). CONCLUSIONS: This nationwide survey provides updated estimates of the prevalence of IgAN and the number of the IgAN patients with high disease activity in Japan. The prevalence of IgAN has risen substantially over the past twenty years.