Distribution of nephrologists and regional variation in the clinical severity of IgA nephropathy at biopsy diagnosis in Japan: a cross-sectional study

OBJECTIVES: The clinical severity of IgA nephropathy (IgAN) at the time of biopsy diagnosis differs significantly among cases. One possible determinant of any such difference is the time taken for referral from the primary care physician to a nephrologist, but the definitive cause remains unclear. This study examined the contribution of the number of nephrologists per regional population as a potential social factor influencing the clinical severity at diagnosis among patients with IgAN in Japan, which has an ethnically homogeneous population. DESIGN: A cross-sectional study. SETTING AND PARTICIPANTS: Patients were registered in the Japan Renal Biopsy Registry (J-RBR), a nationwide multicentre registry, and 6426 patients diagnosed with IgAN were analysed. The facilities registered to the J-RBR were divided into 10 regions and the clinical features of IgAN at biopsy diagnosis, including renal function and level of proteinuria, were examined. MAIN OUTCOME MEASURES: Renal prognosis risk at the time of biopsy diagnosis defined by Kidney Disease Improving Global Outcomes guideline 2012. RESULTS: Among the regions, there were significant differences in the estimated glomerular filtration rate (67.5–91.4 mL/min/1.73 m(2)), urinary protein excretion rate (0.93–1.93 g/day) and renal prognosis risk group distribution at diagnosis. The severity of all clinical parameters was inversely correlated with the number of nephrologists per regional population, which showed an up to 2.7-fold difference among regions. A generalised linear mixed model revealed that a low number of nephrologists per regional population were significantly associated with fulfilment of clinical criteria indicating a very-high-risk renal prognosis (β=−0.484, 95% CI −0.959 to −0.010). CONCLUSIONS: Among Japanese patients with IgAN, significant regional differences were detected in clinical severity at the time of diagnosis. Social factors, such as an uneven distribution of nephrologists across regions, may influence the timing of biopsy and determine such differences.