Japan Diabetic Nephropathy Cohort Study: study design, methods, and implementation Kengo FuruichiMiho ShimizuTadashi ToyamaDaisuke KoyaYoshitaka KoshinoHideharu Abe • Kiyoshi MoriHiroaki SatohMasahito ImanishiMasayuki IwanoHiroyuki YamauchiEiji Kusano • Shouichi FujimotoYoshiki SuzukiSeiya OkudaKiyoki KitagawaYasunori IwataShuichi Kaneko • Shinichi NishiHitoshi YokoyamaYoshihiko UedaMasakazu HanedaHirofumi MakinoTakashi Wada • Research Group of Diabetic Nephropathy, Ministry of Health, Labour, and Welfare of Japan

Background Diabetic nephropathy, leading to end-stage renal disease, has a considerable impact on public health and the social economy. However, there are few national registries of diabetic nephropathy in Japan. The aims of this prospective cohort study are to obtain clinical data and urine samples for revising the clinical staging of diabetic nephropathy, and developing new diagnostic markers for early diabetic nephropathy. Methods The Japanese Society of Nephrology established a nationwide, web-based, and prospective registry system. On the system, there are two basic registries; the Japan Renal Biopsy Registry (JRBR), and the Japan Kidney Disease Registry (JKDR). In addition to the two basic registries, we established a new prospective registry to the system; the Japan Diabetic Nephropathy Cohort Study (JDNCS), which collected physical and laboratory data. Results We analyzed the data of 321 participants (106 female, 215 male; average age 65 years) in the JDNCS. Systolic and diastolic blood pressure was 130.1 and 72.3 mmHg, respectively. Median estimated glomerular filtration rate (eGFR) was 33.3 ml/min/1.73 m 2 . Proteinuria was 1.8 g/gCr, and serum levels of albumin were 3.6 g/dl. The majority of the JDNCS patients presented with preserved eGFR and low albuminuria or low eGFR and advanced proteinuria. In the JRBR and JKDR registries, 484