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Lay Summary The Flemish Collaborative Glomerulonephritis Group (FCGG) registry collects information on patients that undergo kidney biopsy in the region of Flanders in Belgium. The registry summarizes the underlying diagnoses in patients that present with symptoms of kidney disease (e.g. blood and/or protein in the urine or decreased kidney function). Additionally, the registry also collects information on the degree of chronic damage on kidney biopsy. This is important because chronic damage may lead to kidney failure. From 2017 until 2019, a total of 2054 adult biopsies were analyzed, while chronic damage could be analyzed in 898 biopsies. We found that the underlying causes of severe kidney disease were similar to studies performed in other European countries. Importantly, we found that increasing age, reduced kidney function and certain diagnoses are associated with more chronic damage on kidney biopsy. This information may be useful to doctors in clinical practice, in both Belgium and Europe. Background The Flemish Collaborative Glomerulonephritis Group (FCGG) registry provides complete population data on kidney disease epidemiology in the region of Flanders (Belgium), as it captures all native kidney biopsies performed in its population of 6.5 million inhabitants. Methods From 2017 until 2019, 2054 adult kidney biopsies were included from 26 nephrology centers (one biopsy per patient). Data on nephrotic and nephritic syndrome were available in 1992 and 2026 biopsies, respectively. In a subgroup of 898 biopsies containing >= 10 glomeruli from 2018 to 2019, disease chronicity was graded using the Mayo Clinic Chronicity Score (MCCS). The association between clinical variables and MCCS was determined using simple and multiple linear regression models. Results Nephrotic syndrome (present in 378 patients, 19.0%) was most frequently caused by minimal change disease in younger patients (18-44 years), membranous nephropathy in older patients (45-74 years) and amyloidosis in the elderly (>75 years). Nephritic syndrome (present in 421 patients, 20.8%) was most frequently caused by immunoglobulin A nephropathy (IgAN) in younger patients (18-64 years) and ANCA-associated vasculitis (AAV) in older patients (>64 years). AAV and IgAN were the most frequent underlying diagnoses in biopsies in which crescents were identified. In multivariable analysis, acute and chronic kidney disease and diagnoses of diabetic kidney disease, nephrosclerosis and hyperoxaluria/hypercalcemic nephropathy were associated with the highest MCCS increases. Conclusions The FCGG registry validates data from previous Western European registries and provides a snapshot of disease chronicity in the whole biopsied Flemish population. D.D. is supported by a PhD Fellowship grant fundamental research from the Research Foundation Flanders (F.W.O., grant number 11L5622N). B.S. is a senior clinical investigator of The Research Foundation Flanders (F.W.O., grant number 1 842 919 N). The FCGG registry is funded by the Nederlandstalige Belgische Vereniging voor Nefrologie (NBVN). The authors wish to thank all collaborating nephrologists in Flanders and Brussels and responsible persons at the data entry centers (Elsie De Man, Sabine Verhofstede, Ben Sprangers) for their participation in the FCGG registry. The FCGG registry was initiated in collaboration with the Nederlandstalige Belgische Vereniging voor Nefrologie (NBVN), the organization that represents the majority of nephrologists in the region of Flanders. The study was approved by the Ethical Committee of the University Hospitals Leuven (study reference S59182) and local committees of all participating centers.

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