Characteristics of glomerulonephritis with dominant C1q precipitation compared to corresponding glomerulonephritis without C1q staining on immunofluorescent examination

Background: The clinical significance and renal outcomes of C1q nephropathy (C1qN) are unclear; therefore, the implications of C1qN as a new pathological entity are uncertain. We compared the clinical characteristics of glomerulonephritis reclassified into cases that meet the definition of C1qN and glomerulonephritis not included in the definition of C1qN. Methods: In total, 21,697 patients who underwent native kidney biopsy at 18 hospitals throughout Korea between 1979 and 2018 were retrospectively enrolled. A total of 77 patients were selected from the group that met the definition of C1qN after reclassification; however, six patients were excluded because of secondary systemic disease. Characteristics of pathological findings classified into C1qN, without C1q stain, and with nondominant C1q stain that did not fulfill the criteria for C1qN were compared. Results: No differences in clinicopathological findings and incidence of ESRD were evident (matched by age and sex) between glo-merulonephritis cases reclassified into the group that met the definition of C1qN and those without C1q staining. Decreased protein-uria in patients with membranous nephropathies reclassified into the group that met the definition of C1qN was the only significant finding. Immunoglobulins showed higher intensity on immunofluorescence staining of the group that met the definition of C1qN. Additionally, C3 intensity was higher in reclassified immunoglobulin A nephropathy and membranous nephropathies. Conclusion: Overall, reclassification into the group that met the definition of C1qN did not indicate a different clinicopathological iden-tity. C1q activation and presumed classical complement pathway activation in kidney tissues in C1qN could not be confirmed. Hence, further studies are needed.