Type III Collagen Glomerulopathy Presenting With Divergent Histopathological Patterns: Report of Two Adult Cases and Diagnostic Pitfalls.

Type III collagen glomerulopathy (collagenofibrotic glomerulopathy) is a rare idiopathic renal disorder. It is characterized by abnormal deposition of Type III collagen fibrils within the mesangial and subendothelial compartments of glomeruli. Due to its nonspecific clinical presentation and histopathologic overlap with more common glomerular diseases, it is frequently underdiagnosed unless electron microscopy is performed. We report two adult male patients who presented with progressive renal dysfunction, proteinuria, hematuria, and hypertension. In both cases, initial renal biopsies were misleading. The first patient received a diagnosis of tubulointerstitial nephritis; the second was diagnosed with focal segmental glomerulosclerosis (FSGS). Repeat kidney biopsies revealed mesangial expansion, glomerular basement membrane thickening, and variable degrees of chronic glomerulosclerosis on light microscopy. Immunofluorescence was negative or nonspecific in both cases, with no significant immune complex deposition. Definitive diagnosis was established by electron microscopy, which demonstrated characteristic curvilinear collagen fibrils, approximately 60 nm in diameter, within the mesangial and subendothelial spaces, without evidence of immune complex deposits. The two cases represented different stages of disease. One showed relatively preserved glomerular architecture consistent with earlier-stage involvement, while the other exhibited advanced diffuse glomerulosclerosis and severe tubulointerstitial fibrosis. Together, they illustrate the broad morphologic spectrum of this entity. These cases highlight the diagnostic challenge posed by Type III collagen glomerulopathy and its frequent misclassification as other glomerular diseases, particularly FSGS and membranoproliferative patterns. Our findings underscore the essential role of electron microscopy in reaching a definitive diagnosis. Given how often this condition is missed, it may be underrecognized rather than truly rare. Early and accurate diagnosis is critical to avoid mismanagement in patients with unexplained proteinuric kidney disease.