A case of mistaken identity: fibrillary glomerulonephritis masquerading as crescentic anti-glomerular basement membrane disease.

Fibrillary glomerulonephritis (FGN) is a rare cause of rapidly progressive glomerulonephritis (RPGN). We report a case of FGN in which the patient presented with a clinical pulmonary-renal syndrome and whose kidney biopsy showed > 90% crescents on light microscopy. Immunofluorescence microscopy showed pseudo-linear IgG and C3 staining of the glomerular capillary walls resulting in an initial diagnosis of crescentic glomerulonephritis of anti-glomerular basement membrane (anti-GBM) antibody etiology. Electron microscopy showed fibrillary deposits permeating the glomerular capillary walls, characteristic of FGN. Although dialysis dependent at presentation and anuric at discharge, the patient recovered adequate renal function and urine output to come off dialysis at 20 weeks. A follow up biopsy performed at this stage showed progression of the underlying chronic kidney disease. This is the third reported case of FGN with a clinical presentation and histologic and immunofluorescence microscopic findings that closely mimicked anti-GBM antibody mediated disease. These cases demonstrate that FGN is a rare but important consideration in the differential diagnosis of RPGN.