Ninad Parekh, Edward Epstein, Suzanne El-Sayegh Department of Medicine, Division of Nephrology, Staten Island University Hospital, Staten Island, NY, USA Introduction: Diagnosing the etiology of a rapidly progressive glomerulonephritis is of vital importance to guide appropriate therapeutic management. This case highlights the complexity involved in establishing diagnosis when presentation is atypical. In certain cases diagnosis cannot be established based on clinical presentation or biopsy findings alone, and critical analysis of biopsy findings in context of clinical presentation is crucial to guide the clinical decision-making process.Case presentation: A 47-year-old Hispanic male with history of granulomatosis with polyangiitis (GPA) in remission on azathioprine, presented with fatigue and lethargy. Physical examination was unremarkable. Laboratory data revealed elevated creatinine and otherwise normal electrolytes. Urinalysis showed numerous dysmorphic red blood cells with few red cell casts. His serologic results were all negative except anti-proteinase-3 antibody at very low titers. Kidney biopsy showed necrotizing crescentic glomerulonephritis with linear immunoglobulin G staining along the basement membrane.Conclusion: This case presented conflicting serologic and histopathologic findings. The presence of anti-proteinase-3 antibody supported diagnosis of recurrence of GPA. However, linear staining of immunoglobulin G (IgG) on immunofluorescence (IF) staining of renal biopsy supported anti-glomerular basement membrane (GBM) disease. The treatment of anti-GBM disease and GPA both involve immunosuppression with prednisone and cyclophosphamide. However, patients with anti-GBM disease are also treated with plasmapheresis early in the disease presentation to prevent further damage. The patient with GPA, on the other hand, was shown to benefit from plasmapheresis only in the case of severe renal disease (serum creatinine level more than 5 mg/dL) or pulmonary hemorrhage. In this case, since the patient did not have detectable circulating anti-GBM antibody, the decision was made not to proceed with plasmapheresis. The patient was treated with a standard immunosuppressive regimen consisting of prednisone and cyclophosphamide with partial renal recovery at 2 months. Keywords: Necrotizing RPGN, Anti-GBM disease, GPA, ANCA - associated vasculitis, dual antibody-positive disease