When Serology Fails: Biopsy-Proven Dual-Positive Anti-GBM and ANCA-Associated Vasculitis with Negative Serum Anti-GBM Antibodies

Introduction: Dual-positive anti-GBM and ANCA-associated vasculitis is a rare and aggressive autoimmune condition with a poor prognosis. To date, only two reported cases have demonstrated negative anti-GBM serology despite biopsy-confirmed dual positivity. We present a similar case demonstrating clinical improvement following treatment. Case Description: A 72-year-old female with Atrial fibrillation, Hypertension, dermatomyositis (25 y remission), came with 30lb weight loss, decreased appetite, fatigue, nausea, and dysgeusia of 3 months. She had history of crack cocaine use but has been sober for 25 years. Physical examination is significant for only hypotension (BP 82/55 mm Hg). Patient had non-oliguric acute kidney injury (creatinine 5.24 mg/dL; baseline- 1.3mg/dL), anion gap metabolic acidosis, anemia (Hb 9.1 gm/dL), UA with hematuria, proteinuria, hyaline and granular casts, UPCR of 2.1g, ANA (1:160), P-ANCA (1:1280), anti-MPO antibody (40.1) with normal C3/C4 levels, negative for anti-GBM antibodies. Sonogram did not show any pathology. Renal biopsy showed necrotizing crescentic glomerulonephritis, linear staining of glomerular basement membrane with IgG3+, 8 cellular crescents and 2 fibrocellular crescents with 20% IFTA. Patient was diagnosed with anti-GBM disease with vasculitis and was treated with steroids, PLEX, and cyclophosphamide with symptomatic resolution and improvement in creatinine (2.5 mg/dL) Discussion: Anti-GBM disease with ANCA positivity is well described in literature. Here we have seronegative anti-GBM disease and ANCA vasculitis. Different epitopes on IgG antibodies against the anti-GBM antigen (Goodpasture antigen) could be checked in different labs. Elisa method can cause false negative results, and Immunofluorescence is better. Apart from IgG, anti-GBM antibody could be of class IgM, IgA, IgD as well. We usually do not check other classes in routine screening commercially.