Diagnostic Dilemma: When Pyelonephritis Masks Anti-GBM Disease

Introduction: Anti-glomerular basement membrane (anti-GBM) disease is a rare, rapidly progressive vasculitis targeting type IV collagen in glomerular and alveolar capillaries. While viral infections, smoking, and hydrocarbon exposure have been implicated, their pathogenic role remains unclear. We present a case of a young female with an atypical presentation of anti-GBM disease initially diagnosed as pyelonephritis. Case Description: A 37-year-old woman with no significant medical history presented with lower abdominal discomfort and dark urine. Her only recent exposure was a COVID booster vaccine three months prior. Physical exam revealed left costovertebral angle tenderness. CT abdomen showed left-sided pyelonephritis without hydroureteronephrosis. Urine culture confirmed complicated UTI, and treated with intravenous fluids and antibiotics. Despite therapy, creatinine worsened from 4.0 to 7.0 mg/dL. On day 5, she developed mild hemoptysis without respiratory distress. CT chest revealed bilateral pleural effusions and ground-glass opacities. Anti-GBM antibody titers were >8.0. Kidney biopsy revealed 100% crescentic glomerulonephritis with fibrinoid necrosis and linear IgG staining on immunofluorescence. Treated with pulse-dose methylprednisolone followed by tapering dose, plasmaphoresis and rituximab. She was discharged on prophylactic antimicrobials and a proton pump inhibitor. She remained dialysis-dependent at 3-month follow-up. Renal transplant counseling was initiated, contingent on pulmonary resolution and sustained anti-GBM seronegativity. Discussion: The nature of this systemic disorder illustrates the diagnostic challenge of anti-GBM disease mimicking infection, leading to misdiagnosis as pyelonephritis despite no traditional risk factors. Worsening AKI despite appropriate therapy should prompt additional vasculitis evaluation, as delayed recognition contributes to poor renal outcomes. While plasmapheresis, steroids, and cyclophosphamide are standard therapy, rituximab was chosen due to gonadotoxic concerns, highlighting individualized treatment approaches. Emerging therapies like Imlifidase offer potential renal benefit, reinforcing the importance of early diagnosis in atypical presentations. Although the patient received a COVID booster vaccination three months ago, no causal association has been described in the literature.