Anti-Glomerular Basement Membrane Disease without Linear IgG Deposits in an Elderly Patient with Metastatic Rectal Cancer: A Case Report.

Introduction: Anti-glomerular basement membrane (anti-GBM) disease is classically diagnosed based on presence of rapidly progressive glomerulonephritis, circulating anti-GBM antibodies, and the hallmark finding of linear IgG deposition along the glomerular basement membrane on kidney biopsy. These deposits are considered a defining feature of the disease, aiding in its differentiation from other pauci-immune glomerulonephritis. However, rare cases lacking IgG deposits on kidney biopsies challenge this paradigm, complicating both diagnosis and management. Case Presentation: We report the case of an 80-year-old male with metastatic rectal cancer, who presented with acute kidney injury, hematuria, and proteinuria. Serological testing confirmed elevated anti-GBM antibody levels. Surprisingly, renal biopsy revealed crescentic glomerulonephritis without characteristic linear IgG deposition, which raised diagnostic uncertainty. Despite the absence of this classic finding, the patient's clinical course was consistent with anti-GBM disease. Given his malignancy, treatment was tailored to balance immunosuppression with oncologic and infectious considerations. The patient received a modified immunosuppressive regimen, including plasma exchange and corticosteroids, but IV cyclophosphamide was withheld due to recurrent infections. Initially, from a nephrology standpoint, his clinical course was notable for partial renal recovery, resolution of hematuria, and improvement in both creatinine and proteinuria levels. However, he ultimately passed away due to severe infectious complications. Conclusion: This case underscores the diagnostic challenges and therapeutic decision-making in patients who present with features suggestive of anti-GBM disease, characterized by positive anti-GBM antibodies but absent immune deposits, especially when accompanied by malignancy, prior exposure to immunosuppressive therapy, and concurrent infections.