Bacillus Calmette-Guérin Gone Systemic: A Rare Case of Granulomatous Nephritis and AKI Following Intravesical BCG Therapy for Bladder Cancer

Introduction: Intravesical Bacillus Calmette-Guérin (BCG) remains the standard of care for high-grade non-muscle invasive bladder cancer (NMIBC). While typically well tolerated, systemic complications—especially renal involvement—are rare but potentially severe. Granulomatous interstitial nephritis (GIN) following intravesical BCG is sparsely reported and underrecognized. Case Description: 84-year-old man with hypertension and MGUS who developed acute kidney injury (AKI) three months after completing a six-week course of intravesical BCG. Presenting symptoms included failure to thrive, night sweats, and weight loss. Initial evaluation revealed elevated serum creatinine (2.57 mg/dL), bland urinalysis, and positive acid-fast bacillus (AFB) blood cultures. Despite empiric anti-mycobacterial therapy, renal function continued to decline (peak sCr 5.17 mg/dL). Kidney biopsy revealed granulomatous interstitial nephritis with moderate-to-severe interstitial fibrosis and tubular atrophy. Empiric corticosteroid therapy was initiated, resulting in a marked improvement in renal function. Discussion: This case highlights granulomatous interstitial nephritis as a rare but significant complication of intravesical BCG, likely driven by a delayed-type hypersensitivity reaction. The pathogenesis may involve T-cell activation and macrophage-driven cytokine responses similar to those seen in immune checkpoint inhibitor–related nephritis. Early recognition and a combined treatment approach with antimycobacterial agents and corticosteroids can promote renal recovery. Clinicians should maintain a high index of suspicion for BCG-related renal complications in patients presenting with unexplained AKI following intravesical therapy. Timely diagnosis and treatment may prevent irreversible kidney damage.