Persistence of pulmonary infiltrates despite effective anti-infective therapy as a diagnostic clue for pembrolizumab-induced pneumonitis: a case report

Introduction Pembrolizumab-based bladder preservation strategies are increasingly applied in invasive high-grade urothelial carcinoma, yet checkpoint inhibitor pneumonitis (CIP) remains an underrecognized, potentially fatal adverse event. Diagnosis can be markedly delayed when concurrent infection is present. Case presentation A 63-year-old man with invasive high-grade urothelial carcinoma developed complete tumor response after receiving disitamab vedotin plus pembrolizumab following chemotherapy intolerance. He subsequently presented with urinary tract infection and septic shock. Aggressive anti-infective and hemodynamic support promptly reversed shock and normalized systemic inflammatory markers. Nevertheless, follow-up chest computed tomography demonstrated persistent bilateral ground-glass opacities and consolidation without any improvement. Pre-treatment chest radiography had been normal, positron emission tomography–computed tomography showed marked bilateral pulmonary fluorodeoxyglucose uptake without bladder recurrence, and extensive microbiological workup excluded uncontrolled infection. The dissociation between well-controlled infection and stationary pulmonary lesions, together with typical imaging features, raised strong suspicion of pembrolizumab-induced CIP. Conclusion In patients receiving PD-1 inhibitors, persistent pulmonary infiltrates despite effective anti-infective therapy should raise strong suspicion of checkpoint inhibitor pneumonitis and prompt dedicated diagnostic evaluation. This phenotype—the dissociation between resolving systemic infection and stationary lung lesions—may serve as a practical clue to consider CIP earlier, particularly when invasive diagnostic tools are unavailable; however, it is a suggestive rather than confirmatory finding and requires validation in larger cohorts.