Abstract Checkpoint inhibitor pneumonitis is a recognized immune-related adverse event in patients receiving immune checkpoint inhibitors (ICIs), with high-dose corticosteroids as the cornerstone of management. While the risk of Pneumocystis jirovecii pneumonia (PJP) is well established with prolonged high-dose steroid use, the additive risk from ICIs alone remains poorly defined and is primarily supported by case series and reports. The clinical and radiographic presentation of checkpoint inhibitor pneumonitis can closely mimic PJP, necessitating a high index of suspicion and thorough diagnostic evaluation prior to steroid initiation. An 85-year-old male with poorly differentiated lung adenocarcinoma, previously treated with neoadjuvant CarboTaxol and nivolumab, and subsequent radiation, presented with fever and acute respiratory failure. Imaging revealed diffuse interstitial fibrotic changes and patchy ground-glass opacities bilaterally, superimposed on known malignancy. Initial infectious workup, including respiratory, blood, and urine cultures, Legionella and pneumococcal urine antigens, MRSA nares PCR, serum cryptococcus antigen, and tuberculosis QuantiFERON, was negative. Laboratory evaluation showed an elevated LDH of 427 IU/L. Given the clinical scenario and abnormal laboratory findings, empiric treatment for PJP was initiated with intravenous (IV) clindamycin 900 mg every 8 hours, and oral primaquine 52.6 mg oral daily due to a history of severe trimethoprim/sulfamethoxazole allergy in a addition to empiric broad spectrum antibacterial therapy with IV vancomycin and IV meropenem. Despite aggressive management, the patient experienced progressive respiratory failure. After discussion with the patient and family, a decision was made to transition to hospice care, and he died during that hospitalization. Notably, key laboratory results—including an elevated (1,3) Beta-D-Glucan (Fungitell assay), a negative PJP direct fluorescent assay, and a positive PJP quantitative PCR (2000 copies/mL)—returned after his death.This case highlights the diagnostic and management challenges in distinguishing checkpoint inhibitor pneumonitis from PJP, the importance of ruling out PJP prior to corticosteroid initiation, and the need for empiric therapy and prophylaxis in high-risk patients. Delayed laboratory confirmation is common and underscores the necessity of early empiric intervention when clinical suspicion is high. This abstract is funded by: N/A
Diedhiou et al. (Fri,) studied this question.