A Case of Legionella Pneumonia Suspected as Typical Pneumonia After Lung Cancer Surgery, Diagnosed by a Repeat Urinary Legionella Antigen Test.

Legionella pneumophila is a leading cause of severe atypical pneumonia and is associated with high mortality when initial treatment is inadequate. The urinary Legionella antigen test serves as a rapid, first-line diagnostic tool; however, early results may be negative, and antigen shedding can be intermittent. In these situations, timely retesting is essential, especially in postoperative or high-risk patients. We report the case of a 74-year-old man, a former smoker with diabetes and moderate aortic stenosis, who underwent thoracoscopic left lower lobectomy and was discharged on postoperative day (POD) six. He developed a fever on POD 27, accompanied by right upper lobe pneumonia. Outpatient urinary antigen tests for pneumococcus and Legionella were negative, and oral garenoxacin was initiated. However, hypoxemia and inflammation worsened, resulting in urgent admission on POD 30. Broad-spectrum β-lactams (sulbactam/ampicillin, followed by piperacillin/tazobactam) were ineffective. Chest CT revealed enlarging infiltrates and a parapneumonic effusion requiring drainage, despite negative pleural fluid cultures. Given the persistent deterioration of the patient's condition, levofloxacin was added, and the urinary Legionella antigen test (R70829, MIZUHO MEDY Co., Ltd., Tosu, Japan) was repeated on hospital day five (POD 35), which returned positive, confirming Legionella pneumonia. Subsequent targeted therapy with azithromycin (days 6-8 and 13-15), alongside levofloxacin, resulted in clinical improvement. Despite complications during the clinical course, including acute kidney injury necessitating two hemodialysis sessions and a creatine kinase peak of 1,563 U/L, the chest drain was removed on day 21, oxygen was discontinued by day 31, and the patient was transferred to long-term care on day 54. The sequence of renal failure preceding creatine kinase elevation suggests direct renal involvement by L. pneumophila rather than primary rhabdomyolysis. Vigilant retesting enabled timely pathogen-directed therapy and a favorable outcome. This case report highlights that a single negative urinary antigen test does not rule out Legionella infection. We accordingly recommend early repeat antigen testing, ideally within one week, for postoperative pneumonia unresponsive to β-lactams, together with prompt macrolide or fluoroquinolone therapy.