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

<i>Legionella</i> <i>pneumophila</i> is a leading cause of severe atypical pneumonia and is associated with high mortality when initial treatment is inadequate. The urinary <i>Legionella</i> 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 <i>Legionella</i> 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 <i>Legionella</i> antigen test (R70829, MIZUHO MEDY Co., Ltd., Tosu, Japan) was repeated on hospital day five (POD 35), which returned positive, confirming <i>Legionella</i> 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 <i>L. pneumophila</i> 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 <i>Legionella</i> 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.