Background: Pulmonary infections caused by Nocardia spp. exhibit diverse clinical manifestations. However, the clinical manifestations and imaging examinations of the disease lack specificity, leading to an increased misdiagnosis rate. Therefore, the etiological diagnosis is crucial for the confirmation of this disease. Nocardia cyriacige-orgica is an aerobic, weak acid-fast, Gram-positive opportunistic pathogen characterized by slow growth.

Methods: In March 2024, a sputum specimen was sent for examination by the Department of Respiratory and Critical Care Medicine. The appearance was yellow purulent sputum, and no obvious "sulfur particles" were found. Gram staining of sputum revealed numerous Gram-positive bacilli with 90° branching and even tangled filamentous forms, and weak acid-fast staining was positive. Under aerobic conditions at a constant temperature of 37℃, we observed slow-growing colonies that were white, dry, and wrinkled on blood agar.

Results: We used MALDI-TOF MS for identification, and by comparing the mass spectrum peaks of the isolate with the known mass spectrum peaks in the database (MS-IVD database version 3.0), the result was Nocardia cyriacigeorgica with 99.9% confidence. The patient was started on empiric antibiotic therapy with piperacillintazobactam (4.5 g, q 8 hour), along with supportive measures such as nebulized treatments, cough suppressants, bronchodilators, oxygen therapy, and gastric acid suppression. Due to the patient's acute exacerbation of chronic obstructive pulmonary disease (COPD), accompanied by multiple underlying diseases such as heart failure, chronic bronchitis, and gastrointestinal bleeding, the clinical focus was mainly on improving the main symptoms. The patient was kept under observation for Nocardia infection and did not receive targeted treatment.

Conclusions: Through the review of this case and related literature, close cooperation between clinicians, laboratory departments, and imaging departments is essential in the diagnosis and treatment of this atypical pulmonary infection. Therefore, it is necessary that we analyze the laboratory diagnosis of this case to enhance the understanding of the disease and increase the actual positive rate.