Pulmonary infection by Nocardia saintgeorgesii mimicking lung cancer with concurrent pulmonary embolism in an immunocompetent host: a case highlighting the diagnostic role of mNGS

Background Pulmonary nocardiosis presents a diagnostic challenge due to its frequent mimicry of lung cancer on imaging and the low sensitivity of conventional cultures. We report a case initially misdiagnosed as malignancy in an immunocompetent host, where metagenomic next-generation sequencing (mNGS) provided a definitive diagnosis and revealed a concurrent pulmonary embolism, suggesting a potential underrecognized association. Case presentation This report describes a case of PN in an immunocompetent patient who was initially misdiagnosed with lung cancer based on imaging findings but later confirmed as pulmonary nocardiosis via mNGS. Notably, the patient also developed pulmonary embolism (PE). Empirical antibiotic therapy with piperacillin-tazobactam was initiated initially, supplemented with inhaled ipratropium bromide and expectorants to alleviate symptoms. Based on imaging findings suggestive of lung cancer, an invasive procedure was scheduled. mNGS was subsequently performed for further diagnosis. The subsequent results, along with CT scans, indicated no evidence of malignancy, leading to a consideration of Nocardia infection. The treatment regimen was then adjusted to ceftriaxone sodium combined with compound sulfamethoxazole, and the surgical schedule was canceled. The patient’s condition showed significant improvement, and he was discharged without fever or dyspnea. Some literature suggests that many PN patients present with concurrent deep vein thrombosis (DVT), suggesting a potential yet underrecognized association between Nocardia infection and thrombotic events. However, this correlation has not been fully reported before.