Pulmonary Mycobacterium kiyosense infection presenting with elevated serum carbohydrate antigen 19-9: a case report

Abstract Background Pulmonary nontuberculous mycobacterial (NTM) infections are increasing worldwide; however, data on rare species remain limited. We report a case of pulmonary Mycobacterium kiyosense infection diagnosed following an elevation in the tumor marker carbohydrate antigen 19-9 (CA19-9), which was successfully treated. Case presentation A 72-year-old woman with a history of subtotal gastrectomy for gastric adenocarcinoma 7 years prior had been followed up for 3 years for chronic bronchitis. Six months before referral, she developed worsening radiographic findings and elevated CA19-9 levels. Upper and lower gastrointestinal endoscopy and positron emission tomography–computed tomography (PET-CT) revealed no evidence of malignancy, except for progressive consolidation in the right lower lobe. Bronchoscopy performed at a previous hospital revealed a slowly growing mycobacterium that could not be identified at the species level, and the patient was referred to our hospital for further evaluation. Sputum cultures obtained at referral and 2 months later were positive for NTM and were initially identified as Mycobacterium gordonae by DNA–DNA hybridization. Whole-genome sequencing (WGS) of these isolates and the earlier strain confirmed Mycobacterium kiyosense (average nucleotide identity, 99.8%). Based on progressive clinical and imaging findings, combination therapy with clarithromycin, ethambutol, and rifampicin was initiated 4 months after referral. Her CA19-9 levels decreased in parallel with radiographic improvement, and sputum culture was negative 8 months after treatment initiation. Treatment was completed after 24 months without recurrence. Conclusions This is the first reported case of pulmonary M. kiyosense infection successfully treated with a macrolide-based regimen for slowly growing mycobacteria. Clinicians should consider pulmonary NTM infection as a possible cause of elevated CA19-9 levels in the absence of malignancy.