P054

1:International Reference Laboratory of Mycobacteriology, Statens Serum Institute, 2300 Copenhagen S, Denmark.; 2:Department of Infectious Diseases, Aarhus University Hospital, 8200 Aarhus N, Denmark.; 3:Mycobacteriology/Institute of Tropical Medicine, Antwerp, Belgium.; 4:Department of Biosciences, Nottingham Trent University, Nottingham, UK.; 5:Department of Biotechnology and Biomedicine, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.; 6:Global Health Section, Department of Public Health, University of Copenhagen, 1353 Copenhagen K, Denmark.

Infections due to nontuberculous mycobacteria (NTM) are increasingly recognized worldwide, with NTM pulmonary disease (NTM-PD) being the most frequent clinical presentation. At the International Reference Laboratory of Mycobacteriology (IRLM), the most consistently identified NTM species is Mycobacterium avium, which is also among the most prevalent pathogens responsible for NTM-PD. Treatment is challenging and requires long-term, multidrug therapy, thereby increasing the risk of developing resistance. In this study, we investigate resistance during long-term treatment of M. avium patients with follow-up samples received at the IRLM between 2020 and 2024 (n _isolates = 670; n _patients = 98). Genotypic drug susceptibility testing (gDST) with GenoType NTM-DR was conducted on 225 M. avium isolates from 95 patients, showing resistance to amikacin and clarithromycin in 10.7% and 14.7% isolates, respectively. Phenotypic DST (pDST) using the MGIT 960 system was subsequently carried out on a subset of isolates (n=39), revealing high resistance rates to ethambutol (79.5%) and rifampicin (92.3%). Additionally, a pilot study was conducted to investigate the relationship between resistance, treatment, and clinical outcomes in 15 patients diagnosed with NTM-PD. The median patient age was 70 years, and 8 patients had a previous episode of NTM infection. Culture conversion was achieved in 3 patients, while clinical improvement was observed in 6. Most patients (12/15) received antibiotic therapy; however, 4 were deceased during the study period. The observed resistance rates and variable treatment outcomes emphasize the importance of continued research into resistance development and more effective therapeutic approaches.