Between January 1, 2018 and December 31, 2020, the records of all patients who were admitted to the Shanghai Pulmonary Hospital and diagnosed with MAB-PD were examined. This study was approved by the Ethics Committees of Shanghai Pulmonary Hospital (Ethics No. fk22-005). The inclusion criteria were age 18 years or older, diagnosis of MAB-PD, completion of treatment as prescribed and follow-up for at least 1 year after drug withdrawal, HIV negativity, and receipt of linezolid treatment for more than 12 months (linezolid group only). The exclusion criteria were pregnancy or breastfeeding, pulmonary tuberculosis or co-infection with another NTM species, active autoimmune disease requiring corticosteroid or immunosuppressant agents, advanced stage of malignancy, and incomplete clinical data. All patients who received linezolid were included in the linezolid group, and patients who received therapy that did not include linezolid were in the control group.

The diagnosis of MAB-PD was based on a previous consensus report (“An official ATS/ IDSA Statement: Diagnosis, Treatment, and Prevention of Nontuberculous Mycobacterial Diseases”). M. abscessus was isolated from at least two separate expectorated sputum samples or at least one bronchoalveolar lavage fluid (BALF) sample [18].

Mycobacterial cultures were performed using mycobacterial growth indicator tubes (BACTEC MGIT 960 System, Becton Dickinson Life Sciences) according to the manufacturer’s instructions. Briefly, a 1 mL treated sample was added into an MGIT culture tube with 0.8 mL additives, and incubated for 6 weeks. The NTM isolates were then confirmed using MPB64 detection, and the p-nitrobenzoic acid and thiophene-2-carboxylic acid hydrazine tests.

Identification of NTM isolates was performed by PCR reverse dot-blot hybridization (Mycobacterium species identification gene detection kit, Shanghai Xin Peijing Medical Inspection Center, batch No.201,707,002) and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS; Shanghai Kangli Medical Inspection Institute ® System).

Antibiotic susceptibility was tested according to the M24-A2 guidelines issued by the American Clinical and Laboratory Standards Institute (CLSI). The minimum inhibitory concentration (MIC) values of 11 antibiotics against M. abscessus (amikacin, cefoxitin, ciprofloxacin, clarithromycin, doxycycline, imipenem, linezolid, minocycline, moxifloxacin, tobramycin, and sulfamethoxazole) were determined by the broth microdilution method (Sensititre, Thermo Scientific). All results were interpreted using the criteria recommended in the CLSI M24-A2 guideline.

Treatments were determined by the antibiotic susceptibility results, the guideline of the British Thoracic Society [1], and the 2012 Expert Consensus for the Diagnosis and Treatment of Non-tuberculous Mycobacteria Disease from the Chinese Medical Association. For the initial phase, at least three of the following drugs were used: amikacin (intravenous, 10–15 mg/kg/day), clarithromycin (oral, 1000 mg/day) or azithromycin (oral, 500 mg/day), imipenem (intravenous, 2000 mg/day) or cefoxitin (intravenous, 6–12 g/day), linezolid (oral, 600 mg/day), doxycycline (oral, 200 mg/day), clofazimine (oral, 100 mg/day), and moxifloxacin (oral, 400 mg/day). During the continuation phase, at least two of the following drugs were used: amikacin (400 mg/day, nebulised), clarithromycin or azithromycin (doses as above), linezolid (dose as above), doxycycline (dose as above), and clofazimine and moxifloxacin (doses as above). In all patients, treatment was given for at least one year after sputum culture conversion or for 2 years.

Patients were followed from initiation of anti-NTM therapy to death or censoring from loss to follow-up or the decision to terminate treatment. The demographic characteristics, clinical symptoms, laboratory results, radiological features, antibiotic susceptibility test results, outcomes, and adverse effects and events were recorded. Each patient was followed up in an outpatient clinic or an inpatient department after discharge.

The baseline characteristics of enrolled patients were recorded, including the results of NTM cultures, antibiotic susceptibility tests, and chest computed tomography (CT), all of which were performed within 1 month before the start of antibiotic therapy. During therapy, sputum culture was performed at least once every 2 months and chest CT examination every 3 months. To monitor adverse effects and events during therapy, an electrocardiogram and measurements of whole blood cell count, blood chemistry (indicators of liver and kidney function), and electrolytes were performed monthly. Patients on amikacin were screened for hearing loss every 3 months or when indicated. Patients in the linezolid group had a routine blood test every 2 weeks during the first 3 months of treatment. All patients were encouraged to report all adverse events (such as nausea, vomiting, appetite loss, vision loss, and hearing loss) to their physicians during the follow-up period, and symptomatic treatment was offered following corresponding guidelines.

Culture conversion was defined as the finding of at least three consecutive negative mycobacterial cultures from sputum collected at least 4 weeks apart during antimycobacterial treatment. The sample date of the first negative culture was considered the date of culture conversion.

Treatment effectiveness [20] was defined as microbiological cure (finding multiple consecutive negative but no positive cultures with the causative species from sputum after culture conversion and until the end of antimycobacterial treatment) or as microbiological failure (re-emergence of multiple positive cultures or persistence of positive cultures with the causative species from sputum after 12  or more months of antimycobacterial treatment while the patient was still receiving treatment). In this study, treatment success was defined by microbiological cure, and all other outcomes were defined as treatment failure.

Chest CT was performed using 64-slice examinations (1-mm section thickness) and the results were independently evaluated by 2 radiologists and 1 infectious diseases physician. Any discrepancy was resolved by their discussion. The maximum diameter of a measured cavity was used to determine cavity changes (closure, smaller, no change, larger or new cavities), and changes in shape, location, and size were evaluated for all lesions (absorption, no change, or progression).

All patients received anti-NTM treatment until at least 1 year after the sputum culture conversion or up to 2 years if cultures did not turn negative.

Statistical analyses were performed using SPSS version 20.0 (SPSS Inc., Chicago, IL, USA). Continuous variables with normal distributions are presented as means ± standard deviations and compared using Student’s t-test. Continuous variables with non-normal distributions are presented as medians ± inter-quartile ranges and compared using the Mann-Whitney U test. Categorical variables are reported as frequencies (percentages) and compared using the Chi-square test or using Fisher’s exact test (when the theoretical frequency was less than 5). A P value below 0.05 in a two-sided test was considered to indicate statistical significance. Kaplan-Meier curves were plotted to compare the two groups in terms of sputum culture conversion and cavity closure, and the statistical significance of differences was determined using the log-rank test. Unconditional binary logistic regression (forward conditional method) was used to identify risk factors for treatment failure, and adjusted odds ratios (aORs) and 95% CIs were then calculated for each antibiotic.