What are the diagnostic methods for Mycobacterium avium complex (MAC) infection?

From the Guidelines

Diagnosing Mycobacterium avium-intracellulare (MAI) infection requires a combination of clinical evaluation, imaging studies, and microbiological testing, with the definitive diagnosis relying on isolating the organism from respiratory specimens through acid-fast bacilli (AFB) smears and cultures, as recommended by the American Thoracic Society criteria 1. The diagnosis begins with recognizing symptoms such as persistent cough, fatigue, weight loss, and fever, particularly in immunocompromised patients or those with underlying lung disease.

• Chest imaging, including CT scans, is essential to identify nodular bronchiectasis or cavitary lesions characteristic of MAI infection.
• Multiple sputum samples should be collected on separate days to increase diagnostic yield, with at least three sputum specimens recommended for optimal positive predictive value of sputum analysis 1.
• Molecular methods like PCR can provide faster identification once mycobacteria are isolated.
• Blood cultures may be necessary for disseminated MAI infection, particularly in HIV patients with CD4 counts below 50 cells/μL.
• Bronchoscopy with bronchoalveolar lavage or transbronchial biopsy may be required if sputum samples are non-diagnostic. Key considerations in the diagnostic process include:
• The use of culture and smears for AFB from sputum for NTM screening, as recommended by the CF Foundation and the ECFS 1.
• The importance of molecular identification of NTM isolates to the species level, except for M. intracellulare, M. avium, and M. chimaera, where identification can be limited to MAC 1.
• The need for drug susceptibility testing, including clarithromycin susceptibility testing for MAC, and susceptibility testing for M. abscessus complex, to guide treatment and prevent resistance 1.

From the Research

Diagnostic Methods for Mycobacterium Avium Complex (MAC) Infection

The diagnostic methods for MAC infection include:

• Clinical evaluation: patients with risk factors and who meet the diagnostic criteria should be considered for treatment 2
• Radiological findings: chest CT scan or high-resolution computed tomographic scan with multifocal bronchiectasis and multiple small nodules 3, 4
• Microbiologic criteria: 2 or more positive sputum cultures or 1 bronchoscopic specimen culture 2, 3
• Macrogenomic next-generation sequencing (mNGS) of bronchoscopic alveolar lavage fluid (BALF) samples: has high specificity in detecting pathogens of infectious diseases, especially complex mixed infectious disease pathogens 5
• Acid-fast bacilli identification: presence of acid-fast bacilli in sputum results, which can be identified as Mycobacterium avium complex 6

Laboratory Tests

Laboratory tests used to diagnose MAC infection include:

• Sputum cultures: 2 or more positive sputum cultures are required for diagnosis 2, 3
• Bronchoscopic specimen culture: 1 bronchoscopic specimen culture is required for diagnosis 2, 3
• mNGS of BALF samples: has high specificity in detecting pathogens of infectious diseases, especially complex mixed infectious disease pathogens 5
• Acid-fast bacilli identification: presence of acid-fast bacilli in sputum results, which can be identified as Mycobacterium avium complex 6

Imaging Studies

Imaging studies used to diagnose MAC infection include:

• Chest CT scan: shows multiple inflammatory lesions, multifocal bronchiectasis, and multiple small nodules 5, 3, 4
• High-resolution computed tomographic scan: shows multifocal bronchiectasis and multiple small nodules 3, 4