How do you diagnose Mycobacterium avium complex (MAC) in a patient with cough?

Diagnosing Mycobacterium Avium Complex in a Patient with Cough

The diagnosis of Mycobacterium avium complex (MAC) requires meeting specific clinical, radiographic, and microbiologic criteria, with the collection of at least three sputum specimens on separate days for acid-fast bacilli (AFB) smear and culture being essential for diagnosis. 1

Diagnostic Criteria

The American Thoracic Society (ATS) and Infectious Diseases Society of America (IDSA) have established specific criteria for diagnosing MAC pulmonary disease:

• Clinical criteria: Pulmonary symptoms such as persistent cough, sputum production 1
• Radiographic criteria: Nodular or cavitary opacities on chest radiograph, or high-resolution computed tomography (HRCT) showing bronchiectasis with multiple small nodules 1
• Microbiologic criteria: Positive cultures from respiratory specimens 1

Microbiologic Testing

Sputum Collection and Analysis

• Collect at least three sputum specimens on separate days 1
• Process samples using both solid and liquid media 1
• Incubate cultures for a minimum of 6 weeks 1
• Process specimens within 24 hours of collection (refrigerate if delay anticipated) 1
• Decontaminate samples using standard N-Acetyl L-cysteine (0.5%), NaOH (2%) method 1
• For contaminated samples, further treatment with 5% oxalic acid or 0.5% chlorhexidine may be needed 1

Bronchoscopy

• Consider bronchoscopy if patient cannot produce sputum or has negative sputum cultures despite clinical suspicion 1
• Bronchial washings or bronchoalveolar lavage samples can be used 1, 2
• Bronchial washings are more sensitive than expectorated sputum and less likely to be affected by environmental contamination 1
• Transbronchial biopsies are not routinely recommended 1

Diagnostic Thresholds

• A single positive sputum culture is generally considered indeterminate for diagnosis 1
• Two or more positive sputum cultures are required for diagnosis 1, 3
• A single positive bronchoscopic specimen culture may be sufficient 3
• If a lung biopsy yields MAC and shows histopathologic changes typical of mycobacterial disease (granulomatous inflammation), this is sufficient for diagnosis 1

Radiographic Evaluation

• Plain chest radiograph may be adequate for evaluating fibrocavitary disease 1
• HRCT is recommended to demonstrate characteristic abnormalities of nodular/bronchiectatic MAC lung disease 1
• Common radiographic findings include:
  • Nodular opacities (often in mid and lower lung fields) 1
  • Multifocal bronchiectasis (present in up to 90% of patients) 1
  • Clusters of small nodules (5 mm) 1
  • Cavitation may be present 1

Species Identification

• All NTM isolates should be identified to the species level 1
• For M. avium complex, identification can be limited to MAC level, but subspeciation is recommended 1
• Molecular identification methods should be used 1

Clinical Considerations

• MAC pulmonary disease can present with persistent cough and purulent sputum production, usually without fever or weight loss 4
• Cough may be present for many weeks before correct diagnosis is made 4
• Prevalence of cough varies with disease severity and presence of comorbid lung conditions 1
• Consider MAC in patients with nodular bronchiectasis, particularly in middle-aged or elderly women who are nonsmokers 1
• Other CF pathogens and co-morbidities should be considered as potential contributors to symptoms 1

Common Pitfalls and Caveats

• Environmental contamination of specimens can occur since NTM are ubiquitous in nature 1
• Oropharyngeal swabs should not be used for NTM screening or diagnosis 1
• Non-culture based methods are not recommended for detecting NTM in respiratory specimens 1
• MAC can sometimes obfuscate or delay diagnosis of other important lung diseases 1
• Patients with negative sputum smears, no symptoms, no abnormal findings in the left upper division, or fewer affected lobes on CT may require bronchoscopy for diagnosis 2
• Diagnosis may be delayed due to the indolent nature of MAC disease 4

By following these diagnostic criteria and procedures, clinicians can accurately diagnose MAC pulmonary disease in patients presenting with cough and initiate appropriate treatment when indicated.