What is the recommended approach for testing for Mycobacterium Avium Complex (MAC) infection?

Recommended Approach for Testing for Mycobacterium Avium Complex (MAC) Infection

For diagnosing MAC infection, collect at least three separate sputum samples for acid-fast bacilli (AFB) analysis, with blood cultures being essential for suspected disseminated disease in immunocompromised patients. 1, 2

Specimen Collection Based on Clinical Presentation

Pulmonary MAC

1. Respiratory specimens:

   • Collect at least three separate expectorated sputum samples on different days 1, 2
   • Process specimens within 24 hours of collection; refrigerate at <8°C if delay is anticipated 2
   • For patients unable to produce sputum, obtain bronchial wash or lavage specimens 1, 2
   • Protect bronchoscopic specimens from tap water contamination to prevent false positives 1

2. Transbronchial biopsy:

   • Consider when sputum results are nondiagnostic or when other diseases are suspected 1
   • Particularly useful for patients with nodular/bronchiectatic disease on CT 3, 4
   • Histopathologic examination for granulomatous inflammation and AFB 1

Disseminated MAC

1. Blood cultures:

   • Essential for diagnosing disseminated MAC, particularly in immunocompromised patients 1, 2
   • Over 90% of patients with disseminated MAC have positive blood cultures 1
   • One positive blood culture is sufficient for diagnosis in HIV patients with symptoms 2

2. Tissue sampling:

   • For patients with lymphadenopathy, excision or fine needle aspiration of accessible lymph nodes 1
   • Consider bone marrow or liver biopsy for symptomatic patients with two negative blood cultures 1

Laboratory Processing

1. Specimen decontamination:

   • Use N-Acetyl L-cysteine (NALC) (0.5%), NaOH (2%) method for respiratory specimens 2
   • For persistent contamination with gram-negative bacteria, use 5% oxalic acid or 0.5% chlorhexidine 2

2. Staining:

   • Fluorochrome method is preferred for AFB staining 1, 2

3. Culture methods:

   • Use both liquid and solid media for culturing MAC 1, 2
   • Incubate cultures for a minimum of 6 weeks 2
   • Report time to detection of mycobacterial growth 2

4. Identification:

   • Identify MAC isolates to the species level using commercial DNA probes, HPLC, PCR restriction endonuclease assay, or DNA sequencing 1, 2
   • Perform clarithromycin susceptibility testing on isolates recovered prior to treatment initiation 1, 2

Diagnostic Criteria for MAC Pulmonary Disease

To diagnose MAC pulmonary disease (not just colonization), all of the following criteria must be met 1:

1. Clinical criteria: Compatible symptoms (cough, fatigue, malaise, dyspnea, occasionally hemoptysis)

2. Radiological criteria:

   • Nodular/cavitary opacities on chest radiograph, OR
   • HRCT showing multifocal bronchiectasis with multiple small nodules

3. Microbiological criteria (one of the following):

   • Positive culture results from at least two separate expectorated sputum samples
   • Positive culture from at least one bronchial wash or lavage
   • Transbronchial/lung biopsy with mycobacterial histopathologic features AND positive culture for MAC
   • Biopsy showing mycobacterial histopathologic features AND one or more sputum/bronchial washings culture-positive for MAC

Common Pitfalls and How to Avoid Them

1. Environmental contamination:

   • Avoid tap water exposure of surgical wounds, injection sites, and intravenous catheters 1
   • Protect bronchoscopes and clinical specimens from tap water contamination 1, 2

2. Inadequate sampling:

   • Single respiratory specimens are insufficient for diagnosis 1, 2
   • Bronchial washing is more sensitive than routine expectorated sputum for MAC isolation 3, 4

3. Improper specimen handling:

   • Delays beyond 24 hours without refrigeration can compromise results 2
   • Premature termination of cultures can lead to false negatives 2

4. Misdiagnosis:

   • Failure to distinguish colonization from disease can lead to unnecessary treatment 1, 2
   • MAC isolation can sometimes obscure other important lung diseases 1
   • Disseminated MAC can mimic malignancies, requiring thorough examination 5

5. Susceptibility testing limitations:

   • Routine susceptibility testing of MAC isolates is recommended for clarithromycin only 1
   • Extended susceptibility testing for macrolide-resistant isolates is unproven but may be considered 1

By following these guidelines for specimen collection, processing, and interpretation, clinicians can accurately diagnose MAC infection and distinguish between colonization and disease requiring treatment.