From the Guidelines
Diagnosing Mycobacterium avium complex (MAC) infection requires a combination of clinical evaluation, imaging studies, and microbiological testing, with the most recent guidelines recommending the use of ATS/IDSA criteria for diagnosis, as stated in the 2020 study 1. The diagnostic process typically begins with a thorough clinical assessment, looking for symptoms such as persistent cough, fatigue, weight loss, and fever, particularly in immunocompromised patients.
- Chest imaging, including chest X-rays and high-resolution CT scans, is essential to identify nodular bronchiectasis or cavitary lesions characteristic of MAC infection.
- The definitive diagnosis requires isolation and identification of the organism from respiratory specimens (sputum, bronchial wash, or bronchoalveolar lavage) or from blood, tissue, or other body fluids.
- Multiple sputum samples should be collected on separate days to increase diagnostic yield, as recommended by the US Cystic Fibrosis Foundation and European Cystic Fibrosis Society consensus recommendations 1.
- Specimens are cultured on specialized media for mycobacteria, with growth typically taking 2-3 weeks, and once growth is detected, DNA probes or PCR-based methods can identify MAC specifically.
- In patients unable to produce sputum, bronchoscopy with bronchoalveolar lavage may be necessary, and blood cultures using specialized mycobacterial blood culture bottles are particularly important for diagnosing disseminated disease in immunocompromised patients.
- Drug susceptibility testing should be performed on isolates to guide treatment, especially for macrolides and amikacin, as recommended by the ATS/IDSA guidelines 1. The diagnosis of MAC requires meeting specific criteria:
- Compatible clinical and radiographic presentation
- Exclusion of other diseases
- Microbiological confirmation from respiratory specimens or sterile sites, as stated in the 2020 study 1.
From the Research
Diagnosis of Mycobacterium avium complex (MAC) infection
- The diagnosis of MAC lung disease can be confirmed by multiple positive sputum culture findings in patients with typical symptoms and radiologic findings 2.
- Blood culture is also an easy and safe way of diagnosing the infection, especially in HIV-infected patients 3.
- A real-time RNA isothermal amplification and detection method (SAT-MAC assay) can be used to identify and discern M. avium and M. intracellulare, with high specificity and sensitivity 4.
- The SAT-MAC assay can detect isolated strains and clinical sputum specimens, with a limit of detection (LOD) of 30 CFU/mL for M. avium and 20 CFU/mL for M. intracellulare 4.
- Culture tests on Lowenstein-Jensen (L-J) culture medium and PCR-sequencing can also be used to test clinical isolated strains and sputum specimens 4.
Diagnostic Methods
- Sputum culture 2, 4
- Blood culture 3
- Real-time RNA isothermal amplification and detection method (SAT-MAC assay) 4
- Culture tests on Lowenstein-Jensen (L-J) culture medium 4
- PCR-sequencing 4
Clinical Features
- The clinical manifestations of MAC infection are non-specific, with fever, respiratory and gastrointestinal tract symptoms, and weight loss 3.
- M. avium cases are more frequently detected by health examination, while M. intracellulare cases have a better prognosis when compared to patients with positive tuberculin skin test 5.