From the Guidelines
To diagnose Mycobacterium avium complex (MAC) infection, at least two positive respiratory cultures from separate samples are needed, as recommended by the most recent guidelines 1. This requirement helps distinguish between true infection and colonization or contamination, which can occur with a single positive culture. For patients with nodular bronchiectatic disease, three positive sputum cultures are often recommended for a more definitive diagnosis. The multiple culture requirement is important because MAC organisms are commonly found in the environment, including water and soil, making contamination of specimens possible. In addition to the cultures, diagnosis should be supported by compatible clinical symptoms (such as chronic cough, fatigue, weight loss) and radiographic findings (such as nodular opacities or bronchiectasis). Some key points to consider in the diagnosis of MAC infection include:
- The use of at least two separate expectorated sputum samples for culture, as recommended by the US Cystic Fibrosis Foundation and European Cystic Fibrosis Society 1
- The importance of species identification to confirm MAC specifically, as opposed to other mycobacterial species
- The need for expert consultation when infrequently encountered NTM or those usually representing environmental contamination are recovered 1
- The consideration of clinical and radiographic criteria, in addition to microbiologic criteria, for diagnosing NTM pulmonary disease 1
From the Research
Diagnosis of MAC
- The diagnosis of Mycobacterium avium complex (MAC) pulmonary disease requires a positive culture from at least two separate expectorated sputum specimens or one bronchoscopic specimen culture 2, 3.
- The optimal number of sputum examinations remains unclear, but a study found that the diagnostic yield with 3 sputum specimens was 16.5% 4.
- Increasing the number of sputum specimens to 6 improved the diagnostic yield by 7.2% 4.
Number of Cultures Needed
- According to the guidelines, 2 or more positive sputum cultures or 1 bronchoscopic specimen culture are required to diagnose MAC 2.
- Another study suggests that 2 sputum specimens or 1 bronchoscopic specimen are consistent with MAC pulmonary disease 3.
- There is no evidence to suggest that a specific number of cultures is needed, but rather that the diagnosis requires a positive culture from at least two separate expectorated sputum specimens or one bronchoscopic specimen culture 2, 3.