Tests
The goals of testing are to detect nontuberculous mycobacteria (NTM) infections and to distinguish between mycobacteria species. It is not possible to distinguish between TB and NTM infections without testing. The sample(s) collected for analysis depend on the part(s) of the body that the doctor suspects are infected. For pulmonary infections, 3 to 5
sputum specimens are collected first thing in the morning on different days when they are most likely to contain the most mycobacteria. For other parts of the body, washings/aspirates, swabs of the infected area, fluids and/or tissue samples may be collected.
Because of their unique cell wall, all species of mycobacteria will appear as acid fast bacteria (AFB) when a smear of the patient's specimen is treated with a special stain and examined under the microscope. Positive AFB smears are presumed to be potential TB infections until more laboratory data is available.
AFB cultures are performed on samples that have been decontaminated of other bacteria, digested of mucus, and concentrated to increase the ability to detect them in the culture. Nutrients and incubation at appropriate temperature provide a supportive environment for the slow growing mycobacteria. The results of cultures are definitive: they can tell your doctor what organisms are present and what drugs are likely to kill them, but they take time - days to several weeks for positive samples. Cultures are held for six to eight weeks before being reported as negative. M. leprae cannot be detected with this method. It is diagnosed primarily through clinical signs as this species will not grow on culture media.
Once the mycobacteria species has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.
AFB smears and cultures are the primary methods used to detect NTM infections. Other more rapid methods, such as the molecular detection of the organism’s genetic material (DNA/RNA), may be performed on the primary specimen and also used as a means to identify the species of mycobacteria once the bacteria are grown in culture. Characterization of the mycolic acids (cells wall constituents) may also be used to identify the species of mycobacteria.
Non-Laboratory Tests
X-rays may be ordered to look for changes caused by a mycobacterial infection. NTM infections (and TB infections) can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys.
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