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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. Mycobacterial species sometimes colonise the lungs of patients with significant pre-existing lung damage without causing substantial disease. At present there is no laboratory test that can distinguish between colonisation and infection with an NTM.

Laboratory Tests

  • AFB smears and cultures. These are the primary methods used to detect mycobacterial infections. 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 as this is when sputum is likely to contain the most mycobacteria. For other parts of the body, washings/aspirates, swabs of the infected area, fluids and/or tissue samples (biopsy) may be collected.

Because of their unique cell wall, acid will not remove a stain called carbol fuchsin from mycobacteria. Bacteria with this unique staining property are referred to as "acid fast" (AFB) and can be detected when the stained slide is viewed under the microscope.

AFB cultures are performed on samples that have been treated to liquefy mucus and reduce contaminating bacteria. This process also concentrates any AFB in the sample to enhance recovery of organisms in culture. Nutrients and incubation at appropriate temperature provide a supportive environment for the slow-growing mycobacteria. The results of positive cultures tell the doctor what organisms are present and may provide some guidance on appropriate drugs to treat an infection. However, as mycobacteria are slowly growing compared to other bacteria (5-20 hours for each bacteria to divide, compared to 20 minutes for Escherichia coli), results from mycobacterial culture take time — usually several weeks for final results. Cultures are held for six to eight weeks before being reported as negative. 

M. leprae and M. lepromatosis are infrequently detected by culture as these species will not grow on culture media. They are diagnosed primarily through clinical signs and AFB stain of split skin smears. In specialised circumstances PCR tests may be performed to identify these bacteria from samples submitted for testing, or the organism may be cultured in tissues of the nine-banded armadillo, or in the footpads of particular strains of immunocompromised mice.

Once the mycobacteria species has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.

  • Susceptibility testing. This may be performed. The treatment of NTM infections is difficult and for many infections susceptibility testing can only act as a guide to therapy.
  • Molecular tests. 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.

Non-Laboratory Tests
X-rays or CTs 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.


Last Review Date: June 7, 2017