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Nontuberculous Mycobacteria

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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.

Laboratory Tests

  • AFB smears and cultures. These are the primary methods used to detect NTM 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 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 (biopsy) may be collected.

    Because of their unique cell wall, all species of mycobacteria will resist decolorizing with acid during special staining procedures. 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 are definitive; they can tell the 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.

  • Susceptibility testing. This may be performed to determine which antimicrobial drugs will be most effective in treating the infection.
  • 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 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. Chest CT imaging may be ordered and has been found to have greater sensitivity for detecting bronchiectasis and cavities than chest x-ray. People with positive findings should undergo further evaluation, which may include bronchoscopy and lung biopsy.

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