The diagnosis of nontuberculous mycobacteria (NTM) disease can be complex. Since nearly everyone is exposed to NTM, the goals of testing are to distinguish between evidence of previous exposure to NTM and a true NTM infection that requires treatment. According to a joint statement published by the American Thoracic Society and the Infectious Diseases Society of North America, a workup for patients suspected of having NTM lung disease should include, at minimum, the following:
- Chest x-ray and/or high-resolution CT scan
- Three of more sputum samples for acid-fast bacilli (AFB) analysis
- Exams and tests to rule out other disorders, such as tuberculosis
Mycobacterial testing — these are the primary methods used to detect NTM infections and to rule out tuberculosis as a possible cause of a person's symptoms. (See the article on acid-fast bacilli (AFB) testing.) The sample(s) collected for analysis depend on the part(s) of the body that the healthcare practitioner suspects are infected. For suspected lung infections, three or more sputum samples are collected early in the morning on different days. If the affected person is unable to produce sputum, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. For suspected infections in other parts of the body, washings/aspirates, swabs of the infected area, fluids and/or tissue samples (biopsy) may be collected for testing. Testing may include:
- AFB smear — a microscopic examination of a specimen that has been stained to detect acid-fast bacteria, such as NTM organisms. This test can provide probable (presumptive) results within a few hours. It is a valuable tool in helping make decisions about treatment while culture results are pending.
- Molecular tests for NTM (e.g., nucleic acid amplification test, NAAT) — detect the genetic components of NTM organisms and are often done when the AFB smear is positive or NTM infection is highly suspected. Like AFB smears, molecular tests can provide a presumptive diagnosis, which can aid in the decision of whether to begin treatment before culture results are available. Results of molecular tests are typically available several hours after a sample is collected.
- AFB cultures to grow the bacteria are set up at the same time as the AFB smears. Though more sensitive than AFB smears, results of cultures may take days to several weeks.
- Susceptibility testing on the acid-fast bacteria grown in the cultures that are positive will determine the bacteria's susceptibility or resistance to drugs most commonly used to treat NTM infections.
Once the mycobacteria species has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.
X-rays may be ordered to look for changes caused by a mycobacterial infection. NTM infections (and Mycobacterium tuberculosis infections) can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys. High-resolution 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.