Tests
Laboratory Tests
Latent TB infection
Testing for
Mycobacterium tuberculosis may begin with a
TB skin test for latent TB infection. This is not used as a general screen but is targeted at those who are at a high risk for contracting the disease and at those who work or live with high-risk patients. The TB skin test may also be done as part of a physical examination prior to starting school or a new job. The test is performed on the patient’s skin by injecting a purified protein derivative (PPD) solution just under the skin. This provokes a hypersensitivity skin reaction (a red raised bump) in those who may have been infected by
M. tuberculosis. The reaction is evaluated by a healthcare worker at 48 or 72 hours. Positive results may indicate a latent TB infection and should be followed by other tests, such as chest X-rays, to look for signs of active disease.
The QFT-G (QuantiFERON® - TB Gold Test) is a relatively new blood test that can be used as an alternative or follow-up to the TB skin test to help diagnose a latent TB infection. It is not affected by previous QFT-G, TB skin tests, or by BCG (Bacille Calmette-Guérin) vaccination (see
Treatment section). It does not require the patient to return in 48 to 72 hours for evaluation, and there is no local skin reaction. However, the blood sample must be collected and processed for testing within 12 hours, and there is limited data on its use with children and those with suppressed immune systems. A positive QFT-G must be followed up in a similar fashion to a positive TB skin test.
Active Tuberculosis
To diagnose TB of the respiratory tract, 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. If extrapulmonary TB is suspected, samples are collected based upon where in the body the infection is likely to be. Multiple samples of gastric (stomach) washings/aspirates or urine may be collected and submitted to the laboratory. Sometimes cerebrospinal fluid (CSF), biopsied tissue, or other body fluids are also collected.
A presumptive diagnosis of TB can be made by examining a smear of the patient's specimen under the microscope after it has been treated with a special stain to detect acid fast bacteria (AFB). Positive AFB smears are likely to indicate a TB infection, since M. tuberculosis is the most common acid-fast bacillus in the lungs, but the smears cannot distinguish between the different species of "acid-fast" bacilli.
A genetic probe or molecular TB test can add additional information. It amplifies/replicates genetic components of the bacteria and can narrow the identification to a group of mycobacteria (of which M. tuberculosis is the most common). While AFB smears and genetic tests may be available the same day that the samples are submitted, both positive and negative results must be confirmed with AFB cultures.
AFB cultures are performed on respiratory samples that are decontaminated of normal respiratory bacteria, digested of mucus, and concentrated to increase the ability to detect them in the culture. Nutrients and incubation 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.
A new liquid culture method called Microscopic-Observation Drug-Susceptibility (MODS) assay is in development. This method takes only about 7 days to diagnose TB and finds the best antibiotic treatment at the same time. It can recognize the presence of mycobacteria much more quickly than routine culture and can help health care providers diagnose and treat the disease at an earlier stage. It has the potential to help control the spread of infectious TB, but the benefits and limitations of this test are still being evaluated. (See In the News: Faster lab test for tuberculosis in development.)
Once M. tuberculosis has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.
Non-Laboratory Tests
X-rays are often used as a follow-up to positive TB skin tests to look for signs of mycobacteria growth and to help determine whether someone has active tuberculosis or a latent TB infection. Infection with TB can cause a number of characteristic findings on x-rays, including cavities (holes) and
calcification in organs such as the lungs and kidneys. More information on radiological tests can be found at
RadiologyInfo.
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