Tests for Tuberculosis
Testing for Mycobacterium tuberculosis infection may begin with a TB screening test. These screening tests are not diagnostic; they do not tell whether a person has latent TB or an active infection. Additional exams and tests (see Tests to Diagnose Active TB below) must be performed in follow up to a positive screening result.
TB screening is not used as a general screening test for all people but is targeted at those who are at a high risk for contracting the disease and those who have signs and symptoms consistent with an active TB infection. TB screening may also be done as part of a physical examination prior to starting school or a new job. There are two types of tests used to screen people at high risk for TB; usually only one of the tests is performed:
- The tuberculin skin test (TST) is performed by injecting a small amount a substance called purified protein derivative (PPD) just under the skin on a person's forearm. The person must return to the healthcare provider's office 2 or 3 days after the PPD is injected so the injection site can be examined. If the person is infected with M. tuberculosis, a firm red bump will form at the injection site.
- The interferon gamma release assay (IGRA) is a blood test that measures how strongly a person's immune system reacts to specific TB antigens.
Tests to Diagnose Active TB
Acid-fast bacilli (AFB) testing can be used to identify and confirm active infection with M. tuberculosis as well as other Mycobacterium species. (Mycobacteria are called acid-fast bacilli because they are rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure in which the bacteria retain the color of the stain after an acid wash (acid-fast).) One or more of these tests may be performed when a person has signs and symptoms or TB disease or screened positive for TB infection.
Usually three 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. In children, gastric washings/aspirates may be collected. Depending on symptoms, urine, an aspirate from the site of suspected infection, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be submitted for AFB smear and culture.
- AFB smear — a microscopic examination of a specimen that has been stained to detect acid-fast bacteria, such as M. tuberculosis. 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 TB (nucleic acid amplification test, NAAT) — detect the genetic components of TB bacteria in a sputum sample and are often done when the AFB smear is positive or TB is highly suspected. Like AFB smears, NAATs can provide a presumptive diagnosis, which can aid in the decision of whether to begin treatment before culture results are available. Results of NAAT are typically available several hours after a sample is collected. One type of NAAT detects within two hours the presence of M. tuberculosis and determines if it is resistant to rifampin, one of the most commonly prescribed drugs used to treat TB. However, NAAT testing does not replace AFB cultures. All samples submitted for AFB testing should be cultured to ensure that any mycobacteria that are present are available for further testing, according to the Centers for Disease Control and Prevention.
- 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 TB. Depending on the method used, results may be available in 7 days or can take several weeks. There are molecular tests available that can also be used to detect specific genes in the DNA of the bacteria that confer resistance to certain drugs.
See the article on AFB Testing for additional details on these tests.
Other Laboratory Tests
- The adenosine deaminase (ADA) test is not a diagnostic test, but it may be used along with other tests to help determine whether a person has a TB infection in the lining of the lungs (pleurae). Pleural fluid presents a unique problem with detecting M. tuberculosis because there may be a large volume of fluid with a very low number of bacteria present. Though the ADA test is not definitive, it is a rapid test and may be elevated even when there are few bacteria present. ADA results may be used to help guide treatment until results from a culture are available.
- A culture method called microscopic-observation drug-susceptibility (MODS) assay takes only about 7 days to diagnose TB and detects bacterial resistance to antibiotics. It can recognize the presence of TB bacteria much more quickly than traditional culture and can help healthcare providers diagnose and treat the disease at an earlier stage. It has the potential to help control the spread of TB in resource-limited countries, especially those in which HIV infection and TB are prevalent.
X-rays or CT scans are often performed as a follow-up to positive tuberculin skin or IGRA tests to look for signs of Mycobacterium tuberculosis in the lungs and help determine whether a person has active tuberculosis disease 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.org.