To screen for tuberculosis (TB) active or latent infection
If you have been exposed to a person with TB; if you have a clinical condition or risk factor that makes progression to active TB more likely
A blood sample is drawn by needle from a vein in your arm
Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. TB primarily targets the lungs but may affect any area of the body such as the urinary tract, central nervous system, bones, joints, and/or other organs. An interferon gamma release assay (IGRA) blood test screens for exposure to TB by indirectly measuring the body's immune response to antigens derived from these bacteria.
TB may cause an inactive (latent) infection or an active, progressive disease. The immune system of about 90% of people who become infected with TB manages to control its growth and confine the TB infection to a few cells in the body. The bacteria in these cells are inactive but still alive. The person does not have any symptoms and is not infectious but does have a "latent TB infection."
If, after some time, the immune system of an individual with an inactive infection becomes weakened (compromised), the mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. Active TB does cause illness and can be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, talking, singing, or breathing.
The IGRA blood test can detect M. tuberculosis infections, but cannot distinguish between latent and active infections. Additional tests, such as AFB testing, are required to help establish a diagnosis of an active TB infection.
IGRAs are not used as general population screens but are used to screen people who are at high risk for tuberculosis (TB), such as:
Either an IGRA or a tuberculin skin test (TST) may be performed to screen for TB, but in most cases the IGRA is now the preferred test.
The IGRA test measures the release of a substance called gamma interferon by white blood cells in a sample of blood when the cells are exposed to specific TB antigens. The IGRA test is not performed by all laboratories. The test requires viable white blood cells, so the IGRA blood sample must be received and tested by a laboratory within a designated window of time.
Recommendations from the Centers for Disease Control and Prevention (CDC), the Infectious Diseases Society, of America (IDSA), and the American Thoracic Society (ATS) list a preference for an IGRA test when:
The TST test is considered an acceptable alternative to the IGRA if the IGRA is not available or is considered too costly or burdensome.
These same agencies do NOT recommend testing people who are not likely to be infected with TB, or those who are considered at a low risk for TB infection and disease progression.
However, when testing for latent TB infection is required, such as for employment or a legal requirement, they suggest:
IGRA testing may be ordered:
A positive IGRA test result means that the person is likely to have been exposed to TB and the person may have a latent or active TB infection. If a healthcare practitioner suspects that someone has active tuberculosis, a history and physical examination and other tests, such as chest X-rays and AFB laboratory testing, are used to confirm the diagnosis.
A negative result means that it is likely that the person tested does not have a TB infection. However, it does not entirely rule out tuberculosis. It may mean that it is too early to detect exposure. It takes about 6 weeks after infection before a person demonstrates a positive reaction to an IGRA. If suspicion of TB remains high and a healthcare practitioner wants to confirm a negative or indeterminate result, the practitioner may repeat the IGRA or do a TST as an alternate follow-up test.
Occasionally, a person infected with or exposed to other Mycobacterium species, for example Mycobacterium kansasii, will give a false-positive IGRA result for TB. Positive results must be followed up by other tests such as chest X-rays to look for signs of active TB disease. If active TB disease is suspected, AFB testing including smears and cultures and sensitivity testing, may be used to confirm the diagnosis and determine the drug susceptibility for the M. tuberculosis infecting the person.
Positive TST results are commonly seen in those who have received a BCG (Bacille Calmette-Guérin) vaccination. IGRA results are not affected by BCG.
You may be tested under your healthcare practitioner's supervision if there is a need to do so. Since TB can be passed from mother to child during pregnancy, if you are at an increased risk of contracting TB, your healthcare practitioner may want you to have TB screening done. Both the IGRA and TST are considered safe during pregnancy.
Sources Used in Current Review
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