IGRA TB Test
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.
How is it used?
IGRAs are not used as general population screens but are used to screen people who are at high risk for tuberculosis (TB), such as:
- People with diseases or conditions that weaken the immune system, such as HIV or AIDS, which makes them more vulnerable to a TB infection
- Those who are in confined living conditions such as homeless shelters, migrant farm camps, nursing homes, schools, and correctional facilities
- Healthcare workers and others whose occupations bring them in close contact with those who may have active TB
- Those who have been in close contact with someone who has an active case of TB
- Those who come from or have lived for a period of time in a foreign country where TB may be more common
- Those who inject illegal drugs
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:
- Someone is unlikely to return to have their TST evaluated
- The person is 5 years or older
- Is likely to be infected with latent TB
- Has a low to intermediate risk of progressing to active tuberculosis
- A test for latent tuberculosis infection (LTBI) is warranted
- And/or if the person being tested has received the BCG (Bacille Calmette-Guérin) vaccine that might interfere with the interpretation of a TST. BCG is not used as a vaccine in the United States, but it is often routinely administered in countries with a higher incidence of TB and it is used in the U.S. as a treatment for some cancers.
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:
- An IGRA rather than a TST when someone is 5 years or older
- A second test, either an IGRA or TST, when the initial test is positive and to only consider the person positive if both tests are positive
- A TST rather than an IGRA when a person is less than 5 years old, healthy, and the TB screening is warranted
When is it ordered?
IGRA testing may be ordered:
- On a yearly basis for those who are part of a high-risk group, either because they have a disease that weakens their immune system or because they work or live around others in high-risk groups
- Prior to a person joining an at-risk population, such as healthcare workers
- When someone has been in close contact with someone who has an active case of TB; this would be done a few weeks after a suspected exposure as it usually takes about 6 weeks after contact and initial infection before a positive result would be detected.
- When an individual has lived for an extended time in a country where TB is common
- When a person has signs and symptoms of TB, such as a chronic cough that produces phlegm or sputum, sometimes with bloody streaks, fever, chills, night sweats, and unexplained weight loss
What does the test result mean?
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.
Is there anything else I should know?
Should I get a tuberculosis screening test if I am pregnant?
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.