Tuberculosis (TB) is a bacterial infection that is caused by Mycobacterium Tuberculosis and is a highly infectious disease. The bacteria primarily target the lungs but can affect other areas of the body, such as the central nervous system, urinary tract, bones, joints, and other bodily organs. A blood test called the Interferon Gamma Release Assay (IGRA) measures the immune response to antigens that are derived from the bacteria in order to screen for TB.
TB may either result in an active, progressive disease or inactive (latent) infection. In about 90% of people who have been infected, the immune system can control the spread of the disease and restrict it to just a few cells in the body. While the bacteria in these cells are inactive, they are still alive. This is referred to as an inactive or latent TB infection, and the person will present with none of the symptoms of TB and cannot transmit the disease.
However, should the immune system become compromised or weakened for any reason, the bacteria may become active, causing the mycobacterium to start growing again. This will result in TB that becomes symptomatic of the illness and is infectious, which means that it can be transmitted to others via respiratory secretions such as moisture, aerosols, and sputum that are expelled from the body when coughing, sneezing, breathing, talking, laughing or singing.
Although the IGRA blood test can accurately detect Tuberculosis, it is unable to differentiate between latent or inactive TB and active infections. Additional testing may be required to make this distinction, such as the AFB test for TB infections.
The QuantiFERON® (QFT-Plus) TB Gold Plus, formally known as the IGRA TB Test or Interferon Gamma Release Assay.
This test is not commonly used to screen general population groups but rather to test individuals who present with high-risk factors for TB, including:
- Diseases that compromise the immune system like HIV and AIDS, which places them at a higher risk of contracting an active TB infection.
- Cramped living conditions like prisons, schools, migration camps, homeless shelters, etc. that increase exposure and vulnerability to the disease.
- People working in healthcare or other occupations that may expose them to carriers of active TB.
- Any person who has come into close contact with a person/s who have a confirmed case of active TB.
- People who have traveled to live in or spent extended periods in foreign countries or areas where TB is prevalent.
- People who inject illegal substances or drugs.
The IGRA test has become the preferred means if diagnosing TB, but the Tuberculin Skin Test (TST) may still be performed to screen for TB.
The IGRA test uses a blood sample to measure the amount of gamma interferon that is released by blood cells when they are exposed to TB antigens. Select laboratories only perform the test as viable white blood cells are required for accurate testing, which must be tested within a specified time frame.
The CDC (Centers for Disease Control and Prevention) in conjunction with the American Thoracic Society and the Infectious Diseases Society has compiled a list of recommendations for preferred IGRA testing if:
- A person is not likely to return for their TST to be evaluated.
- A person is five years of age or younger.
- Has a high probability of being infected with latent TB.
- Has little to no risk of becoming immuno-compromised, resulting in the development of an active TB infection.
- There is evidence that warrants a Latent Tuberculosis Infection Test.
- If the person has received the BCG (Bacille Calmette-Guérin) vaccine, which could interfere with the TST interpretation and results. BCG is not commonly administered in the U.S.A. unless for the treatment of some forms of cancer. It is, however, used in countries that do have a high prevalence of TB.
- If the IGRA test is unavailable or when the TST test is determined to be a more suitable option due to cost or other extenuating circumstances.
Under the following circumstances, these agencies do NOT recommend testing for Tuberculosis in those who are unlikely to be infected, people who are at low risk of contracting the disease, or where the disease is not progressing.
If latent TB testing is a requirement for any reason, such as for employment or legal reasons, the following is suggested:
- IGRA testing for those who are five years and older.
- A second test (either IGRA or TST) to confirm a positive result from the first test. Only if both tests are positive can the person be positively diagnosed with TB.
In addition, IGRA testing can be ordered:
- Annually for persons at high risk either because they have the disease and are prone to a weakened immune system or because they live and/or workaround others that are at high risk.
- Before a person joins a group or population that is at high risk like healthcare workers.
- When a person has been exposed to a person with an active infection. This test should only be administered a few weeks after exposure. It can take about six weeks from the time of exposure and initial infection before a positive result can be detected in the blood.
- A person who has lived for an extended period in a country where TB is prevalent.
- A person who presents signs and symptoms of tuberculosis, including chronic cough producing sputum or phlegm that may contain bloody streaks, fever, chills, night sweats, and inexplicable weight loss.
Understanding the TB Test Results
If the IGRA test result is positive, it means that the person has been exposed to TB and that they may have either an active TB infection or a latent TB infection. If there is any suspicion that the infection may be active, the healthcare practitioner must take a full medical history, provide a thorough medical examination and perform any other relevant tests such as a chest x-ray, AFB laboratory testing, and any other tests to confirm the diagnosis.
If the IGRA test result is negative, it is unlikely that the person has been infected with Tuberculosis. However, there is still a possibility that a TB infection may be present. The test may have been performed too early after exposure or infection to detect the infection. A person should only be tested about six weeks after possible exposure or infection in order to get a positive reaction from the IGRA test. If there is still a suspicion that a person may have contracted TB even though the IGRA result was negative, the healthcare practitioner should perform a second IGRA test later or confirm with the alternative TST test.
The IGRA test may provide a false-positive result in rare cases, especially due to exposure to the Mycobacterium species Mycobacterium Kansasii. If there is a concern of a false-positive result, chest x-rays and other tests must be performed to rule out a TB infection. If active TB is suspected, smears, cultures and sensitivity testing as part of the AFB test should be used to confirm a diagnosis as well as the susceptibility to drugs of the strain of Mycobacterium causing the infection.
Pregnant women who are at high risk may be required to be screened for Tuberculosis as the disease is transmittable from mother to child during pregnancy. Both TST and IGRA are safe for TB testing during pregnancy.