What is TB Gamma Interferon - Quantiferon?

Also Known As: TB Blood Test, Interferon-Gamma Release Assay, QuantiFERON®-TB Gold Plus (QFT-Plus), T-SPOT®.TB test (T-Spot)

Formal Name: Interferon Gamma Release Assay

Mycobacterium tuberculosis is the bacteria responsible for the infectious disease known as tuberculosis (TB). It spreads easily from person to person, mainly through coughing and sneezing. Exposure to Mycobacterium tuberculosis can cause tuberculosis infection, which may lead to tuberculosis disease. TB mainly injures the lungs but also damages other vital organs like the brain, spine and kidneys.

TB blood tests, or interferon-gamma release assays (IGRAs), are used to check for TB infection; they are also used to assist with diagnosis when someone is showing signs of TB disease. A TB blood test uses a sample of blood to look for signs of an immune reaction to Mycobacterium tuberculosis.

A TB blood test is used to measure a person’s immune response to the bacteria that causes TB.
A TB blood test is used as a screening tool for TB infection and as a diagnostic tool for TB disease.

TB infection is when a person is carrying TB bacteria, but their immune system is able to keep the infection under control. It is also called latent tuberculosis. People with TB infection do not show symptoms of TB and can’t spread it to others. However, if left untreated, it may develop into a tuberculosis disease.

TB disease is when a person’s immune system is not able to control the infection of TB bacteria; it is sometimes called active tuberculosis. People with TB disease usually show TB symptoms and can spread TB to others.
A TB blood test can be used as a screening test for TB infection or as part of a diagnostic work-up for TB disease.

• Screening for TB infection: Screening for TB infection means testing a person without any symptoms to check if their immune system shows signs of a reaction to Mycobacterium tuberculosis. Screening may be recommended when someone has a high risk of being exposed to TB and would benefit from treatment if a TB infection is detected.
• Diagnosing TB disease: Diagnostic tests are used when a patient has symptoms of tuberculosis. A positive TB test supports a diagnosis of TB disease. In addition to the results of a TB test, doctors consider a patient’s medical history and the results of a physical exam, imaging, and other lab tests to diagnose TB disease.

A TB blood test can only show that a person has immune system reactivity to Mycobacterium tuberculosis. Additional tests are required to determine if the person meets the criteria for a diagnosis of TB infection or TB disease.

What does the test measure?

A TB blood test measures the amount of a specific protein called interferon-gamma (IFN-g) that is produced by a person’s T cells. T cells are a kind of white blood cell and are part of the body’s immune system.

To conduct a TB blood test, a sample of a person’s blood is mixed with deactivated parts of the TB bacteria called antigens. The blood sample is placed under conditions to promote immune cell activity. At the end of the test, the amount of IFN-g is measured. The level of IFN-g corresponds with the amount of immune system activity in response to the TB antigens. Higher levels of IFN-g suggest that previous exposure to Mycobacterium tuberculosis is likely.

The TB blood test is one of two types of tests used to detect TB. The other type of test is a TB skin test.

When should I get an IGRA TB test?

A TB blood test is recommended if your health history, living situation, or working conditions put you at risk for exposure to TB. These risk factors can include:

• Being close contact with someone who has TB
• Being born in or traveling to parts of the world where TB is more common, such as Mexico, the Philippines, or India.
• Living or working in a large group setting such as a health care facility, nursing home, homeless shelter, or correctional facility.
• Having certain health conditions such as diabetes, prior organ transplantation, intravenous drug use, or HIV/AIDS.

If you have been experiencing symptoms of TB, a doctor may order a TB blood test to assist with diagnosing TB disease. TB may cause the following symptoms:

• A persistent cough that has lasted for three weeks or longer.
• A cough that produces blood or mucus.
• Chest pain
• Fatigue
• A lack of appetite and/or weight loss.
• A fever, chills, or night sweats.

A TB blood test is preferred over a TB skin test for certain people, including those who have been vaccinated with the bacilli Calmette-Guérin (BCG) TB vaccine and those who may be unable to attend multiple appointments as required for a TB skin test.