TB Quantiferon (IGRA)

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The Interferon Gamma Release Assay (IGRA) is a blood test used to detect tuberculosis (TB) infection. Unlike the traditional TB skin test, the IGRA measures the immune response to TB bacteria by detecting interferon-gamma, a protein released by white blood cells in response to TB antigens. The IGRA is a two-step process in which a blood sample is drawn and then analyzed in a laboratory. The test is highly specific and can distinguish between TB infection and vaccination with the Bacille Calmette-Guérin (BCG) vaccine, which can cause false-positive results with the skin test. The IGRA is recommended for people who have received the BCG vaccine and for those at high risk of developing active TB disease, such as healthcare workers, immigrants from high-prevalence countries, and people living with HIV/AIDS.

Awaiting result:

One day

Collect material:

Blood

Prepare:

Learn more

Purpose of the test

The interferon-gamma release assay (IGRA) is a valuable tool for diagnosing tuberculosis (TB) infection and disease. The test is used to screen people at high risk for TB, such as healthcare workers, immigrants from high-prevalence countries, and people living with HIV/AIDS. The IGRA may also be used to aid in the diagnosis of TB disease in patients who have symptoms or radiographic evidence suggestive of TB.

The primary goals of the IGRA test are:

1. To detect TB infection: The IGRA is a highly specific test that can differentiate between TB infection and Bacille Calmette-Guérin (BCG) vaccination, which can cause false-positive skin test results. The IGRA measures the immune response to TB bacteria by detecting interferon-gamma, a protein released by white blood cells in response to TB antigens. A positive result indicates that a person has been infected with TB bacteria at some point in their life.

2. To diagnose TB disease: In patients with symptoms or radiographic evidence suggestive of TB, IGRA may be used as an aid in diagnosing active TB disease. A positive result indicates that M. tuberculosis infection is likely; a negative result indicates that infection is unlikely.

3. To monitor treatment: IGRA can be used to monitor the effectiveness of treatment for active TB disease. A decrease in interferon-gamma levels over time indicates that treatment is working.

Identifying TB infection and disease early is critical for effective treatment and control of the disease. If left untreated, TB can cause serious health problems and even death.

Some potential consequences of not having the IGRA test include

- Delayed diagnosis: Without early detection, patients may not receive appropriate treatment until their condition has worsened.
- Increased risk of transmission: People with undiagnosed or untreated active TB disease can spread the infection to others.
- Complications: TB can cause serious complications, including meningitis, bone and joint infections, and kidney failure.

The IGRA test is an important tool for diagnosing TB infection and disease. It can help identify people at high risk for TB and aid in the diagnosis of active TB disease. Early detection is key to effective treatment and control of the disease and can help prevent serious health problems and complications.

Who Should Get Tested

The Interferon Gamma Release Assay (IGRA) is recommended for people at high risk for tuberculosis (TB) infection or disease. This includes

- Healthcare workers: Healthcare workers are at increased risk for TB due to frequent exposure to patients with the disease.
- Immigrants from high-prevalence countries: People who have immigrated from high-prevalence countries, such as India, China, and Mexico, are at increased risk for TB.
- People with HIV/AIDS: HIV weakens the immune system and increases the risk of developing active TB disease.
- Close contacts of people with active TB disease: People who have been in close contact with someone who has active TB disease are at increased risk of infection.

IGRA may also be recommended for

- People who have received the Bacille Calmette-Guérin (BCG) vaccine: The BCG vaccine can cause false-positive results with the traditional TB skin test, but not with the IGRA. Therefore, IGRA is recommended for people who have received the BCG vaccine.
- People who cannot return for a second appointment: The traditional TB skin test requires two appointments, while the IGRA requires only one blood draw. Therefore, IGRA may be more convenient for people who cannot return for a second appointment.

Note that not everyone needs to be tested for TB infection or disease. Your healthcare provider can help determine if you are at increased risk and whether testing is necessary.

Blood Test Preparation Guidelines

Time of day

It is recommended to schedule your appointment for blood tests in the morning hours between 7:00-10:00.

Fasting

It is recommended to fast for approximately 12 hours before blood sampling. The last meal of the previous day should ideally be consumed around 6:00 p.m. On the day before the test, avoid heavy and fatty meals as well as alcohol.

Stay hydrated

Drinking water prior to testing can help with sample collection.

Other factors

Tests should not be performed after a sleepless night or intense physical activity. It is recommended to avoid exercise and stress immediately before blood collection and to not smoke. A short rest is recommended.

Medications and supplements:

Blood samples should be collected before the morning dose. Some drugs can interfere with test results. Consult with your doctor whether you can delay your dose because of lab tests.

Biotin supplements:

High doses of biotin supplements can affect test results, causing false elevation or reduction. It's recommended to avoid taking biotin for at least 72 hours before blood collection. If you are taking biotin, inform the personnel collecting the blood so that they can provide specific instructions.

Interpreting Test Results

Interferon gamma release assay (IGRA) test results are reported as positive, negative, or indeterminate. Interpretation of the test results is based on the amount of interferon-gamma released or the number of cells releasing interferon-gamma. Both the standard qualitative test interpretation and the quantitative assay measurements should be reported.

- Positive result: A positive result indicates that M. tuberculosis infection is likely. The cutoff for a positive result varies depending on the specific IGRA assay used, but generally ranges from 0.35 to 0.7 IU/mL for interferon-gamma concentration or 6 to 10 spot-forming cells (SFCs) per well for spot count.
- Negative result: A negative result indicates that M. tuberculosis infection is unlikely. However, a negative result does not completely rule out TB infection, especially in people with weakened immune systems.
- Indeterminate result: An indeterminate result means that the test could not give a clear answer due to technical problems or problems with the blood sample. Indeterminate results can occur in up to 10% of tests and may require retesting.

You should be aware that multiple negative results from any combination of TB tests cannot rule out M. tuberculosis infection, especially in people with ongoing exposure or symptoms suggestive of TB disease.

Your healthcare provider will interpret your IGRA test results along with other clinical information and may recommend further testing or treatment as needed.

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