Interferon-gamma release assay (IGRA), Quantiferon test, T-spot
This test finds out whether you have been infected with tuberculosis (TB), a highly contagious bacterial infection spread through the air. It's possible to have inactive (latent) TB and not feel sick, or you can have active TB disease with symptoms. People with latent TB are not contagious.
This test is more accurate and more specific than skin tests for TB. Results are available within 24 hours. Also, you can have this screening test if you have been vaccinated against TB. The TB vaccine is called BCG (Bacille Calmette-Guerin). The skin test is not advised if you've been vaccinated.
Two whole blood TB tests known as interferon gamma release assays (IGRAs) have been approved by the FDA and are available in the U.S. for TB screening. They are:
QuantiFERON® TB Gold In-Tube test (QFT-GIT)
T-SPOT® TB test (T-Spot)
You might need this test if you have recently been exposed to someone who has TB or if your healthcare provider suspects you may have a TB infection.
Symptoms of TB include:
Unexplained weight loss
Coughing up blood
TB usually affects the lungs, but it can spread to other parts of your body, including your joints, spine, brain, and kidneys, and cause additional symptoms.
You also might have this test if you:
Have HIV or another disease that weakens your immune system
Use illegal drugs
Live or work in a place with a higher rate of TB infection. This may be a prison or some nursing homes.
Need to start a medicine or medicines that suppress your immune system
Recently emigrated from areas where TB is more common, such as some Eastern European or Latin America countries
If you are a healthcare worker, you might have this test periodically as part of your facility's infection control program.
If you test positive for TB, your healthcare provider will probably order a chest X-ray, sputum smear, and TB culture to find out if you have active or latent TB.
You may also be tested for HIV after a positive TB test.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.
A positive IGRA test suggests you may have one of two types of TB:
Inactive (latent) TB. This means TB bacteria are present in your body, but are not active and are not causing symptoms. You are not contagious, although it's possible for you to develop TB in the future.
Active TB or TB disease. This means TB bacteria are active in your body, and you are contagious.
A negative test means that a TB infection (either active or inactive) is unlikely. But you may get negative results if you have very advanced TB. This is because in later stages the disease can suppress the immune reaction, causing the IGRA test to be positive.
This test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
Your test results may be affected if you have tumors that require treatment with medicines that suppress your immune system. Your results may also be affected if you have HIV, AIDS, or another blood disorder.
You don't need to prepare for this test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.