TB Screening (Whole Blood)
Does this test have other names?
Interferon-gamma release assay (IGRA), Quantiferon test, T-spot
What is this test?
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:
Why do I need this test?
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.
What other tests might I have along with this test?
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.
What do my test results mean?
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
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
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
How is this test done?
This test requires a blood sample, which is drawn through a needle from a vein in
Does this test pose any risks?
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.
What might affect my test results?
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.
How do I get ready for this test?
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.