Health Encyclopedia

TB Screening (Whole Blood) 

Does this test have other names?

Interferon-gamma release assay (IGRA)

What is this test?

This test finds out whether you have been infected with tuberculosis (TB), a highly contagious bacterial infection. It's possible to have latent or inactive TB and not feel sick, or you can have active TB disease with symptoms.

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 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)

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 doctor suspects you may have a TB infection.

Symptoms of TB include:

  • Cough

  • Fever

  • Night sweats

  • Weight loss 

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

  • Recently emigrated from areas where TB is more common, such as some Eastern European or Latin America countries

If you are a health care 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 doctor will probably order a chest X-ray, sputum smear, and TB culture to confirm the results. 

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 health care provider.

A positive IGRA test suggests you may have one of two types of TB:

  • Latent TB or inactive 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. However, you may get negative results if you have very advanced TB because in later stages the disease can suppress the immune reaction, causing the IGRA test to be positive.

Your results may also be affected if you have HIV, AIDS, or another blood disorder.

How is this test done?

This test requires a blood sample, which is drawn through a needle from a vein in your arm.

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 drugs that suppress your immune system.

How do I get ready for this test?

You don't need to prepare for this test.  Be sure your doctor 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.