TB Screening (Whole Blood) 

Does this test have other names?

Interferon-gamma release assay (IGRA), QuantiFERON test, T-spot, T-SPOT

What is this test?

This test shows if you have been infected with tuberculosis (TB). TB is a very contagious bacterial infection that is spread through the air. It's possible to have inactive (latent) TB and not feel sick or have noticeable symptoms. 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 ready in 24 hours. Also, you can have this screening test if you have been vaccinated against TB. The TB vaccine is called bacille Calmette-Guérin (BCG). The skin test is not advised if you've been vaccinated. The skin test can lead to a false-positive test result in people who have had the TB vaccine. The vaccine will not lead to a false-positive IGRA test result.

There are two whole blood TB tests known as interferon gamma release assays (IGRAs) that 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 may need this test if you have recently been exposed to someone who has TB, or if your healthcare provider thinks you may have a TB infection.

Symptoms of TB may include:

  • Cough

  • Fever

  • Night sweats

  • Unexplained weight loss 

  • Coughing up blood

  • Chest pain

  • Fatigue

  • Shortness of breath

TB usually affects the lungs. But it can spread to other parts of your body, such as your joints, spine, brain, and kidneys, and cause more symptoms.

You also may 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, such as a prison or nursing home

  • Need to start a medicine that suppresses your immune system

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

  • Are a healthcare worker and need this test 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, you will likely also need 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?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

A positive IGRA test means 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.

How is this test done?

The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand.

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be 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 illegal drugs you may use.

Online Medical Reviewer: Chad Haldeman-Englert MD
Online Medical Reviewer: Raymond Turley Jr PA-C
Online Medical Reviewer: Tara Novick BSN MSN
Date Last Reviewed: 9/1/2022
© 2000-2024 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.