Definition

The IGRA (Interferon Gamma Release Assay) test is a blood test used to determine whether a person has been exposed to Mycobacterium tuberculosis, the bacteria responsible for tuberculosis (TB). This test measures how the immune system responds to the bacteria. If someone has been infected with M. tuberculosis, their red blood cells will release interferon-gamma (IFN-g) when exposed to antigens from the bacteria.

The IGRA test offers several advantages:

• It requires only a single visit to perform.

• Results are available within 24 hours.

• It does not interfere with repeat tests.

• The test results are unaffected by the Bacillus Calmette-Guérin (BCG) vaccine.

However, there are some limitations to the IGRA test:

• Blood samples must be processed within 8 to 30 hours after collection to ensure accurate results.

• Errors during sample collection or handling can affect the test’s accuracy.

• The IGRA test is not suitable for certain groups, including:

  • Children under 5 years old

  • Individuals recently exposed to tuberculosis bacteria

  • People with weakened immune systems

  • The cost of the test is relatively high

Indication

The IGRA test is typically used to diagnose latent TB infection, not active TB disease.

Latent TB infection occurs when a person has TB bacteria in their body, but their immune system keeps the infection from developing into disease. People with latent TB do not show symptoms and cannot spread the bacteria to others.

Although latent TB infection is usually harmless, there is a small chance it can progress into active TB disease. This is more likely in children, older adults, and individuals with compromised immune systems. Active TB disease occurs when the bacteria begin to multiply, leading to symptoms such as fever, unexplained weight loss, fatigue, and a persistent cough.

The IGRA test is useful for investigating TB exposure, screening during pregnancy, and monitoring healthcare workers or others who require repeated evaluations for M. tuberculosis infection.

It is also the preferred test for people who have received the BCG vaccine.

Contraindication

Contraindications for blood sampling mainly concern conditions at the site where the blood is being drawn. For example, infections in the blood collection area are a contraindication because they can increase the risk of bacteria entering the bloodstream.

Other contraindications include the presence of blood vessel grafts, bruising, hardened arteries, or a history of radical breast surgery on the same side of the arm where the blood will be collected.

Preparation Before the Test

It is important to inform your doctor if you:

• Have a disease that affects your immune system, such as HIV, lymphoma, or kidney disease.

• Take medications that suppress the immune system, including steroids (such as prednisone), cyclosporine, or chemotherapy (cancer drugs).

• Have had a fever or infection in the past month, including illnesses like influenza, measles, or lung infections.

• Have received a vaccine in the past month.

• Have previously had TB disease, have been in contact with someone with TB, or have received the BCG vaccine.

These conditions can influence the results of the test.

Test Procedure

For the IGRA test, a healthcare provider will take a blood sample from a vein in your arm using a small needle. After inserting the needle, a small amount of blood will be collected into a test tube or vial. You may experience mild discomfort when the needle is inserted or removed, but the process usually takes less than 5 minutes.

To perform the IGRA test, the fresh blood sample is mixed with antigens and a control substance.

Normal and Abnormal Values

The interpretation of IGRA test results is based on the amount of interferon-gamma (IFN-g) released or the number of cells that release IFN-g. The results are classified as positive, negative, or indeterminate.

Results and Recommendations (Follow-up Tests)

A positive result suggests that you have likely been exposed to TB bacteria, indicating latent TB infection. A negative result suggests that infection is unlikely. An indeterminate result means that it is uncertain whether you have a TB infection.

IGRA results are often used alongside the tuberculin skin test (Mantoux test) to help diagnose M. tuberculosis infection. For children under 5 years old, the tuberculin skin test is preferred over the IGRA test.

Your doctor will discuss the pros and cons of each test and help determine which is best for your situation. In some cases, both the tuberculin test and IGRA may be necessary. 

Consult the Right Doctor

The diagnosis of latent TB infection should be confirmed by a general practitioner, who will rule out active TB infection. The doctor will assess symptoms, perform a chest X-ray, and, if needed, carry out a sputum or other clinical sample test to check for M. tuberculosis.

Latent TB infection can be treated with antibiotics to prevent it from developing into active TB later.

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