About the Test
Purpose of the test
A TB blood test is used to measure a person’s immune response to the bacteria that causes TB.
A TB blood test is used as a screening tool for TB infection and as a diagnostic tool for TB disease.
TB infection is when a person is carrying TB bacteria, but their immune system is able to keep the infection under control. It is also called latent tuberculosis. People with TB infection do not show symptoms of TB and can’t spread it to others. However, if left untreated, it may develop into tuberculosis disease.
TB disease is when a person’s immune system is not able to control the infection of TB bacteria; it is sometimes called active tuberculosis. People with TB disease usually show TB symptoms and can spread TB to others.
A TB blood test can be used as a screening test for TB infection or as part of a diagnostic work-up for TB disease.
- Screening for TB infection: Screening for TB infection means testing a person without any symptoms to check if their immune system shows signs of a reaction to Mycobacterium tuberculosis. Screening may be recommended when someone has a high risk of being exposed to TB and would benefit from treatment if a TB infection is detected.
- Diagnosing TB disease: Diagnostic tests are used when a patient has symptoms of tuberculosis. A positive TB test supports a diagnosis of TB disease. In addition to the results of a TB test, doctors consider a patient’s medical history and the results of a physical exam, imaging, and other lab tests to diagnose TB disease.
A TB blood test can only show that a person has immune system reactivity to Mycobacterium tuberculosis. Additional tests are required to determine if the person meets the criteria for a diagnosis of TB infection or TB disease.
What does the test measure?
A TB blood test measures the amount of a specific protein called interferon-gamma (IFN-g) that is produced by a person’s T cells. T cells are a kind of white blood cell and are part of the body’s immune system.
To conduct a TB blood test, a sample of a person’s blood is mixed with deactivated parts of the TB bacteria called antigens. The blood sample is placed under conditions to promote immune cell activity. At the end of the test, the amount of IFN-g is measured. The level of IFN-g corresponds with the amount of immune system activity in response to the TB antigens. Higher levels of IFN-g suggest that previous exposure to Mycobacterium tuberculosis is likely.
The TB blood test is one of two types of tests used to detect TB. The other type of test is a TB skin test.
When should I get an IGRA TB test?
A TB blood test is recommended if your health history, living situation, or working conditions put you at risk for exposure to TB. These risk factors can include:
- Being close contact with someone who has TB
- Being born in or travelling to parts of the world where TB is more common, such as Mexico, the Philippines, or India.
- Living or working in a large group setting such as a health care facility, nursing home, homeless shelter, or correctional facility.
- Having certain health conditions such as diabetes, prior organ transplantation, intravenous drug use, or HIV/AIDS.
If you have been experiencing symptoms of TB, a doctor may order a TB blood test to assist with diagnosing TB disease. TB may cause the following symptoms:
- A persistent cough that has lasted for three weeks or longer.
- A cough that produces blood or mucus.
- Chest pain
- Fatigue
- A lack of appetite and/or weight loss.
- A fever, chills, or night sweats.
A TB blood test is preferred over a TB skin test for certain people, including those who have been vaccinated with the bacille Calmette-Guérin (BCG) TB vaccine and those who may be unable to attend multiple appointments as required for a TB skin test.
Finding an IGRA TB Test
How to get tested
A TB blood test is usually ordered by a doctor and can be completed at a hospital, clinic, doctor’s office, or health department. Getting a TB blood test may be required if you work in a health care setting or a childcare center.
Can I take the test at home?
A TB blood test cannot be completed at home.
How much does the test cost?
The cost of a TB blood test may vary depending on the circumstances. If an employer is requiring a TB blood test to meet certain health and safety regulations, the cost of the test may be covered by the employer.
In other situations, asking a patient’s insurance provider or doctor about the specific costs, copays, and deductibles may be helpful.
Taking an IGRA TB Test
A TB blood test is performed by a health care professional who completes a blood draw and sends the blood samples to a laboratory for incubation and analysis.
A TB blood test takes a single visit to complete, and the test results are typically available within 24 to 48 hours after the blood sample is collected.
Before the test
A TB blood test does not require any preparation beforehand. If you are someone who is uncomfortable with the sight of blood or blood being drawn, you may wish to let the professional collecting the sample know.
During the test
A TB blood test requires a sample of blood, which is typically taken from a vein in the forearm. A nurse or phlebotomist will clean the skin with an alcohol wipe to sanitize the blood draw site. A band made of stretchy or elastic material is placed around the upper arm to help make the vein more accessible and restrict blood flow. A small needle is inserted into the vein, and a sample of blood is collected into up to 3 vials.
After the sample is collected, an adhesive bandage or cotton swab and medical tape is placed over the puncture to protect and keep the site clean; and the elastic band is removed. The initial needle stick and needle removal may cause a few moments of discomfort and stinging.
After the test
If you are feeling queasy, faint, or light-headed after the blood draw, you should let the health care professional know. Otherwise, you can continue with your normal daily activities. If you notice prolonged pain, swelling, or bleeding from the blood draw puncture site, you should report this to the health care professional as soon as possible.
IGRA TB Test Results
Receiving test results
The results of the TB blood test may be available within 24 to 48 hours after taking the blood sample. They may share the test results over the phone, through a secure messaging platform, or at a follow-up appointment.
Interpreting test results
The results from a TB blood test are typically reported in one of three ways:
- A positive result means that there was an immune reaction to the TB bacteria and that infection is likely.
- A negative result means that there was no immune reaction and that TB infection is not likely.
- An uninterpretable result means that the test result was indeterminate, invalid, or borderline.
TB blood test results may also include a number reflecting the specific level of interferon-gamma produced by the immune cells in the blood sample.
TB blood tests do not diagnose TB disease on their own. TB blood tests can only show that a person has been infected by Mycobacterium tuberculosis bacteria. Doctors consider other factors, such as the patient’s health history, symptoms, and other test results to establish a diagnosis of TB disease.
Are test results accurate?
TB blood tests are reliable and widely used. False positives, in which a person tests positive despite not having an infection, are uncommon in TB blood tests.
TB blood tests do have limitations and can produce false negatives, in which a person has a negative test result despite having a TB infection. This can occur in individuals with immunosuppressive conditions, including: lHIV infection, known TB disease, or anergy, which is a lack of a typical immune response.
The timing of the test can also affect the accuracy of results. If a person is tested too soon after their exposure to tuberculosis bacteria, they may have a false negative result. Experts recommend testing at least 6 to 8 weeks after the exposure to TB.
Do I need follow-up tests?
If a TB blood test shows a positive result, follow-up tests and other clinical assessments are used to determine whether TB infection has progressed to active TB disease. Follow-up care usually includes a review of a person’s health history, a physical exam, and a chest x-ray.
If symptoms of TB disease are present, additional diagnostic testing may be ordered to check for the presence of TB bacteria in different parts of the body. A sputum culture can test phlegm from the lungs. Urine or tissue samples may also be collected, if there are signs that TB is affecting other organs.
Questions for your doctor about test results
A conversation with your health care provider can help you understand your TB test results. You may wish to ask the following questions:
- What does the result of my TB blood test mean?
- Do more tests need to be done because of this result?
- Do any factors such as my age, lifestyle, or overall health help explain this test result?
- Are there any precautions I need to take following this test result?
Comparing and contrasting an IGRA TB test and a TB skin test
The IGRA TB test and the TB skin test are both used to check for a TB infection. While the IGRA test uses a blood sample, a TB skin test works by injecting a small amount of TB testing solution, called Tuberculin, just under the skin of the forearm and checking for a localized immune reaction. This test does not require any samples, but does require a follow-up visit within a 48 to 72 hour window to obtain an accurate reading of the results.
While the TB skin test and TB blood test are both effective, factors such as a patient’s age, overall health, TB vaccination status, along with test cost and availability are taken into consideration when determining which test is appropriate for the patient’s situation.
