At a glance
Also known as
Purified Protein Derivative; PPD; Mantoux; Latent Tuberculosis Infection Test; Interferon-gamma Release Assays; IGRA; T-Spot®.TB; QuantiFERON®-TB Gold (QFT-G); QuantiFERON®-TB Gold In-Tube (QFT-GIT); QuantiFERON®-TB Gold Plus
Why get tested?
TB screening tests help to determine whether you may have been infected with the Mycobacterium tuberculosis bacteria. It does not distinguish the difference between active or latent infection
When to get tested?
When you have been in close contact with someone who is known or suspected to have tuberculosis (TB); when you have diseases or conditions that weaken your and put you at a greater risk of developing active TB; when you are about to start a treatment that will weaken your immune system and put you at risk of developing active TB; when you are confined in living conditions such as a nursing home, school, homeless shelter, migrant hostel, detention centre, or correctional facility; when you inject illegal drugs; when you have lived for a period of time in or come from a foreign country where TB is more common; when you have an occupation, such as health care worker, that will bring you into close contact with those who may have active TB; sometimes as part of an examination prior to starting school or a new job where you could spread TB
For a tuberculin skin test, no sample is required. A small amount of purified protein derivative (PPD) solution is injected just under the first layer of skin of your inner forearm. For an interferon gamma release assay, a blood sample is drawn by needle from a vein in the arm
Test preparation needed?
What is being tested?
Tuberculosis (TB) screening tests help to determine whether a person has been infected with Mycobacterium tuberculosis , the cause of TB. The screening tests measure the body's immune response to derived from the bacteria – either directly as a skin reaction to a tuberculin skin test (TST) or indirectly with an interferon gamma release assay (IGRA) blood test.
TB, once called "consumption", has been recognised as causing illness for thousands of years. This bacterial infection may affect many body organs, but it primarily targets the lungs. TB may cause an inactive () infection or an active, progressive disease. The of about 90% of the people who become infected with TB manage to control its growth and confine the TB infection to a few cells in the body. The bacteria in these cells are inactive but still alive. The person does not have any symptoms and they are not infectious, but they do have a "latent TB infection." If, after some time, the person's immune system becomes weakened (), the mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. Active TB does cause illness in the person and it may be passed to others through respiratory secretions such as or aerosols released by coughing, sneezing, laughing, talking, singing or breathing.
The tuberculin skin test involves two steps: the injection of a small amount of purified protein derivative (PPD) solution under the first layer of skin of the forearm and an evaluation of the injection site conducted by a health care worker at 48 and/or 72 hours to see if a local skin reaction has occurred.
The IGRA test measures the release of gamma interferon by white blood cells in a sample of blood when the cells are exposed to specific TB antigens.
Both tests can detect M. tuberculosis infections, but neither can distinguish between latent and active infections.
How is the sample collected for testing?
For the tuberculin skin test, no sample is required. The test is performed on a person's skin. A purified protein derivative (PPD) solution that contains M. tuberculosis antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red raised bump) in those who have been infected by TB.
A health care worker will wipe the inner forearm with alcohol and let the skin dry. Using a syringe and small gauge needle, they will inject a small amount of PPD solution just under the first layer of the skin. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed. The site must be examined by a trained health care worker at 48 and/or 72 hours to see if a local skin reaction has occurred.
For an interferon gamma release assay, a blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading the following articles under: Coping with discomfort and anxiety
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.