Tuberculin Skin Test
Once the immune system has been exposed to tuberculosis, the blood cells involved in immune response (T-lymphocytes) become sensitized. The immune system reaction peaks after about 2 months.
Tuberculin is obtained from a human strain of purified Mycobacterium Tuberculosis. The TB skin test involves the intradermal injection of a small amount of this substance. If the individual developed an immune response to the bacteria responsible for tuberculosis some time in the past, the injection will stimulate sensitized T lymphocytes and activate the infiltration of lymphocytes into the site of the injection. Test results are based on the area of induration caused by the reaction which is evaluated during a second visit 48 to 72 hours after the injection.
Why is this test performed?
Firstly, the TB skin test is used to determine whether an individual has developed an immune response to the bacterium that causes tuberculosis.
The test is usually performed on individuals who:
This test may also be performed to establish a baseline in those likely to come into contact with tuberculosis and for whom the tuberculin skin test may be repeated in the future. In this context, the test applies to health care workers and to those working with at-risk populations.
In some circumstances, the IGRA (Interferon Gamma Release Assay) blood test is preferred over the tuberculin skin test. For example, when the second visit is compromised or when the skin test results may be altered as seen in those who have received the Bacille Calmette-Guérin vaccine (vaccine or chemotherapy).
What does an abnormal test result mean?
A reaction is deemed positive based on the diameter of the induration. Induration > 10 mm is considered a positive skin test.
However, in certain at-risk populations, the diameter of induration may be smaller.
A negative skin test does not always mean that a person is free of tuberculosis. Certain factors can lead to false negative results in patients with an active infection:
What you need to know before the test
Before going for blood tests, a procedure or other exam, it is best to always bring a list of all the drugs you take (prescription, OTC and natural health products). Unless told otherwise, you should take your medication as usual on the day of the test. When in doubt, ask your pharmacist for more information.
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The patient information leaflets are provided by Vigilance Santé Inc. This content is for information purposes only and does not in any manner whatsoever replace the opinion or advice of your health care professional. Always consult a health care professional before making a decision about your medication or treatment.