Tuberculin skin test

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Tuberculin skin test

Tuberculin Skin Test

Tuberculosis is an infection caused by a bacterium called Mycobacterium tuberculosis that generally affects the lungs. During active disease, the infected person may present various symptoms (e.g., cough, fever, weight loss). Latent tuberculosis occurs when a person is infected with the tuberculosis bacterium, but the infection is dormant (asleep). People with latent tuberculosis have no symptoms, but are at risk of developing active disease.

It is possible to detect an immune response in infected individuals using the tuberculin skin test (TST), also known as Mantoux tuberculin test. Tuberculin is obtained from a strain of purified Mycobacterium Tuberculosis. The tuberculin skin test involves the injection under the skin of a small amount of this substance to induce a local immune response. An induration may then be observed at the injection site in people who have already been in contact with the tuberculosis microbe.

What is the purpose of this test?	The test is used to determine whether an individual has developed an immune response to the bacterium that causes tuberculosis. The TST may be used for several reasons, including in individuals who: have signs and symptoms of tuberculosis have been in recent contact with an active case of tuberculosis are arriving from countries with a high incidence of tuberculosis have a condition associated with increased risk (e.g., immunosuppression) 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 (e.g., healthcare workers).
How to prepare?	No preparation needed.
Optimal result	Normal / Negative The usual threshold for determining a positive reaction is an induration diameter ≥ 10 mm. However, this threshold may be lower in some patients.
Associated tests	The IGRA (Interferon Gamma Release Assay) blood test may be used in some circumstances. 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-GuÃ©rin vaccine (tuberculosis vaccine). A lung X-ray may be performed if the TST is positive, even in the absence of symptoms.
How is it done?	Tuberculin is injected under the skin surface during an initial appointment. During a second visit (48 to 72 hours after the injection), the area of induration is evaluated.

What does an abnormal test result mean?

If the induration diameter is greater than the threshold, this indicates that the person has previously been in contact with the tuberculosis bacteria.

Factors that can interfere with test accuracy

Certain factors can lead to false negative results in patients with an active infection, such as:

improper storage of tuberculin
insufficient amount of antigen injected
injection administered too deeply
improper reading of test results
certain active viral infections (e.g., chickenpox, measles, whooping cough)
taking certain immunosuppressive medications
certain diseases (e.g., chronic renal insufficiency, sarcoidosis)
recent administration of a live virus vaccine
malnutrition
neoplasms in the lymphatic and reticuloendothelial systems

What you should know before going for this test

When you go in for a blood test or any other medical exam, it's always a good idea to bring a complete list of any prescriptions, over-the-counter medications, or natural health products you're taking. Unless otherwise advised, it's best to take your medications as usual on blood test days. If in doubt, don't hesitate to talk to your healthcare professional.

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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.