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Tuberculin (TB) Skin Test

Why do I need a TST?

Early diagnosis is very important in the control of tuberculosis (TB). A TST can assist in the diagnosis by showing whether someone has been exposed to Mycobacterium tuberculosis bacteria.

You may need a TST for employment, travel or because you have been in contact with a person who has active TB. If you have been exposed to TB you may need more than one TST.

What is a TST?

A tiny amount of test fluid called tuberculin will be injected just under the skin of your forearm. This is not a vaccination. You may have slight discomfort, such as itching and redness at the test site. You should not touch or cover the site. You will need to have your arm checked 48 to 72 hours after the TST was given to measure the result and determine if the test is positive or negative.

If the TST is positive, it means that you may have been exposed to the TB bacteria. A TST cannot tell if you have active TB disease. A chest x-ray will be necessary to determine if the TB bacteria have damaged your lungs. Your doctor may prescribe medication to prevent or cure TB.

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"Two-step" TST

Sometimes the immune system of some individuals cannot remember past TB infection. This often occurs in the elderly and in persons whose immune system is weak. These people can have a negative reaction to the first TST. A second TST, given at least one week to as long as one year after the original TST, may produce a much greater response.

Two-step testing is recommended for:

  • People who will be re-tested periodically (i.e. healthcare workings, correctional facilities)
  • Residents (<65 years of age) and staff of long- term care homes, correctional facilities, homeless shelters and drop-in centres
  • Travellers planning a prolonged trip to an area with a high rate of TB

Who can have a TST?

  • People recently immunized with an inactivated vaccine
  • Pregnant women
  • People who have had a previous Bacillus Calmette–Guérin (BCG) vaccination
  • People who have had a previous TST and have no record of the results or the test results were not recorded in “millimetres” of induration

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Who should not have a TST?

  • People who have had a previous severe reaction to the TST (i.e. blistering)
  • People with extensive burns or eczema
  • People who are known to have had active TB or have a record of TB treatment
  • People with a record of a positive TST recorded in “millimetres” of induration

Is the TST safe?

  • Yes. Most people will have no side effects. The TST can cause pain, redness, rash, swelling, blistering, hemorrhage, haematoma, ulceration or necrosis at the injection site, however these side effects are rare.

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Who should delay having a TST?

  • People who have had a viral infection in the past four to six weeks (i.e. chickenpox, influenza)
  • People who have received live virus vaccines in the past four to six weeks (i.e. mumps, measles, chickenpox or yellow fever vaccines)

Who do I talk to if I have questions about this test or TB disease?

If you have any questions about the TST or about TB disease, talk to your doctor or call the Halton Region Health Department by dialing 311 to speak with a nurse.

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