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Facts About...

Tuberculin Skin Test (TST)
Purified Protein Derivative (PPD)

What is it?

The Tuberculin Skin Test (TST) assists doctors/health care providers in diagnosing Tuberculosis (TB) infection. A very small amount (0.1ml) of Purified Protein Derivative (PPD) test solution is injected just under the skin on the forearm to perform a TST. A tiny raised bump (“wheal” or “bleb”) will appear where the PPD has been injected, which soon fades away. You must return back to your health care provider within 48-72 hours, as directed, to have this site examined and “read” to determine if the test is negative or positive. If this site is not read in the specified time frame, by your doctor/health care provider, the test results will not be reliable. This is not a vaccination. The skin test cannot give you TB.

How effective is a TST?

A TST is one of the best tests that doctors/health care providers have to assist in diagnosing TB infection. A positive skin test means that you have been exposed to the TB bacteria at some time in your life and that you have TB infection. It does not necessarily mean you have TB disease. If you have a positive result, you will be referred to your doctor/health care provider for further assessment which may include a chest x-ray and medication to prevent TB disease.

Are there side effects with a TST?

There may be swelling or redness at the test site and slight discomfort such as itching.  Severe blistering reactions are very rare. The Health Department recommends that persons wait in a designated area 15 minutes after the TST to make sure no reaction occurs.

Who should receive a TST?

Individuals who should be considered for TST include:

  • Household and close contacts of a person who has respiratory TB disease
  • People with medical conditions, such as:
    • HIV infection and AIDS
    • Transplantation recipient
    • Silicosis
    • Kidney disease
    • Cancer
    • Diabetes
    • Treatment with glucocorticoids (more than or equal to 15mg per day)
    • Heavy alcohol drinker (more than or equal to 3 drinks per day)
    • Heavy cigarette smoker (more than one pack per day)
    • Underweight (less than 90% ideal body weight)
  • People born in other countries with high rates of TB (especially those who lived more than 20 years in country with high rates of TB, and those who arrived Canada in the last two years)
  • People with old or healed TB without proper TB treatment
  • People who are homeless or under-housed
  • People who abuse alcohol or recreation drugs
  • Travellers who are visiting country with high rates of TB
  • Staff and residents of correctional facilities
  • Health care and daycare workers and volunteers
  • Aboriginal persons in communities with high rates of TB
  • Travellers who are visiting a country with a high rate of TB

Your doctor/health care provider may recommend a one step or a two-step TST. The two-step TST is indicated for persons working in certain occupations where TST testing is done more frequently (usually once a year) due to their work environment. The two-step establishes a base-line result before frequent testing is done. The first TST is done followed by a second TST 1-4 weeks after the first test. The second TST is only done if the first test was negative.

If you are a contact of a person who has TB disease you will have the first TST done followed by the second TST no sooner than 8 weeks after the exposure to TB.

Who should NOT get a TST?

The following persons should not receive a TST:

  • Those who have had a previous positive TST result
  • Those who had history of TB infection of TB disease in the past
  • Those with severe blistering TST reactions (very rare) in the past or with extensive burns or eczema present over TST testing sites
  • Those with symptoms of TB disease (such as fever, weight loss, cough, night sweats, fatigue)
  • Those who are ill with major viral infections (not a common cold)
  • Those who have received measles, mumps, rubella, varicella (chickenpox), oral polio and yellow fever immunization in the past 4 weeks; defer for 4 weeks
  • Those who have had an anaphylactic or other allergic reaction to a previous TST test
  • Those who have a severe allergy (anaphylaxis) to a previous dose of any vaccine(s)
    [Note: this may give rise to an allergy to any of the components of Tubersol®:
    • Purified protein derivative of M. tuberculosis (PPD)
    • Phenol
    • Polysorbate 80 (Tween 80)
  • People can have a TST even if they:
    • Have a common cold
    • Immunized with any vaccine on the same day
    • Immunized greater than 4 weeks prior with live attenuated virus vaccines
    • Are pregnant or breastfeeding
    • Received BCG (Bacille Calmette-Guerin) vaccination in the past

Your record of protection

Keep a record of your TB skin test.  A positive TST is never to be repeated as the test results will always remain positive and there may be an increased reaction at the injection site.

Keep your records in a safe place!

June 2015