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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:

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:

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