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Tuberculosis (TB) Skin Test

Why do I need a TB skin test?

The TB skin test (TST) is a way to find out if someone has been infected with the TB bacteria.

You may need a TST for employment, school, volunteering, travel, medical reasons or because you have been in contact with a person who has active TB.

What is a TB skin test?

A TST involves injecting a small amount of testing material just below the top layers of skin, usually on the arm.

It may cause some swelling, itching or tenderness at the injection site. You can participate in normal activities following the test.

Two or three days later, a health care worker checks the arm to see if a bump has developed. The appearance and size of the bump determines the result of the test. If there is a bump, the health care worker will measure it and decide if the reaction is positive.

What is a “two-step” TB skin test?

Sometimes the immune system of some individuals cannot ‘remember’ a past TB infection. These people may not have a reaction after the first TST, but a second TST may produce a positive response. This is called the ‘booster effect’.

A second TB skin test repeated 1–4 weeks after the first test is often required to get an accurate baseline for people who are going to be working or volunteering in a health care field.

What does a “NEGATIVE” reaction mean?

A negative result means there is no reaction and that TB bacteria have not entered the body in the past.

It could also mean that the body has forgotten to react, such as when someone’s immune system is lowered due to medication, medical conditions or aging.

What does a “POSITIVE” reaction mean?

At some time in the past, TB bacteria have entered the body.

The health care provider will need to determine if the individual has inactive TB infection or active TB disease and will order a chest x-ray and possibly sputum tests (mucous/phlegm) if the person has symptoms.

Having a positive TB skin test does NOT mean that the infection can be passed to other people. The bacteria can only be transmitted if the person has signs of active TB disease such as coughing, night sweats, chills, weakness, weight loss and chest pain.

Medication may be prescribed which will slowly fight the TB bacteria and allow the body to heal. TB drugs are provided free of charge from the Leeds, Grenville and Lanark District Health Unit.