TB Intradermal Test

The TB Intradermal Test, also called the Mantoux Tuberculin Skin Test (TST) or PPD skin test, is used to determine whether a person has been infected with the bacteria that cause tuberculosis (TB).

How it works

A healthcare professional injects a small amount of purified protein derivative (PPD) just under the skin of the forearm.
A small, pale bump (called a wheal) should appear immediately after the injection.

The patient must return 48 hours later to have the test read.

Reading the test

The provider measures the induration (raised, firm area)—not the redness.

5 mm or more: Positive for people at highest risk (such as those with HIV, recent TB contacts, or certain immunocompromised individuals).
10 mm or more: Positive for people with increased risk (such as healthcare workers, residents of congregate settings, or people with certain medical conditions).
15 mm or more: Positive for people with no known TB risk factors.

Important points

A positive test does not mean the person has active TB disease. Additional evaluation, such as a chest X-ray and medical assessment, is needed.
A negative test does not completely rule out TB, especially in people with weakened immune systems or those recently exposed.
Anyone who has had a positive TB skin test in the past should generally not receive another TB skin test. Instead, they should inform healthcare providers of their previous positive result.

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