The Mantoux test, also known as the tuberculin skin test (TST), is used to determine if someone has been infected with Mycobacterium tuberculosis, the bacteria that cause tuberculosis (TB).

Procedure:

• A small amount (0.1 mL) of purified protein derivative (PPD) is injected intradermally into the inner surface of the forearm.
• A raised, pale wheal of 6–10 mm should appear at the injection site.
• The test site is examined 48 to 72 hours later.

Reading the Results:

The result is based on induration (swelling), not redness. The size of the induration is measured in millimeters:

• ≥5 mm: Positive in high-risk individuals (e.g., HIV-positive, recent TB contacts).
• ≥10 mm: Positive in moderate-risk groups (e.g., healthcare workers, recent immigrants from high-prevalence countries).
• ≥15 mm: Positive in people with no known risk factors for TB.

Important Notes:

• False positives may occur in people vaccinated with BCG.
• False negatives can happen in immunocompromised individuals or recent TB infection (within 8 weeks).