How Does the Mantoux Tuberculin Skin Test Work?

The Mantoux tuberculin skin test works by causing a reaction in the skin of a person who is infected with Mycobacterium tuberculosis, says the Centers for Disease Control and Prevention. A health care worker injects a very small amount of tuberculin purified protein derivative, or PPD, beneath the patient's skin, which eventually raises a welt.

A trained technician administers the PPD with a tuberculin needle, says the CDC. The bevel should be facing up, and the technician inserts it into the patient's inner forearm. The reaction to the PPD is ready to be evaluated 48 to 72 hours after the test. Health care workers evaluate the reaction by the size in millimeters of the induration, which is a hardened, raised welt, and whether the patient is at high risk of contracting the disease.

For example, an induration that is 5 millimeters across or more is a positive indicator in people who have HIV, who've recently been around someone who has tuberculosis or whose chest X-ray shows changes that are consistent with having had tuberculosis previously, claims the CDC. Health care workers consider an induration of 10 millimeters or more to be a positive indicator in a recent immigrant from a country where tuberculosis is common, a person who injects street drugs and children under 4. An induration that is 15 millimeters and above is positive for everyone.