Tuberculin skin test

Tuberculin skin test (also known as the Mantoux test) is a diagnostic tool for tuberculosis (TB). It involves the injection of a small amount of tuberculin into the skin and observing the reaction.

Procedure

The test is performed by injecting 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of the forearm. The injection should be made with a tuberculin syringe, with the needle bevel facing upward. The healthcare professional performing the test must ensure that the injection produces a pale elevation of the skin (a wheal) 6 to 10 mm in diameter.

Interpretation

The reaction to the test is read 48 to 72 hours after the injection, by measuring the diameter of the induration (hard, raised area) across the forearm. The result is recorded in millimeters. If there is no induration, the result is recorded as "0 mm". Erythema (redness) should not be measured.

Results

The interpretation of the test depends on the person's risk factors for TB and the size of the induration. For example, in people with no risk factors, an induration of 15 mm or more is considered positive. In people with risk factors, an induration of 10 mm or more is considered positive. In people with HIV or other conditions that weaken the immune system, an induration of 5 mm or more is considered positive.

Limitations

The tuberculin skin test has several limitations. It may produce false-positive results in people who have been vaccinated with the BCG vaccine or who have been infected with non-tuberculosis mycobacteria. It may produce false-negative results in people with weakened immune systems, in the elderly, and in people who are severely ill. The test also requires a return visit to a healthcare provider to read the results, which can be inconvenient for some people.

See also

• Tuberculosis
• BCG vaccine
• Tuberculin
• Mycobacteria

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