TB skin test

TB Skin Test

The TB skin test, also known as the Mantoux tuberculin skin test, is a diagnostic tool used to detect tuberculosis (TB). It involves injecting a small amount of tuberculin purified protein derivative (PPD) into the skin and observing the reaction.

Procedure

The TB skin test is performed by injecting 0.1 ml of tuberculin PPD into the inner surface of the forearm. The injection should produce a pale elevation of the skin (a wheal) 6 to 10 mm in diameter. The patient is instructed to return in 48 to 72 hours so that the reaction can be read.

Interpretation

The skin test reaction should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will need to be rescheduled for another skin test. The reaction is measured in millimeters of the induration (palpable, raised, hardened area or swelling). The reader does not measure erythema (redness). The diameter of the indurated area is measured across the forearm (perpendicular to the long axis).

Positive Result

A positive result indicates TB infection, but does not confirm active TB disease. Additional tests are required for diagnosis. The size of the induration is used to determine whether the result is positive, with larger indurations indicating higher likelihood of TB infection.

Negative Result

A negative result usually means the person is not infected. However, the test may be falsely negative in a person who is recently infected or in a person with immunosuppression.

False Positive

False positive results can occur due to previous BCG vaccination or infection with non-tuberculosis mycobacteria.

False Negative

False negative results can occur in certain conditions such as HIV infection, malnutrition, or severe TB disease.

See Also

• Tuberculosis
• BCG vaccine
• Immunosuppression
• HIV

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