Tuberculin Skin Test (TST, PPD, Mantoux)

The tuberculin skin test (TST) is one way to identify a patient with tuberculosis.

PPD = purified protein derivative = tuberculin

Performance: A small amount of tuberculin (5 tuberculin units, or 5-TU) is injected into the superficial dermis of the forearm.

If a repeat test is indicated, then select a different site away from the previous.

Reading time: 48-72 hours

Measurement: diameter of the raised zone of induration in mm (erythema is not counted)

The diameter of the zone of induration is interpreted based on the population group.

Patient	Positive if
HIV-positive	>= 5 mm
recent contact with a TB case	>= 5 mm
fibrotic changes on chest X-ray consistent with remote tuberculosis	>= 5 mm
organ transplant recipient	>= 5 mm
immunosuppression ( > 15 mg/day of prednisone for >= 1 month; TNF-alpha antagonist)	>= 5 mm
recent immigrant from high prevalence country (less than 5 years)	>= 10 mm
IV drug user	>= 10 mm
resident or employee of high-risk setting (prison, nursing home, homeless shelter, hospital, health care facility)	>= 10 mm
Mycobacteriology lab personnel	>= 10 mm
child < 5 years of age	>= 10 mm
child or adolescent exposed to an adult in a high-risk category	>= 10 mm
person with no known risk factors for tuberculosis and living in a low prevalence region	>= 15 mm

Performance:

• Both false positive and false negative results can occur.

• Previous receipt of BCG can cause a very strong false positive reaction.

• The gamma-interferon blood test is more sensitive but more expensive.

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