The tuberculin skin test is based on the fact that infection with M. tuberculosis produces a delayed-type hypersensitivity skin reaction to certain components of the organism. The components of the organism are contained in extracts of culture filtrates and are the core elements of the classic tuberculin PPD (also known as purified protein derivative). This PPD material is used for skin testing for tuberculosis. Reaction in the skin to tuberculin PPD is initiated when specialized immune cells, called T cells, which have been sensitized by prior infection, are recruited to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration (a hard, raised area with clearly defined margins at and around the injection site) through local vasodilation edema , fibrin deposition, and recruitment of other inflammatory cells to the area.

The standard recommended tuberculin test is administered by injecting 0.1mL of 5 TU (tuberculin units) PPD into the top layers of skin (intradermally) of the forearm . The use of a skin area that is free of lesions and away from veins is recommended. The injection is typically made using a one-quarter to one-half inch, 27-guage needle and a tuberculin syringe . The tuberculin PPD is injected just beneath the surface of the skin. A discrete, pale elevation of the skin (a wheal ) 6 to 10 mm in diameter should be produced when the injection is done correctly. This wheal or "bleb" is generally quickly absorbed. If it is recognized that the first test was improperly administered, another test can be given at once, selecting a site several centimeters away from the original injection.

"Reading" the skin test means detecting a raised, thickened local area of skin reaction, referred to as induration. Induration is the key item to detect, not redness or bruising. Skin tests should be read between 48 and 72 hours after the injection when the size of the induration is maximal. Tests read after 72 hours tend to underestimate the size of the induration.

Interpretation of Skin Test Results

The basis of the reading of the skin test is the presence or absence of induration and its diameter. The diameter of the induration should be measured transversely (i.e. perpendicular) to the long axis of the forearm and recorded in millimeters. The area of induration ( palpable , raised, hardened area) around the site of injection is the reaction to tuberculin. Again, redness is not measured. A tuberculin reaction is classified as positive based on the diameter of the induration in conjunction with certain patient-specific risk factors. In a healthy person who is not immunocompromised, induration greater than or equal to 15mm is considered a positive skin test. If blisters are present (vesiculation), the test is also considered positive. In a person with underlying kidney disease, diabetes, or a healthcare worker, 10mm of induration is considered a positive skin test. 5 mm is considered a positive skin test result for patients who are immunocompromised , such as with rheumatoid arthritis patients and Crohn's disease patients. Induration of less than 2mm, without blistering, is considered a negative skin test.

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