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結核菌素皮膚試驗 (Tuberculosis Skin Test - PPD Skin Test)

Q:What is the tuberculosis skin test?
A:The tuberculosis skin test (also known as the tuberculin or PPD test) is a test used to determine if someone has developed an immune response to the bacterium that causes tuberculosis (TB). This response can occur if someone currently has TB or if they were exposed to it in the past. 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 bacterium. 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 begins when specialized immune cells, called T cells, which have been sensitized by prior infection, are recruited by the immune system 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.

Q:How is the tuberculosis skin test administered?
A:The standard recommended tuberculin test is administered by injecting 0.1mL of 5 TU (tuberculin units) PPD into the top layers of skin (intradermally, immediately under the surface of the skin) 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.

Q:What is the method of reading the tuberculosis skin test?
A:"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 and the amount of induration (localized swelling). 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.

http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm

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Two-step Mantoux test

Some people who were previously infected with TB may have a negative reaction when tested years after infection, as the immune system response may gradually wane. This initial skin test, though negative, may stimulate (boost) the body's ability to react to tuberculin in future tests. Thus, a positive reaction to a subsequent test may be misinterpreted as a new infection, when in fact it is the result of the boosted reaction to an old infection.
Use two-step testing for initial skin testing of adults who will be retested periodically (e.g., health care workers). This ensures that any future positive tests can be interpreted as being caused by a new infection, rather than simply a reaction to an old infection.

  • Return to have first test read 48–72 hours after injection
  • If first test is positive, consider the person infected.
  • If first test is negative, give second test 1–3 weeks after first injection
  • Return to have second test read 48–72 hours after injection
  • If second test is positive, consider person previously infected
  • If second test is negative, consider person uninfected

A person who is diagnosed as "infected" on two-step testing is called a "tuberculin converter". The US recommendation that prior BCG-vaccination be ignored results in almost universal false diagnosis of tuberculosis infection in people who have had BCG (mostly foreign nationals). 

http://www.currytbcenter.ucsf.edu/abouttb/tbcontrol_faqs/2_how_is_two_step_tst_done.pdf

 

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