Tuberculosis Skin Test (PPD Skin Test) (cont.)
How is the tuberculosis skin test administered?
The standard recommended tuberculin test, known as the Mantoux test, is administered by injecting a 0.1 mL volume containing 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 abnormalities and away from veins is recommended. The injection is typically made using a ¼- to ½-inch, 27-gauge 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.
What is the method of reading the tuberculosis skin test?
"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.
How are skin test results interpreted?
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 (for example, 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 whose immune system is normal, induration greater than or equal to 15 mm 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 health-care worker or personal contact of someone with active TB, 10 mm of induration
is considered a positive skin test. In patients who are immunocompromised, such as people with rheumatoid arthritis or Crohn's
disease, 5 mm of induration is considered a positive skin test result. Induration of less than 2 mm, without blistering, is considered a negative skin test.
On the other hand, a negative test does not always mean that a person is free of tuberculosis. People who have been infected with TB may not have a positive skin test if their immune function is compromised by chronic medical conditions, cancer chemotherapy, or AIDS. Additionally, 10%-25% of people with newly diagnosed tuberculosis of the lungs will also have a negative result, possibly due to poor immune function, poor nutrition, accompanying viral infection, or steroid therapy. Over 50% of patients with widespread, disseminated TB (spread throughout the body, known as miliary TB) will also have a negative TB test.
A person who received a BCG vaccine (administered in some countries but not the U.S.) against tuberculosis may also have a positive skin reaction to the TB test, although this is not always the case. The positive reaction that is due to the vaccine may persist for years. Those who were vaccinated after the first year of life or who had more than one dose of the vaccine have the greatest likelihood of having a persistent positive result than those who were vaccinated as infants.
Last Editorial Review: 9/2/2008
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