Tuberculosis Skin Test
(PPD
Skin Test)
Medical Editor:
William C. Shiel, Jr., MD, FACP, FACR
What is the tuberculosis skin test?
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.
How is the tuberculosis skin test
administered?
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.
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.
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.
Last Editorial Review: 8/1/2006
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