The tuberculosis skin test (PPD skin test) is used to determine if a person has developed an immune response to the bacterium that causes tuberculosis (TB).
The test is positive in a person who currently has TB, in a person who was exposed to it in the past, or in a person who have received the BCG vaccine against TB (which is not performed in the U.S.) because infection with mycobacterium tuberculosis produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium.
Everyone can have this test including infants, pregnant women or HIV-infected people.
During the test, an injection of 0.1 mL volume containing 5 TU (tuberculin units) PPD is inserted into the top layers of skin (intradermally, immediately under the surface of the skin) of the forearm. A skin area must be free of abnormalities and away from veins.
If the injection is done correctly, a discrete, pale elevation of the skin (a wheal) 6 mm-10 mm in diameter will be visible but it is quickly absorbed. After 48-72, hours the skin test should be read by detecting a raised, thickened local area of skin reaction, referred to as induration.
In a healthy person induration equal or greater than 15 mm, it is considered that the test is positive. In a person with diabetes, kidney disease or in someone who had contact with a person with active TB value is 10 mm and in patients with autoimmune diseases value is 5 mm.
The test is negative if induration is less than 2 mm but if this does not always mean that a person is free of tuberculosis.
A false negative result can be in people with chronic medical conditions, cancer chemotherapy, or AIDS, in 10%-25% of people with newly diagnosed tuberculosis of the lungs will also have a negative result because of poor immune function, poor nutrition, accompanying viral infection, or steroid therapy and over 50% of patients with widespread, disseminated TB, known as military TB.
A false positive result can be in people who received a BCG vaccine (administered in some countries but not the U.S.) against tuberculosis.
There is a very low risk of having a severe reaction to the test which includes swelling and redness of the arm but there is no risk of developing tuberculosis from the test because live bacteria are not used for the test.