| Subject | Contents |
| Definition | The purified protein derivative is an antigen used to aid in the diagnosis of tuberculosis '>tuberculosis infection. This test is based on the observation that infection with the bacteria that causes tuberculosis '>tuberculosis frequently leads to a sensitivity to these antigens . |
| Alternative Names | Purified protein derivative standard; TB skin test; Tuberculin skin test |
| How the test is performed | The test site (usually the forearm) is cleansed. The PPD extract is then injected into the most superficial layer under the skin resulting in a transient bleb on the skin. As the reaction will take 48-72 hours to develop, you must return to your health care provider at 48-72 hours for a proper evaluation of the test site to determine whether you have had a significant reaction to the PPD test. A reaction is measured in millimeters of induration (hard swelling ) at the site. |
| How to prepare for the test | There is no special preparation for this test. Inform your health care provider if you have ever had a positive PPD skin test -- if so, you should not have a repeat PPD test. Also, notify your health care provider if you are taking certain drugs such as steroids or have a medical condition which can affect your immune system such as cancer, as your PPD may be falsely negative or the cutoffs when interpreting a reaction may be different. Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:infant test or procedure preparation (birth to 1 year) toddler test or procedure preparation (1 to 3 years) preschooler test or procedure preparation (3 to 6 years) schoolage test or procedure preparation (6 to 12 years) adolescent test or procedure preparation (12 to 18 years) |
| How the test will feel | There will be a brief sting as a needle is inserted just below the skin surface. |
| Why the test is performed | The test is performed to evaluate infection with tuberculosis. |
| Normal Values | A negative reaction (no induration) or a level of induration below the cutoff for each risk group (e.g., different cutoffs for children, people with HIV, etc.) may indicate lack of infection with the bacteria which causes TB. Unfortunately, this is not a perfect test and up to 20% of people infected with tuberculosis may not have a reaction on the PPD skin test. In addition, individuals with certain conditions which affect the immune system may also have a false negative test (e.g., cancer, recent chemotherapy, late-stage AIDS). |
| What abnormal results mean | The significance of the reaction depends on the size of the reaction on the skin and the individual. The reaction is measured in millimeters of induration (hard swelling ) at the site. For example, a small reaction (5 mm) is considered a positive reaction in individuals with HIV, in individuals on chronic steroids or in close contacts of a person with active tuberculosis. Larger reactions (greater than or equal to 10 mm) are considered positive in individuals with diabetes, renal failure and health care workers, among others. In individuals with no known risks for tuberculosis, a positive reaction requires a 15 mm induration. |
| What the risks are | There is a very small risk of severe redness and swelling of the arm in individuals who have had a previous positive PPD test and who undergo repeat testing; yet there have been a few cases of this reaction also occurring in individuals who have not been previously tested. |
| Special considerations | This test is widely used -- however its limitations include the fact that there are false negatives and false positives. In addition, a positive skin test does not necessarily mean that an individual has active tuberculosis. Additional studies are needed to verify whether active disease is present. |
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