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 blister on the skin.

Because the reaction will take 48-72 hours to develop, you must return to your health care provider within that time for a proper evaluation of the test site. This will 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.

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. 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. Your PPD may be falsely negative or the cutoffs when interpreting a reaction may be different.

Infants and children:
The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following:

• 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)

A negative reaction (no induration) or a level of induration below the cutoff for each risk group may indicate lack of infection with the bacteria which causes TB. There are different cutoffs for children, for people with HIV, and other risks groups.

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 (cancer, recent chemotherapy, late-stage AIDS) may also have a false negative test result.

Significance 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 injection site.

A small reaction (5 mm) is considered to be positive in individuals with HIV, in individuals on steroid therapy, or in individuals in close contact with 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 or greater induration.