The PPD is a skin test that screens for the presence of antibodies against tuberculosis proteins.
Tests are normally negative. A positive test indicates current active infection, previous infection, or enough previous exposure (without clinical infection) to develop antibodies against tuberculosis.
The test is read 48-72 hours after intradermal injection. The diameter of induration (the "zone") is measured.
A variety of factors can lead to false positives or false negatives
Normal Values*
Zone diameter
Negative
< 5 mm
Intermediate
5-10 mm
Positive
>10 mm
*These are general values taken from a variety of sources. The actual normal values may vary from lab to lab and from one type of testing protocol to another.
