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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
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Normal Values*
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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.
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