Page last updated August 2018

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PPD

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.