What is the definition or description of: positive ppd?

See below: Ppd stands for purified protein derivitive and is from the bacteria found in tb. A small quantity is placed in the layers of your skin and if swelling develops in that area over the proceeding 72 hrs this could indicate that you have been exposed to tb. A positive depends on the size and risk factors that one has for exposure to tb. If not high risk and healthy it needs to be > 15 mm in size.

Positive PPD: Positive PPD means that a person has been exposed to tuberculosis.