A positive Purified Protein Derivative, or PPD, test for tuberculosis indicates exposure to the tuberculosis bacterium and potential latent infection, writes MedlinePlus. A positive test manifests as a swelling around the injection site 48 to 72 hours after the test is placed under the skin. A swelling larger than 5 millimeters in diameter is usually necessary for a positive test, but smaller swellings may indicate infection in people with weakened immune systems.
Medical providers perform the PPD test because tuberculosis infections can persist in a latent state for many years after initial exposure to the tuberculosis bacteria, states MedlinPlus. Though carriers of tuberculosis are symptom-free, the latent infection can reactivate many years after the initial exposure, causing tuberculosis symptoms in the infected person and potentially leading to infection of others. People with weakened immune systems from HIV infection, chemotherapy, diabetes or the side effects of certain medications are particularly at risk for tuberculosis reactivation, as are infants and elderly people.
Though the PPD test indicates tuberculosis exposure, it does not necessarily indicate infection, according to MedlinePlus. An individual exposed to tuberculosis may have a positive skin test reaction but may not have an active or latent infection. Rather, the presence of a positive test screens individuals for follow-up tests, such as chest X-rays, that more definitively determine their infection status.