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Tuberculosis PPD Skin Test

A tuberculosis skin test is also known as a purified protein derivative (PPD) skin test. It is a test used to check the body’s current sensitivity to certain elements of the tuberculosis bacteria. During a clinic visit, a healthcare practitioner will inject a small amount of PPD under the top layer of the skin on the forearm. The individual must return to the clinic for a reading 48-72 after their visit. A nurse or other medical professional will check the area where the PPD was placed to see if there is any reaction. 

After 48-72 hours post-injection, a practitioner will examine the injection site. If the area where the PPD has been injected is not swollen, or only slightly swollen, then the test results are negative. If the area is swollen, with an induration of 10 mm or more, the test is considered to be positive. A doctor will typically order a chest x-ray to check for active or latent TB infection after a positive result.

A PPD skin test requires at least 2 visits. However, in the case of a two-step PPD skin test, there are four visits. The first visit is for the injection while the second is for the reading. With a two-step, a third visit is scheduled 7 days after the first for a second injection. The second read is done 48-72 hours after the third visit.

Individuals are expected to continue having positive results on all future skin tests. In this case, a Quantiferon TB blood test may be utilized in place of the skin test. An annual chest x-ray and TB questionnaire is also an option. 

To learn more about the difference between TB test types view our blog post, TB Testing – The Differences Between QuantiFERON Gold, QuantiFERON Gold Plus and PPD Skin Testing.

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