How Do You Read a TB Skin Test?


Quick Answer

A TB skin test is read between 48 and 72 hours after administering the injection by measuring the size of any swelling found at the site of the test. If little or no swelling is present, the test is negative, according to the Centers for Disease Control.

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Full Answer

If the firm swelling at the site of the test measures 5 millimeters or greater, this is considered a positive reaction in people who have had recent exposure to TB or who have HIV, an organ transplant, or other condition that suppresses the immune system, as explained by the Centers for Disease Control. Swelling that measures 10 millimeters or more is a positive result in people originating from countries with a high-risk of TB, children under the age of 4, and other individuals at high risk of contracting TB. If the swelling measures 15 millimeters or more, this is a positive result in all individuals.

The TB skin test is also called a PPD, or purified protein derivative, skin test, as stated by MedlinePlus. Medical professionals conduct this test by injecting a small amount of tuberculin purified protein derivative under the skin of the patient's forearm. The test diagnoses tuberculosis, a contagious bacterial disease that affects the lungs.

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