How to Tell If a TB Test Is Positive?

The skin test for tuberculosis (TB) also is called a PPD (Purified Protein Derivative) or the Mantoux test. A small amount of protein from the TB bacterium is injected underneath the skin of the forearm. This test is used to find out if someone ever has been infected with tuberculosis.

Instructions

• 1

  Take the test. A health-care provider will inject the TB test under the skin of your forearm.

• 2

  Go back to where the test was performed within 48 and 72 hours to have your arm checked for test results. The size of swelling at the injection site indicates a positive or negative test.

• 3

  Measure swelling at the injection site. A positive test produces a large, hard swelling of 15mm, or slightly more than 1/2 inch. Swelling of about 10mm, or slightly less than 1/2 inch, is considered positive in health-care workers and people who have risk factors such as kidney failure or diabetes. Swelling of about 5mm is considered positive in people who have immune disorders such as human immunodeficiency virus (HIV) and rheumatoid arthritis.