Skin tests are frequently performed especially on health care workers to test for the presence of Mycobacterium tuberculosis. The tuberculin skin test is administered by a health care professional who injects a substance called purified protein derivative, or PPD, underneath the skin on the forearm, the Centers for Disease Control and Prevention explains. The test sites are then read at a later date. For accuracy, skin tests should only be read by a qualified health care professional.
Wait 48 to 72 hours after the skin test was administered to read the test. Tuberculin skin tests should be read within this time frame for accuracy. If more than 72 hours elapses after the test was administered, it must be repeated.
Examine the skin where the test was administered for the presence of a raised bump. If no bump exists, or if the bump is less than 5mm, the test is considered negative. Only bumps should be measured, not areas of redness. The skin test can cause irritation to the skin that normally results in a reddened area, but the redness is not to be confused with a positive test.
Measure the bump, also known as an induration, if it exists. MedlinePlus, a publication of the National Institutes of Health, explains that a bump that is 5mm in size or larger indicates a positive reaction in people who are HIV positive, anyone who is being treated with steroids and people who have had contact with individuals infected with tuberculosis. An induration that is 10mm or larger may appear in people with underlying medical conditions such as diabetes and also in health care workers. In people with no unusual risks for tuberculosis exposure, MedlinePlus explains that a positive reaction is considered 15mm or larger.