Tuberculosis, or TB, is a highly contagious respiratory infection. It is spread through contact with respiratory droplets released into the air when someone who is infected coughs or sneezes. TB is no longer common in the United States. In fact, more than half the reported cases in the U.S. are in people from countries where it is common, according to the University of Michigan. Doctors can test whether someone is infected with TB by administering the purified protein derivative skin test, or PPD test. The test involves injecting PPD under the uppermost skin layer. It takes several days for test results to become evident.
Step 1
Examine the PPD test site between 48 hours and 72 hours after the test was administered. Swelling is generally the mark of a positive test result, but some swelling can develop even in people with a negative test results, MedlinePlus reports.
Step 2
Measure the amount of swelling at the test site. The greater the area of swelling, the greater indication of a positive test result for tuberculosis, according to Harvard Medical School. Reactions of 5 mm are considered a positive result in people with HIV or on steroid therapy, as well as those who have been exposed to TB. Reactions of 10 mm are considered positive in health-care workers, those with kidney failure or diabetes and IV drug users. For people who have no known risk for TB, a positive test result is one that is 15 mm or greater.
Step 3
Go to your scheduled appointment to have the test result read. Because result valuations vary for different populations, you must have your TB test evaluated by a medical professional in the specified time period, the University of Michigan University Health Services warns.
Tips and Warnings
- Your doctor may draw a circle around the test site to better read the results. If your doctor does, do not wash it off until after it has been evaluated. Tuberculosis skin tests are not foolproof. As many as 20 percent of people may have a negative test result while still being infected with TB bacteria, MedlinePlus advises. False negatives are also common in people with conditions that affect the immune system. A positive skin test does not necessarily mean you have an active tuberculosis infection. Your doctor will likely order more tests, including blood tests, urine tests and chest X-rays, before deciding on a course of treatment, according to Harvard Medical School.
- In rare cases, people who have previously tested positive for TB with the PPD test can develop severe arm swelling and redness in subsequent tests, MedlinePlus reports. This reaction also rarely develops in people who have never before had the test.
