Why the Test May Be Ordered
This particular skin test is useful in determining whether or not someone has been infected with the bacteria responsible for causing tuberculosis. The physician may order this test when the patient has had significant exposure to this bacterium and to rule out or confirm a possible diagnosis of tuberculosis. Healthcare employees are required to have the TBskin test prior to employment and annually thereafter. Students pursuing a career in healthcare are also required to have the PPD skin test before participating in clinical rotations.
How the PPD Skin Test is Performed
The PPD skin test can be performed at health centers such as local health departments, hospitals and physicians offices. There is no special preparation required for this test. The injection site is properly cleaned using an alcohol swab and a needle is inserted barely under the skin, where the extract will be injected. It is normal to have a small, raised spot following this injection. The patient is then sent home until results can be read.
The Importance of the PPD Skin Test Results
Obtaining results of the PPD skin test works a little bit differently than with other tests. Nothing has to be sent off or analyzed in the lab. Rather, the patient returns to the physician’s office or clinic anywhere from forty eight to seventy two hours following the injection. The healthcare provider will then look for signs of reaction, such as significant swelling at the site. According to the National Institute of Health, this is usually measured in millimeters and redness does not affect the reading.
When a measurable result is present, this does not necessarily mean that the patient has tested positive for infection of Mycobacterium tuberculosis. As a matter of fact, measurement sizes and positive results are based on health conditions of the patient. For example, those with close contact with TB, HIV patients and individuals on steroid therapy will be considered positive if swelling measures at least five millimeters. Healthcare workers, those with a history of drug abuse via needles and diabetics have positive results when swelling is at least ten millimeters, while others with no known exposure must measure fifteen millimeters or more before the test result is considered positive.
Lack of reaction is deemed a negative result.
Accuracy of Results
Contrary to popular belief, a positive result does not necessarily mean that the patient has tuberculosis. Several factors can contribute to positive results.
This can include a type of tuberculosis vaccine called a BCG vaccination, which has been used in certain countries outside of the United States. Certain health conditions can also cause positive TB skin test results. The National Institute of Health indicates that almost twenty percent of those with tuberculosis may not have a positive result. As with other tests, it is not always one hundred percent accurate. For this reason, other tests may commonly be performed to confirm such a diagnosis. It is not recommended that the PPD skin test be repeated when a positive reaction is noted.
Tests Associated With the PPD
The physician will often order additional tests for those that have a positive PPD skin test result or when tuberculosis is suspected. These tests may include chest x-rays, blood tests and cultures of sputum. Tuberculosis may be more difficult to diagnose in those who have recently contracted the disease, children and those with a weakened immune system.
PPD Skin Test. Medline Plus. National Institute of Health. 1, December 2009. Viewed 9, July 2010. https://www.nlm.nih.gov/medlineplus/ency/article/003839.htm
Tuberculosis: Tests and Diagnosis. Mayo Clinic. 28, January 2009. Viewed 9, July 2010. https://www.mayoclinic.com/health/tuberculosis/DS00372/DSECTION=tests-and-diagnosis