The TB skin test is an injection of purified protein derivative, or PPD, given just under the skin. The purpose of the test is to determine if an individual has been exposed to tuberculosis. Tuberculosis is a serious infection that primarily effects the lungs. The TB test is considered safe; however, there can be side effects.
Typical Side Effects
It is not uncommon for individuals to have redness, itching and discomfort at the site of the injection. These side effects do not indicate a positive TB test and should not interfere with repeating the test if necessary.
Misconceptions
To determine a positive TB test, a health care provider will inspect the injection site 48 to 72 hours after the test was administered. For a test to be considered positive, the amount of induration, or hardness, surrounding the injection site will be measured. The MayoClinic.com reports that the amount of induration combined with a person's TB risk factors determines the significance of the result. Redness and swelling are potential side effects of the TB skin test, but are not considered significantly important when determining the outcome of the test.
False Negative TB Tests
Even when infected, patients with severely immune compromised systems, children and the elderly may have lesser, delayed, or even negative reactions to a TB skin test. Other measures may be needed to diagnose infection such as blood tests, x-rays and cultures. Drugs.com reports that 20 percent of individuals infected with TB will not have a positive response. Those who have recently been infected with a bacteria similar to TB, have undergone chemotherapy, have HIV/AIDS or have cancer may have a false negative result.
BCG Vaccine and the TB Skin Test
The use of the BCG, or bacillus Calmette-Guerin, vaccination is not common in the United States, but is routinely used in other countries. If someone has been vaccinated with the BCG vaccine, it may be cause for a false-positive skin test. If a BCG vaccination has been administered to an individual who has subsequently had a positive TB skin test, further testing needs to be done to determine possible tuberculosis infection.
Immediate Reaction
Because the TB skin test is an shot just under the skin, a small welt, or wheel, should develop upon injection. This welt, according to Drugs.com, should disappear in a few hours after the test. The initial welt resulting from the administration of the PPD is not a positive reaction.
