Who Is Tested?
A tuberculosis (TB) test is only administered to populations at high risk for exposure to the bacteria, according to Labtestsonline.org. These populations include those with weakened immune systems, like those with HIV or AIDS; individuals confined to a living space, like prison or a nursing home; health care workers who come in contact with individuals with TB; people who frequently interact with someone with TB; and those coming from a foreign country where TB may be more common. Less frequently, TB testing is done prior to admission to a new school or job and to screen pregnant women. The test is used to diagnose latent TB infections or active disease, according to Labtestsonline.org.
How Are They Tested?
The most commonly used method for checking for a TB infection or active TB is the PPD skin test. PPD is an acronym for purified protein derivative. According to the National Institutes of Health (NIH), the skin test begins with a cleansing of the test site, usually the forearm. PPD is a liquid extract that is injected into the skin. According to the NIH, the injection penetrates the first layer of skin, creating a blister on the surface. The patient can return home for 48 to 72 hours. At that point they must return to the doctor and have the test site examined. A reaction to the PPD is measured by how hard the swelling is at the site of the blister.
Results
The smaller the swelling measurement, the less the risk of TB infection. The swelling is measured in millimeters (mm). A small reaction of 5 mm is considered to be a positive TB result in patients with HIV, those participating in steroid therapy, or those who have been in close contact with a person with active TB, according to the NIH. Individuals with no known risks for TB require a reaction of at least 15 mm at the injection site in order to be diagnosed with TB.
