Tuberculosis, or TB, is most commonly tested with the TB skin test. The test consists of an injection just under the skin of the inside of the forearm of purified protein derivative, or PPD. The test must be read within 48 to 72 hours by a licensed health care provider. According to Lab Tests Online, the test is considered positive if the reaction to the injection is a hard, raised bump of a predetermined size according to age and health status of the person. The test does not test for active TB infection but exposure to TB at some point. However, there are several problems with the administration and interpretation of the TB skin test.
Active TB May Produce False Negative
According to the National Lung Health Education Program, "substantial numbers of patients with active TB do not have significant reactions to the test." A negative test result is potentially dangerous for someone who's infected with TB but is testing negatively, when only a TB skin test is used as a parameter for diagnosing TB. All matters of clinical importance should be considered when TB is suspected. Health care providers should not be solely reliant on the TB skin test.
Allergies to TB Skin Test
Some immediate allergic reactions have been observed with the TB skin test. These reactions usually resolve within 24 hours and are very rare.
False Positives
MayoClinic.com reports that false positives are most likely if you're infected with a different type of mycobacterium than tuberculosis or have been inoculated with bacillus Calmette-Guerin, or BCG, vaccine. The BCG vaccine is not commonly used in the United States but is utilized in countries with high rates of TB. People who have been vaccinated with BCG should alert health care providers before TB skin test administration.
Delayed Reaction
If a person has been exposed to someone with TB infection, the initial TB skin test may not produce a positive result. According to the Centers for Disease Control and Prevention, a second TB test may be needed eight to 10 weeks after the exposure.
