Tuberculin Skin Test
The tuberculin skin test (TST), also known as the Mantoux skin test,
involves injecting 0.5ml of tuberculin antigen under the skin in the lower part of the forearm and waiting for an antigen-antibody reaction to take place within 2-3 days. This reaction is interpreted by a trained health care professional that measures the size of the swelling or raised area (not redness). The decision whether the test is positive or negative is made by considering the size of the swelling as well as the patient’s medical history and risk of tuberculosis infection. (1)
The tuberculin skin test can be performed in infants, children,
pregnant women, persons that have received the BCG vaccine or persons with HIV. (2)
It is only contraindicated in persons that have previously had an adverse
reaction to it (ulceration, blistering, necrosis, etc.). Since receiving a live
virus vaccination might alter the TST results, the TST should be performed on
the same day as the live virus vaccine or 4-6 weeks after. (2)
Persons that were infected with tuberculosis long before getting tested
might obtain a negative result on the tuberculin skin test. By administering
tuberculin to their forearm, their ability to react to a subsequent tuberculin
test will be enhanced. This is called a “boosted reaction” or “two-step testing”
and is useful to perform in all health care personnel in order to eliminate the
risk of erroneously interpreting a positive tuberculin screen test as a recent
infection. (2)
The following table shows how TST results are interpreted: (2)
References:
1. CDC. Tuberculosis testing & diagnosis. http://www.cdc.gov/tb/topic/testing/default.htm. Updated April 12, 2013.
Accessed May 10, 2013.
2.CDC. Tuberculosis fact sheets. Tuberculin skin testing. http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm Updated September 1, 2012. Accessed May 17, 2013.
Images
http://upload.wikimedia.org/wikipedia/commons/f/fa/Mantoux_tuberculin_skin_test.jpg
http://www.healthyinfo.com/clinical/tb/index.shtml
ttp://www.lung.ca/tb/tbtoday/tbdiagnosis/skin_test.html