Signs of a Positive TB Test

Injection
Patient getting TB test (Image: Alexander Raths/iStock/Getty Images)

Tuberculosis, commonly called TB, is an infectious disease that is caused by the Mycobacterium tuberculosis bacteria. It is usually found in its chronic, latent form, and most individuals appear without symptoms. It is contagious, and is spread air borne by the infected individual. According to the World Health Organization, it is mostly common in the developing world, with new cases seen in sub-Saharan Africa and Southeast Asia.

Tuberculin Skin Test

The Mantoux skin test can be performed to determine if a person has or ever has had a TB infection. A small amount of the TB antigen, known as a purified protein derivative, is injected under the top layer of skin on the patient's forearm. A follow-up is conducted within three days examine the injection site for an immune response. If the test is positive for TB, the skin is red, raised, palpable and hardened. The reaction is measured in millimeters. According to the Centers for Disease Control and Prevention, 5 mm or more is seen as positive in people who are immune compromised, such as HIV patients. Ten mm or more is seen as positive in people such as recent immigrants or children. Fifteen mm or more is positive in people with no known risk factors.

Disadvantages of TB Skin Test.

The main disadvantage of the Mantoux testing is that it does not tell when the person was infected, and if it is latent or active. It only shows if the person has ever been exposed to TB. People from countries that administer the TB vaccine known as Bacillus Calmette-Guerin usually test positive, and they can be falsely diagnosed as having TB. A person with a severely compromised immune system such as an AIDS patient can react to foreign substances such as purified protein derivative. The Mantoux tuberculin skin test remains the most common test for TB.

Facts regarding TB Skin Test

The TB skin test can be repeated several times, and in cases where a person does not return to have it read within three days, another one can be performed. The Centers for Disease Control and Prevention encourages people receiving live vaccines, to either conduct the TB skin test on the same day as the vaccination, or four to six weeks afterward.

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