Skin Reactions to TB Tests
A tuberculosis skin test, or Mantoux tuberculin skin test, is conducted to determine if you're infected with Mycobacterium tuberculosis. The test is necessary if you think you have been exposed to TB, travel to countries that have a high TB rate, are in the health care professions, or have a compromised immune system (such as HIV/AIDS patients). A TB test is important in the early detection of this infectious disease in order to provide effective treatment. Keep in mind that the TB test is not a vaccine, and will not provide protection against tuberculosis.
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Administration
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When the test is administered, a very small amount of purified tuberculin protein is injected into your forearm. According to the Centers for Disease Control and Prevention, this amount equates to about 0.1 ml. No live bacteria exists within the injection, and you cannot receive TB from and injection. It will take between 48 to 72 hours before signs of the TB test become apparent.
Skin Reaction
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TB tests can create bumps upon the injection site a few days after having received a shot. Small bumps are normal. However, large bumps are not. Your clinician will measure the size of your bump, how hard the bump is, and how raised or swollen the bump is. Redness at the injection site may also occur. According to Rutgers, some individuals may experience adverse reactions to a skin test, such as abnormal swelling of the arm or skin death.
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Measurement
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If your injection site presents itself with a large bump, the diameter of the induration (hardness of the bump) will be measured in millimeters. According to the CDC, a positive test can result in an induration of between 5 to 15 millimeters. An induration of 5 ml is classified for those who are immunosuppressed, patients who've had transplants, or if you have recently come into contact with an infected TB individual. A result of 10 ml are reserved for those who are IV users, children, clinical professionals, lab personnel, and immigrants. A result of 15 ml are reserved for individuals who have no known risks for contracting TB.
Negative Test
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In most people who have a negative test, there will be no outward appearance of a bump on your forearm. However, it is possible to not show any signs, but still have TB. Some reasons that you may produce a false-negative test include, a viral illness like chickenpox, a recent TB infection that occurred within the last two months, or a TB infection that occurred many years prior to you taking the TB test. According to the Mayo Clinic, children, older adults, and individuals suffering from AIDS "may have a delayed or no response" to the skin test.
False Positive
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It is possible when you take a TB test to create a false positive. Your skin will create a bump, and may give the appearance that you have TB when you actually do not. Certain types of other non-tuberculosis mycobacterium, an improper test administration, or an incorrect interpretation of a test can result in a false positive. The vaccination bacillus Calmette-Guerin vaccine, which is used in countries with a high incidence of TB, can also create a false positive.
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