How to Read a Tuberculosis Skin Test
The tuberculosis skin test is a method used to check if a person is infected with Mycobacterium tuberculosis, which is the bacteria that causes tuberculosis, or TB. The test involves injecting a fluid into a person's arm and then asking him to come back for further evaluation of the injection site in 48 to 72 hours.
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Instructions
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How to Read a Tuberculosis Skin Test
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Monitor the injection site for two to three days. You may notice some redness and induration or swelling in that area. You must measure the amount of swelling on the third day approximately 72 hours after the TB test was administered. If the test is not evaluated at this time, it is considered invalid and will need to be re-administered.
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Measure the amount of swelling at the test area with a flexible ruler calibrated in millimeters. Measure the diameter of just the area of swelling across the arm and not along the long length of the arm. Don't include areas of redness in your measurement.
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Know your risk factors for TB. Interpreting the TB result depends on what your TB risk factors are. Risk factors include being HIV positive, living in crowded quarters, being exposed to persons that have TB disease or having an occupation that exposes you to the bacteria that causes TB, such as working in a laboratory.
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Be aware of the criteria required for a test to be interpreted as positive. An area of swelling greater than 5mm is considered positive in persons who have recently had contact with a person who has TB, persons who have HIV, persons with chest X-rays that show healed TB or whose immune systems have been suppressed due to medication or disease. A 10 mm or greater swelling is considered positive in people who moved from a country with a high prevalence of TB within the past five years. This measurement is also considered positive in people who abuse drugs through injections and people at risk of exposure to TB on the basis of where they live or work. An induration of 15mm or more is positive in anyone, regardless of risk factors.
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Tips & Warnings
A negative TB test could occur due to a weakened immune system, a recent TB infection that is 8 to 10 weeks old or a very old one that is many years old. This could also happen if the person is less than 6 months old, has some viral illnesses like chicken pox, has an overwhelming case of TB or just got a recent live virus vaccination. Incorrect administration or interpretation of the test may also cause this problem. These results are invalid and are also known as false negative results.
A false positive result is possible in people who are infected with non-tuberculosis mycobacteria or who have been vaccinated against TB. Inaccuracies in the administration or measurement of the test may also cause positive results.
