Read a Tuberculosis Skin Test

A tuberculosis skin test is also known as the Mantoux tuberculin test. This test measures your immune system's response to the bacteria that causes tuberculosis. Your results will be interpreted by your doctor a couple of days after the test. If you're curious about how to read a tuberculosis skin test, this article will guide you through the process, but remember: the test must be read by a trained professional. You can interpret the test yourself, but the result needs to be documented by a medical professional to ensure proper treatment and/or follow-up.[1]

Steps

Reading the Test

  1. Go to your doctor for the tuberculosis skin test. You will be given an injection of a purified protein derivative into the inner forearm that will result in a 6-10 mm welt that will disappear within a few hours.[1]
  2. Leave your arm uncovered. Do not place a bandage over the site of the test for the required 48 to 72 hours. You can wash and dry your arm carefully.[2]
    • You also should not scratch or rub the test area on your arm. This can cause redness or swelling that may cause the results to be misread. You can apply a cold washcloth on your arm if it is itchy.[2]
  3. Return to your doctor. The test must be read within 48-72 hours. If you do not come back within 72 hours, the test is considered invalid and will have to be repeated.[3]
  4. Find and mark the induration. Use your fingertips to find the induration. This is a hard, dense, raised formation with definite borders. If there is a firm bump, use a pen to mark the widest edges of the induration on the forearm.[4] The only part that is important for your test results is the hard bump. The reddened area or any slight swelling does not count towards the size of the induration.
    • You cannot always see the induration. You must find the induration with your fingertips.[5]
  5. Measure the induration. The test site may be red, but that does not mean you have TB. You have to measure the induration. This induration is measured across your forearm in millimeters.[4] Use a ruler with millimeter measurements. Place the edge of the ruler with "0" on the left edge of the raised bump where you marked it with a pen. Look where the mark made on the right side of the bump falls on the ruler.[4]
    • If the mark is between two different lines, use the lower measurement.

Interpreting the Test

  1. Determine if the individual is in the high-risk group. An induration of 5mm or more is categorized as positive in individuals in the high-risk group. This group includes people who have:
    • HIV
    • received organ transplants
    • been immunosuppressed for various reasons
    • recent contact with a TB positive person
    • chest x-ray consistent with old healed TB
    • end stages of renal (kidney) disease[6]
  2. Determine if the individual is in the moderate-risk group. An induration of 10mm or more is categorized as positive in people in the moderate-risk group. This includes people who:
    • immigrated recently from a country with widespread TB
    • use injection drugs
    • work in healthcare settings, prisons, nursing homes, or other similar areas
    • have clinical conditions that put them at risk, like diabetes, leukemia, low body weight
    • are children under age 4
    • are children and adolescents exposed to high-risk adults[1]
  3. Look for a large induration for everyone else. For those not in the high or moderate-risk group, an induration of 15mm or more is considered positive. This includes all individuals, regardless of any risk factors they might have. The test is also considered positive if there are blisters present, even if there is little swelling.[6]
  4. Look for a negative result. If there is no firm bump, the result is negative.[7] If there is soft swelling or redness, but no noticeable hardened bump that can be felt at the site, it is a negative.[5]
    • Even if you believe your skin test is negative, you must return to the doctor to have it professionally read.

Tips

  • Go to any further testing your doctor might order if the test is positive or if the test is considered borderline positive.

Warnings

  • False positives and false negatives can happen with this test. If there are any questions about your tuberculosis test results, consult with your doctor as soon as possible.
  • A TB test should always be interpreted by a trained health professional within the 72 hour time span. These professionals receive training and practice to measure the results correctly.

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 1.2 http://www.cdc.gov/tb/publications/factsheets/testing/skintesting.htm
  2. ↑ 2.0 2.1 http://www.webmd.com/a-to-z-guides/tuberculin-skin-tests?page=2
  3. ↑ http://www.cdc.gov/tb/topic/testing/
  4. ↑ 4.0 4.1 4.2 http://www.cdc.gov/tb/publications/Posters/images/Mantoux_wallchart.PDF
  5. ↑ 5.0 5.1 http://www.cdc.gov/tb/education/mantoux/part2.htm
  6. ↑ 6.0 6.1 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3481914/
  7. ↑ http://www.webmd.com/a-to-z-guides/tuberculin-skin-tests?page=3
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