How to Read a Tuberculosis Skin Test

A tuberculosis skin test is also known as the 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. This article will tell you how to read a tuberculosis skin test.

Steps

1. Go to your doctor for the tuberculosis skin test. You will be given an injection of a purified protein derivative that will result in a welt that will disappear within a few hours.
2. Leave your arm unbandaged for the required 48 to 72 hours. You can wash and dry your arm carefully.
   • You also should not scratch or rub the test area on your arm. You can apply a cold washcloth on your arm if it is itchy.
3. Return to your doctor within 72 hours to have your TB test read. If you do not come back in 72 hours, the test is considered invalid and will have to be repeated.
4. Measure the amount of swelling (also called induration) at the test site. This induration is measured across your forearm in millimeters.
5. Compare the amount of swelling to the standardized risk factor classification. This classification chart is listed below.
   • An induration of 5mm or more is categorized as positive in individuals who have HIV, organ transplants, chronic disease (rheumatoid arthritis), contact with a TB positive person or a corresponding chest x-ray.
   • An induration of 10mm or more is categorized as positive in people who immigrated recently from a country with widespread TB, uses injection drugs, works in health-care, or are children or adolescents exposed to a high-risk adults.
6. An induration of 15mm or more is considered positive in 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.
7. Go to any further testing your doctor might order if the test is positive or if the test is considered borderline positive.

Warnings

• 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.
• 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.

Things You'll Need

• Ruler with millimeters