How to Read a Tuberculosis Test

The tuberculosis test (TB Test) is designed to locate individuals who have been exposed to and carry the bacteria that causes tuberculosis but are not yet sick from the disease. It is easier to prevent a case of tuberculosis in someone who has been infected than to treat a case of tuberculosis if it is actually present.
The standard TB test is given by injecting 0.1 ml volume containing 5 TU (tuberculin units) into the top layers of skin. The test is to be administered at the forearm of the test subject and the injection should be made in an area free of obstruction and away from veins.

Things You'll Need

  • Millimeter Ruler
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Instructions

  1. Reading a Tuberculosis Test

    • 1

      Wait 48 to 72 hours to read the reaction after the injection is administered.

    • 2

      Measure the area that is raised, hardened, or swollen across the forearm with the millimeter ruler. Do not measure the red area only.

    • 3

      For a small reaction (5 millimeters or less of hardened swelling) consider the result to be positive if the tuberculosis test patient is a person that has had an organ transplant, is an HIV patient, is on steroid therapy or has made contact with someone who has tuberculosis.

    • 4

      For a large reaction (10 millimeters of hardened swelling) consider the result to be positive if the test subject is a recent (within the last five years) foreign immigrant, a diabetic, suffers from kidney failure, is an IV drug user, is a healthcare worker or is a child under four years of age.

    • 5

      For all other test patients, a reaction site measuring 15 millimeters or more is considered to be positive for tuberculosis.

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