How to Do a Two-Step TB Test


Tuberculin skin testing (TST) is the standard method for screening for tuberculosis (TB) infection. Because the immune system looses its ability to respond to the tuberculin injection during a TB infection, individuals infected with TB may show a false negative result with the initial TST. To avoid misinterpreting a previous infection that didn't show at initial testing with a current infection, many institutes frequently apply a two-step TB testing. The two-step TST is common among health care workers and nursing home residents.

Allow the trained health care personnel to inject 0.1 ml of tuberculin purified protein derivative (PPD) into the inner surface of your forearm. A pale elevation of the skin at the injection spot appears and indicates a correct intradermal injection.

Go back to the health care provider 48 to 72 hours after initial tuberculin injection. Allow the health care personnel to read the test by measuring the test area induration (swelling, redness, raised or hardened area). Depending on your health status, an induration of greater than 5 mm may be considered positive.

Give your health care personnel permission to place a second tuberculin injection at the seventh day after the initial TST when your skin test reads negative. This subsequent testing allows for a boosted reaction from a previous infection to read positive.

Return to your health care provider within 48 to 72 hours from the second TST and get a test reading. A negative two-step TB test indicates no current or previous TB infection. A positive two-step test indicates infection and additional evaluation to rule out current TB infection is performed.

Tips & Warnings

  • Some institutes allow for a three-visit testing with the two-step TB test by reading the initial TST at the seventh day (same day of second injection).
  • Always follow the guidelines of your health care provider for TST reading time frames.

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