A small dose of tuberculin will not sensitize a patient, but it may cause a patient with latent tuberculosis to show an anamnestic response. This "booster effect" can cause a repeat tuberculin test to be positive.
The use of 2 tuberculin skin tests in tandem is referred to as a two-step tuberculin skin test and is typically used in health care workers at the time of initial employment.
Method:
(1) An initial tuberculin skin test is performed and read within 48-72 hours.
(2) If positive, then the patient is evaluated for tuberculosis.
(3) If the initial skin test is negative, then a second skin test is performed (at another site) 1 to 3 weeks after the first.
A patient with latent tuberculosis may show a positive reaction to the second dose.
The time interval between doses is kept small to avoid confusion that might arise from a new tuberculosis exposure with conversion in the interval window.
After the two-step test at hiring subsequent tuberculin testing is a single step test done annually.
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