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What is a two-step test?

Best practice indicates that a two-step tuberculin skin test (TST) involves two tests performed within one to four weeks of each other. The second test can be completed up to one year from the first test, provided there has not been an exposure to infectious tuberculosis (TB) disease during that time period. It cannot be done less than one week from the first test.

Why a two-step test?

A positive TST may gradually wane over the years.  The first skin test may be negative in persons whose TB exposure was many years ago.  However, this initial test may stimulate the individual’s immune response and a positive reaction may occur when the person is retested one or more weeks later. This delayed response is termed the “booster” phenomenon.  The two-step TST provides an accurate baseline for individuals who will have future serial testing.  If a true baseline is not obtained with a two-step test and the individual is tested again at a future date, a positive result may be misinterpreted as a new infection or “conversion”, when it may really represent a “booster” phenomenon.

When should a two-step test be done?

A two-step TST should be performed if subsequent TSTs will be conducted in the future (e.g., among health care or correctional service workers). The two-step protocol needs to be performed once only if properly performed and documented. It never needs to be repeated. Any subsequent TST can be one step, regardless of how long it has been since the last TST.

If the first test in the two-step TST is positive, the second TST is not required. The individual should be medically evaluated with a chest x-ray and symptom review.

Two-Step TST Decision Making Chart

Person with unknown TST

One documented negative TST

Properly documented results of previous two-step TST

A two-step test is required.

If the TST was done in the past 12 months, only a one-step test is required.

If the result is positive, refer to *Person with a positive TST.

If both tests are negative, only a one-step test is required.

If the result is positive, refer to *Person with a positive TST.

If any previous test was positive, refer to *Person with a positive TST.

If both tests are negative, no further testing is required.

If either test is positive, refer to *Person with a positive TST.

*Person with a Positive TST

  • The person must have a medical assessment and chest x-ray.
  • Further skin testing is not recommended.
  • The person should be informed of the signs and symptoms of active TB disease.
  • Any person with symptoms of TB should not work until there is documentation from their health care provider that they do not have active TB disease.

 

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Last modified: 
Tuesday, October 5, 2021 - 3:16pm