Mantoux Test - Two-step Testing

Two-step Testing

Some people who were previously infected with TB may have a negative reaction when tested years after infection, as the immune system response may gradually wane. This initial skin test, though negative, may stimulate (boost) the body's ability to react to tuberculin in future tests. Thus, a positive reaction to a subsequent test may be misinterpreted as a new infection, when in fact it is the result of the boosted reaction to an old infection.

Use of two-step testing is recommended for initial skin testing of adults who will be retested periodically (e.g., health care workers). This ensures any future positive tests can be interpreted as being caused by a new infection, rather than simply a reaction to an old infection.

• The first test is read 48–72 hours after injection.
  • If the first test is positive, consider the person infected.
  • If the first test is negative, give a second test one to three weeks after the first injection.
• The second test is read 48–72 hours after injection.
  • If the second test is positive, consider the person previously infected.
  • If the second test is negative, consider the person uninfected.

A person who is diagnosed as "infected" on two-step testing is called a "tuberculin reactor". The US recommendation that prior BCG vaccination be ignored results in almost universal false diagnosis of tuberculosis infection in people who have had BCG (mostly foreign nationals).