When skin testing persons who were infected with M. tuberculosis a long time ago. In some persons who are infected with M. tuberculosis , the ability to react to tuberculin may wane over time. When given a skin test years after infection, these persons may have a negative reaction. Two-step testing is used to determine whether these persons are truly infected. When skin testing persons who are anergic.
Anergy is the inability to react to skin tests because of a weakened immune system.
In some persons who are infected with M. tuberculosis , the ability to react to tuberculin may wane over time. When given a skin test years after infection, these persons may have a negative reaction. However, this skin test may stimulate the immune system, causing a positive reaction to subsequent tests. This is called the booster phenomenon. Two-step testing is used to distinguish between the booster phenomenon and new infection.