Two-step testing is used to distinguish between the booster phenomenon and new infection. If the reaction to the first test is negative, a second test is given 1 to 3 weeks later. Two-step testing can be useful for the initial skin testing of adults who are going to be retested periodically, such as health care workers or nursing home residents. Some persons may not react to the tuberculin skin test even though they are infected with M. tuberculosis . This may be because of anergy.
Anergy is the inability to react to skin tests because of immunosuppression.
Anergy is often caused by HIV infection, but it can also be caused by other medical conditions. Anergy is determined by administering two delayed-type hypersensitivity antigens, such as tetanus toxoid, mumps, or Candida , by the Mantoux method. Persons who do not react to any of these antigens, including tuberculin, are probably anergic. The use of anergy testing in conjunction with PPD testing is no longer recommended routinely for screening programs for M. tuberculosis infection conducted among HIV-infected persons in the United States. In persons vaccinated with BCG, sensitivity to tuberculin is highly variable, depending upon the strain of BCG used and the group vaccinated.