The ability of persons who have TB infection to react to PPD may gradually wane. For example, if tested with PPD, adults who were infected during their childhood may have a negative reaction. However, the PPD could boost the hypersensitivity, and the size of the reaction could be larger on a subsequent test. This boosted reaction may be misinterpreted as a PPD test conversion from a newly acquired infection. Misinterpretation of a boosted reaction as a new infection could result in unnecessary investigations of laboratory and patient records in an attempt to identify the source case and in unnecessary prescription of preventive therapy for HCW's. Although boosting can occur among persons in any age group, the likelihood of the reaction increases with the age of the person being tested.

When PPD testing of adults is to be repeated periodically (as in HCW skin testing programs), two-step testing can be used to reduce the likelihood that a boosted reaction is misinterpreted as a new infection. Two-step testing should be performed on all newly employed HCW's who have an initial negative PPD test result at the time of employment and have not had a documented negative PPD test result during the 12 months preceding the initial test. A second test should be performed 1-3 weeks after the first test. If the second test result is positive, this is most likely a boosted reaction, and the HCW should be classified as previously infected. If the second test result remains negative, the HCW is classified as uninfected, and a positive reaction to a subsequent test is likely to represent a new infection with M. tuberculosis.

Boosting "Waned" Hypersensitivity

  • 1st PPD not significant -- but provides stimulus to increase size reaction on 2nd test
  • Differentiates between "waned hypersensitivity and recent infection
  • Influences whether INH preventive therapy is started

This information is provided by Tuberculosis Prevention and Control Program within the Department of Public Health.