Interpretation of Repeated Tuberculin Tests: Boosting, Conversion, and Reversion

The tuberculin skin test is one of the few tests developed in the 19th century that is still in present use in clinical medicine. The first tuberculin test material was prepared by Robert Koch (1); its use for detection of tuberculosis (TB) infection was first described in 1907 by von Pirquet (2). Given such a long history of use, it may seem surprising that aspects of interpretation of this test remain controversial. However, this reflects changes in the populations affected with tuberculosis and their relative frequency of true positive tests from TB infection, and false-positive tests associated with bacillus Calmette-Guerin (BCG) vaccination, or nontuberculous mycobacteria, as well as the recent human immunodeficiency virus (HIV) epidemic. Particular problems have arisen with use of repeated tuberculin tests to detect new infection in high-risk populations such as initially tuberculin-negative contacts of active cases, and workers with occupational exposure. This has revealed that tuberculin reactions may decrease in size (reversion) or increase in size because of: ( 1 ) random variability from differences in administration, reading, or biologic response; ( 2 ) immunologic recall of preexisting delayed type hypersensitivity to mycobacterial antigens (boosting); or ( 3 ) new infection (conversion). This review has been undertaken to provide information regarding factors causing changes in the size of repeated tuberculin reactions.