For Students
Infectious Disease Prevention Educational Module
Section 8: Two-Step Mantoux
From: Centers for Disease Control (CDC), Core Curriculum on Tuberculosis (2000)
http://www.cdc.gov/nchstp/tb/pubs/corecurr/Chapter4/Chapter_4_Skin_Testing.htm.
In some people who are infected with M. tuberculosis, delayed-type hypersensitivity to tuberculin may wane over the years. When these people are skin tested many years after infection, they may have a negative reaction. However, this skin test may stimulate (boost) their ability to react to tuberculin, causing a positive reaction to subsequent tests. This boosted reaction may be misinterpreted as a new infection. The booster phenomenon may occur at any age; its frequency increases with age and is highest among older persons. Boosted reactions may occur in persons infected with nontuberculous mycobacteria or in persons who have had a prior BCG vaccination.
Two-step testing is used to reduce the likelihood that a boosted reaction will be misinterpreted as a recent infection. If the reaction to the first test is classified as negative, a second test should be done one to three weeks later. A positive reaction to the second test probably represents a boosted reaction (past infection or prior BCG vaccination). On the basis of this second test result, the person should be classified as previously infected and cared for accordingly. This would not be considered a skin test conversion. If the second test result is also negative, the person should be classified as uninfected. In these persons, a positive reaction to any subsequent test is likely to represent new infection with M. tuberculosis (skin test conversion). Two-step testing should be used for the initial skin testing of adults who will be retested periodically, such as health care workers.
Because of cross-reactions with other mycobacteria, the specificity of the tuberculin test is less when serial skin testing is performed than when a single test is administered. Thus, serial skin-testing programs tend to overestimate the incidence of new TB infection in the tested population. Because of this potential for overestimation of new infections, serial skin-testing programs should be targeted to populations at high risk for continued exposure to infectious persons.
Proceed to Section 9: Self-Assesment Questions.
