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Two-Step TB Skin Test

The two-step test is NOT the usual PPD skin test in which you receive an injection of PPD and the test area is observed 48 hours later. The two-step PPD test is used to detect individuals with past TB infection who now have diminished skin test reactivity. This procedure reduces the likelihood that a boosted reaction is later interpreted as a new infection. The reason for the 2 stage PPD test appears to be the “booster phenomenon.” It occurs in some people who were infected with TB in the past because the body loses its ability to react to the tuberculin solution. Thus, when these people are tested many years after the initial infection they may have a negative reaction. However, if they are tested a second time within up to one year after the first test, they may have a positive reaction. In the 2-step testing procedure a person is given a baseline PPD test. If the test is negative, a second test is administered one to three weeks later. If the second test is negative, the person is considered uninfected. If the second test is positive, then the person is considered to have a “boosted” reaction to an infection that occurred a long time ago.

The number of visits required may be either three or four depending on the protocol of the health care agency administering the test.

The Four Visit Approach

Visit 1, Day 1
The PPD antigen is applied under the skin.
Visit 2, Day 3
The PPD test is read. If the first test is positive, it indicates that the individual is infected with TB. A chest X-ray and evaluation is necessary. If the individual is asymptomatic and the chest X-ray indicates no active disease, the individual can enter patient care areas.
Visit 3, Day 7
A second PPD skin test is applied to those individual in whom the PPD skin test is negative.
Visit 4, Day 9
The second test is read. A positive 2nd test indicates TB infection in the distant past. The individual is referred for a chest X-ray and evaluation by a physician. An asymptomatic individual whose chest X-ray indicates no active disease may enter patient care areas.

The Three Visit Approach

Visit 1, Day 1
The first skin test is applied and the individual returns in 7 days for the test to be read. If the first test is positive, it indicates that the individual is infected with TB. A chest X-ray and evaluation is necessary. If the individual is asymptomatic and the chest X-ray indicates no active disease, the individual can enter patient care areas.
Visit 2, Day 7
A second skin test is applied on individuals whose first test was negative at 7 days.
Visit 3, Day 9 or 10
The second test is read. A positive 2nd test indicates TB infection in the distant past. The individual is referred for a chest X-ray and evaluation by a physician. An asymptomatic individual, whose chest X-ray indicates no active disease may enter patient care areas. The majority of significant PPD skin test reactions will remain “positive” for 7 days after application. Those that have diminished or disappeared by day 7 will be boosted back to positive by the 2nd skin test. Reducing the number of visits from 4 to 3 will not reduce the sensitivity of the two-step test.

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