TB 2-step skin test

In some persons infected with TB, especially older persons, the ability to mount a positive TST reaction may wane over time. An initial TST placed in these individuals may not fully react, and be interpreted as negative. However, a repeat TST within a year or more may react and show a positive reaction. This effect is known as the booster phenomenon, and a positive “boosted” response should be considered the valid baseline for that individual. In a person who has never been infected with TB (assuming no BCG vaccination or non-TB mycobacterial infection), repeated TST testing itself will not elicit a positive reaction.

Two-step testing is a screening method that takes into account a possible booster phenomenon in an individual and should be considered for any person when serial testing is to occur. While boosting is most common in persons aged 55 or older, some employers who require TB screening utilize a two-step test for all new employees regardless of age while others select an age cut-off point of usually 45 to 55 years to reduce the likelihood that a boosted reaction is not misinterpreted as a recent conversion.

Candidates for two-step TST include employees of health care facilities, employees who undergo periodic TB screening, and residents of congregate living settings (details for screening those populations are described in Chapter Seven, page 7-7). Two-step testing should be performed only for initial TB screening. Subsequent periodic testing requires only one TST. Therefore, an individual who can provide documentation of a negative TST by the Mantoux technique within the preceding year has no need for two-step testing, as it is very unlikely the TST result is the result of waning immunity.


 Procedure and interpretation for two-step skin testing (see Figure 2-1, page 2-17):

1. Administer the Mantoux TST.
2. Examine the TST in 48 to 72 hours. If initial TST is negative, repeat TST within one to three weeks using the same dose and strength of tuberculin and have patient return in 48 to 72 hours for the second reading. A positive TST means the individual is considered infected, and should be evaluated for treatment of LTBI.

OR

 As an alternative, the initial TST can be examined in seven days, since a truly positive TST persists for many days beyond 72 hours. If the initial TST is negative, a repeat TST can be administered at the same visit and the individual should be instructed to return in 48 to 72 hours for the second reading. A positive TST means the individual is considered infected, and should be evaluated for treatment of LTBI.

Source: http://publichealth.lacounty.gov/tb/forms/chapter2.pdf   (page 15)