Timing of the Second TB Skin Test in Two-Step Testing
For two-step TB skin testing, the second test should be placed 1-3 weeks after the first test if the initial result is negative. 1, 2
Two-Step Testing for Baseline Screening
The two-step procedure is specifically designed to detect the "booster phenomenon" - where individuals with waned TB immunity from remote infection may have their reactivity restored by an initial PPD test. 1
Key timing parameters:
- Administer the second test 1-3 weeks after the first test if the initial result is negative or shows a small reaction 1, 2
- This interval allows adequate time for the immune system to demonstrate a boosted response without waiting unnecessarily long 1
- Performing the second test too soon can interfere with the immunological response and lead to inaccurate results 2
Primary applications:
- Newly employed healthcare workers with an initial negative PPD and no documented negative test within the preceding 12 months 1, 2
- Individuals in settings requiring periodic TB screening (healthcare facilities, correctional institutions, nursing homes) 1, 2
- Establishing baseline TB status in populations with high rates of prior TB exposure or BCG vaccination 1
Post-Exposure Testing (Different Context)
Important distinction: The 1-3 week interval applies to baseline two-step screening, NOT to post-exposure contact investigations. For contacts of infectious TB patients, different timing applies:
- Initial test: As soon as possible after identifying the exposure 2
- Second test: 8-12 weeks after the last exposure to the infectious patient 3, 2
- The window period is 8-10 weeks because tuberculin sensitivity takes this long to develop after M. tuberculosis infection 3
- A negative test obtained <8 weeks after exposure is unreliable for excluding infection 3
Common Pitfalls
Do not confuse these two distinct scenarios:
- Baseline two-step testing (1-3 weeks between tests) is for detecting the booster phenomenon in individuals without known recent exposure 1, 2
- Post-exposure testing (8-12 weeks between tests) is for detecting new infection in contacts of infectious TB cases 3, 2
Additional considerations:
- The booster phenomenon is more common in older adults, individuals with previous BCG vaccination, and foreign-born persons from high TB prevalence countries 3, 1, 2
- Two-step testing should NOT be used for contact investigations, as a positive second test after initial negative result in that context indicates recent infection, not boosting 3
- Improper timing between tests may affect accuracy of detecting the booster phenomenon 1