Two-Step TB Test Timing
The second tuberculin skin test in a two-step protocol should be administered 1–3 weeks after reading the first test. 1
Standard Two-Step Protocol Timeline
The two-step testing process follows this specific sequence:
- Day 0: Place the first PPD test 1
- Day 2–3 (48–72 hours): Read the first test 1
- Day 9–24 (1–3 weeks after first reading): Place the second PPD test 1
- 48–72 hours after second placement: Read the second test 1
Rationale for Two-Step Testing
Two-step testing is specifically designed to detect the "booster phenomenon"—where individuals with waned TB immunity may have their reactivity restored by an initial PPD test, which could otherwise be misinterpreted as a new infection during subsequent serial testing. 1
- The booster effect can occur years after initial TB infection or BCG vaccination, particularly in older adults, foreign-born persons from high TB prevalence countries, and those previously vaccinated with BCG 2, 1
- Without two-step testing, a boosted reaction on future testing might trigger unnecessary contact investigations and preventive therapy 1
When Two-Step Testing Is Required
Two-step testing should be performed in these specific situations:
- All newly employed healthcare workers with an initial negative PPD and no documented negative PPD within the preceding 12 months 1
- Settings with periodic TB screening (healthcare facilities, correctional institutions, nursing homes) 1
- Establishing baseline TB status in populations with high rates of prior TB exposure or BCG vaccination 1
Critical Timing Considerations
- The 1–3 week interval between tests is essential—performing the second test too soon can interfere with the immunological response and lead to inaccurate results 1
- For contact investigations (different from two-step baseline testing), the window period for detecting new infection is 8–10 weeks after exposure ends, and a negative test obtained <8 weeks after exposure is unreliable 2
Common Pitfalls to Avoid
- Improper timing between first and second test (outside the 1–3 week window) may compromise detection of the booster phenomenon 1
- Confusing two-step baseline testing with post-exposure testing—these serve different purposes with different timing requirements 2, 1
- Failing to implement two-step testing in appropriate populations leads to misclassification of boosted reactions as new infections 1