What is the 2-step Tuberculin Skin Test (TST)?

The Two-Step Tuberculin Skin Test (TST)

The two-step tuberculin skin test (TST) is a screening procedure designed to establish an accurate baseline for tuberculosis infection by identifying boosted reactions that might otherwise be misinterpreted as new infections during subsequent testing. 1, 2

What is the Two-Step TST?

The two-step TST involves:

1. First TST: Administration of standard 0.1 mL of purified protein derivative (PPD) (5 tuberculin units) intradermally, with reading of results 48-72 hours later
2. Second TST: If the first test is negative, a second TST is administered 1-3 weeks after the first test
3. Final interpretation: The result of the second test is considered the "correct" baseline result 2, 1

Purpose and Mechanism

The two-step approach addresses the "booster phenomenon," which occurs when:

• Individuals with remote TB infection experience waning of their tuberculin reactivity over time
• The first TST stimulates or "boosts" the immune response
• Without a two-step baseline, a subsequent positive test might be misinterpreted as a new infection (conversion) rather than a boosted reaction 1, 2

Who Should Receive Two-Step TST?

Two-step testing is recommended for:

• Healthcare workers (HCWs) upon hire, regardless of risk classification 1
• Individuals who will undergo serial TST testing (such as healthcare workers)
• Persons with increased likelihood of boosting:
  • Foreign-born individuals
  • Those with history of BCG vaccination
  • Older adults 2, 3

Interpretation of Results

Interpretation of the TST depends on both the size of induration and risk factors:

• ≥5 mm: Positive for HIV-infected persons, recent contacts of TB cases, and immunocompromised individuals
• ≥10 mm: Positive for healthcare workers and other risk groups
• ≥15 mm: Positive for persons with no known risk factors 1

Important Distinctions

• Two-step testing vs. contact investigation: Two-step testing should NOT be used for contact investigations. For contacts, a positive second test should be considered evidence of recent infection 2
• Two-step testing vs. serial testing: Two-step testing establishes a baseline, while serial testing is periodic screening to detect new infections 1

Clinical Implications

Research shows that:

• In one study, 14.2% of healthcare workers had boosted reactions on the second test 3
• Even after negative two-step testing, foreign birth and history of BCG vaccination remain associated with higher rates of positive results on future tests 4
• The boosting effect is particularly common in individuals with previous BCG vaccination or exposure to environmental mycobacteria 1

Practical Considerations

• The second test should be performed 1-3 weeks after the first test
• If the first test is positive, no second test is needed
• The result of the second test becomes the new baseline
• Documentation of the two-step baseline is important for future comparison 2

Pitfalls to Avoid

• Misinterpreting a boosted reaction as a new infection
• Using two-step testing inappropriately during contact investigations
• Failing to document baseline results properly
• Not allowing sufficient time between tests (1-3 weeks is recommended)
• Incorrectly measuring or interpreting induration size 1, 2

By establishing an accurate baseline through two-step testing, healthcare facilities can better identify true TB infections in subsequent testing, improving occupational health surveillance and preventing unnecessary treatment for presumed TB infection.