What is the recommended two-step tuberculin skin test (TST) protocol for tuberculosis screening in Canada?

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Last updated: October 28, 2026 • View editorial policy

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Two-Step Tuberculin Skin Test Protocol in Canada

For new healthcare workers or employees in settings with potential TB exposure who have not had a TST within the past year, perform an initial TST and, if negative, repeat the test 1-3 weeks later to detect the booster phenomenon and establish a true baseline. 1

Purpose of Two-Step Testing

The two-step TST protocol is designed to distinguish between:

  • Boosted reactions (waning immunity from remote infection that is "awakened" by the first test)
  • True conversions (new TB infections acquired after baseline testing)

Without two-step testing, a boosted reaction on subsequent annual testing could be mistakenly interpreted as a new infection, leading to unnecessary investigations and treatment 2, 3.

When to Use Two-Step Testing

Indicated for:

  • New healthcare workers at time of employment who have an initial negative TST result
  • Employees who have not had a documented negative TST within the previous 12 months
  • Any setting where serial tuberculin testing will be performed (e.g., hospitals, long-term care facilities) 1, 3

NOT indicated for:

  • TB contact investigations (a second positive test after initial negative should be considered recent infection, not boosting) 4
  • Patients with documented prior positive TST or TB disease
  • One-time screening without planned follow-up testing

Step-by-Step Protocol

Step 1: Initial TST

  • Administer 0.1 mL of PPD (5 TU) intradermally on the volar forearm using Mantoux technique
  • Read at 48-72 hours by measuring transverse diameter of induration
  • Interpretation threshold: ≥10 mm is positive for healthcare workers 1

Step 2: Second TST (if Step 1 is negative)

  • Administer 1-3 weeks after the first test 1, 2, 3
  • Use the same technique on either arm
  • Read at 48-72 hours

Interpretation of Results

If Step 1 is positive (≥10 mm):

  • Consider the person previously infected with TB
  • Proceed with chest X-ray and evaluation for active disease
  • Consider treatment for latent TB infection if indicated

If Step 1 is negative but Step 2 is positive (≥10 mm):

  • This represents a boosted reaction from remote infection
  • Classify as previously infected (not a new conversion)
  • The second test result becomes the baseline for future comparisons 2, 3

If both tests are negative:

  • Classify as uninfected
  • Any future positive TST (≥10 mm) represents true conversion from new infection 2

Critical Considerations

Booster Phenomenon Prevalence

The booster phenomenon occurs in approximately 6.9% of healthcare workers in Canadian settings 5. This rate is clinically significant enough to warrant two-step testing to avoid false-positive conversion rates.

BCG Vaccination

For BCG-vaccinated individuals (common in Canada's immigrant population):

  • Still perform two-step testing - the protocol remains valid
  • Nearly 80% of well-documented BCG-vaccinated personnel have interpretable reactions (either clearly negative or significantly positive) 5
  • A positive reaction (≥10 mm) in BCG-vaccinated adults from high TB prevalence countries should be attributed to M. tuberculosis infection, not vaccine effect 2, 3

Current Practice Context

Important update: The 2019 CDC/NTCA guidelines 6 now recommend against routine serial TB testing for healthcare workers in the absence of known exposure or ongoing transmission. However, two-step testing remains appropriate at baseline for new employees to establish a proper reference point should future testing become necessary due to exposure.

Common Pitfalls to Avoid

  1. Using two-step testing in contact investigations - This will cause you to miss true recent infections by attributing positive second tests to boosting 4

  2. Waiting too long between tests - The window is 1-3 weeks; longer intervals may not capture the booster effect properly

  3. Dismissing positive reactions in BCG-vaccinated individuals - Treat these as true infections, especially in adults from endemic areas 2, 3

  4. Performing unnecessary serial testing - After establishing baseline with two-step testing, annual repeat testing is no longer routinely recommended unless there is documented exposure or ongoing transmission 6

References

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.