What is the minimum time interval before repeating a Tuberculin Skin Test (TST)?

Minimum Time Interval Between Tuberculin Skin Tests

The minimum time interval before repeating a Tuberculin Skin Test (TST) is 1-3 weeks when using the two-step testing method, and at least 8-10 weeks after exposure ends when testing for TB infection following exposure. 1

Testing Intervals Based on Clinical Scenario

Post-Exposure Testing

• Initial test: Should be administered as soon as possible after exposure is identified 2
• Follow-up test: Must be performed 8-10 weeks after exposure ends if initial test is negative 2
  • This 8-10 week window period is critical as tests performed earlier may miss infections due to delayed hypersensitivity development
  • Previous recommendations suggested 2-12 weeks, but reanalysis of data indicates 8 weeks is the outer limit of this window period 2

Two-Step Testing (for Baseline Screening)

• Used to detect boosting phenomenon and establish accurate baseline
• Minimum interval between first and second test: 1-3 weeks 1
• Purpose: Identifies individuals with waned immunity who may show boosting on subsequent tests
• Important note: Two-step testing should NOT be used for contact investigations; a positive second test in contacts should be considered evidence of recent infection 2

Routine Surveillance Testing

• Recommended interval: At least 3 months between routine tests 1
• High-risk healthcare workers: Every 6 months
• Standard healthcare workers: Annually

Special Considerations

Boosting Phenomenon

• Nonspecific or remote delayed-type hypersensitivity to tuberculin can wane over time 2
• Subsequent TSTs can restore this responsiveness (boosting)
• Boosting is more common in:
  • Foreign-born individuals 2
  • BCG-vaccinated persons
  • Elderly patients (progressive boosting observed with sequential testing) 3

False Results and Interpretation

• For most accurate results, TST should be measured at 72 hours after placement 4
• Readings at 48 or 96 hours may lead to less accurate results
• Induration ≥5mm is considered positive for contacts of TB cases 2

After Travel to TB-Endemic Areas

• If initial test before travel was negative, repeat testing should be done approximately 8-10 weeks after return 2
• This applies especially to immunosuppressed patients or those traveling for more than one month

Pitfalls to Avoid

1. Misinterpreting boosting as new infection: This is particularly problematic in foreign-born individuals and the elderly 2, 3

2. Testing too early after exposure: A negative test <8 weeks after exposure is unreliable for excluding infection 2

3. Repeating TST in individuals with documented positive results: This is unnecessary and can lead to confusion 1

4. Ignoring the effect of previous TST on interferon-gamma assays: TST can boost subsequent interferon-gamma assay results when performed 2-4 weeks later 5

5. Considering erythema instead of induration: Only induration should be measured when interpreting TST results 6

By following these evidence-based intervals for tuberculin skin testing, clinicians can accurately identify TB infections while avoiding misinterpretation due to boosting phenomena or premature testing.