What is the recommended waiting time between Purified Protein Derivative (PPD) placements for individuals requiring repeated testing?

Recommended Waiting Time Between PPD Placements

For individuals requiring repeated tuberculin skin testing (PPD), a minimum waiting period of 1-3 weeks is recommended between tests when using the two-step testing method, and subsequent tests should be spaced at least 3 months apart for routine surveillance testing. 1

Two-Step Testing Protocol

When initial PPD testing is required (such as for new healthcare workers):

• First PPD test is placed and read at 48-72 hours
• If the first test is negative, a second test should be performed 1-3 weeks after the first test
• The result of the second test is considered the "correct" baseline result
• This two-step approach helps identify the "booster phenomenon" and prevents misinterpreting it as a new infection in future tests 1

Booster Phenomenon Explained

• Some individuals with previous TB infection may have waning skin test reactivity over time
• The first PPD test can stimulate or "boost" the immune response
• Without two-step testing, a boosted reaction on subsequent testing could be misinterpreted as a new infection 1, 2
• Boosted reactions are particularly common in:
  • Older individuals
  • Those previously exposed to nontuberculous mycobacteria
  • Those previously vaccinated with BCG 2

Routine Surveillance Testing Intervals

After establishing a baseline with two-step testing (if indicated), subsequent testing should follow these intervals:

• High-risk healthcare workers (frequent exposure to TB patients or involvement in high-risk procedures like bronchoscopy): Every 6 months 1
• Other healthcare workers in lower-risk settings: Annually 1
• After known exposure to infectious TB without proper precautions: Immediate testing and repeat testing 12 weeks after exposure ended 1

Special Considerations

Previous Positive PPD Results

• Individuals with documented previous positive PPD results should not undergo repeated PPD testing 3
• Instead, these individuals should receive:
  • Symptom screening (cough, fever, night sweats, weight loss)
  • Chest X-ray to rule out active disease 3

BCG Vaccination History

• BCG vaccination history should not affect the interpretation of PPD results or testing intervals 4
• There is little relationship between prior BCG vaccination and PPD positivity, especially as time since vaccination increases 5, 4
• PPD reactions >14mm are unlikely to be due to prior BCG vaccination 5

Post-Exposure Testing Protocol

When unprotected exposure to TB occurs:

1. Immediate PPD testing (unless a negative test is documented within the previous 3 months)
2. If initial test is negative, repeat testing 12 weeks after exposure ended
3. Individuals with reactions ≥5mm or symptoms suggestive of TB should receive chest radiographs 1

Common Pitfalls to Avoid

• Performing repeated PPD tests at intervals shorter than recommended can lead to false interpretations due to the booster phenomenon
• Failing to use two-step testing for initial screening can result in misclassification of boosted reactions as new infections
• Considering BCG vaccination status when interpreting PPD results may lead to undertreating true TB infections 4
• Repeating PPD tests in individuals with documented positive results is unnecessary and can lead to confusion 3

By following these evidence-based guidelines for PPD placement intervals, healthcare providers can accurately identify true tuberculosis infections while avoiding misinterpretations due to boosting or other phenomena.