What is the purpose and procedure of the PPD (Purified Protein Derivative) 2-step process for tuberculosis screening?

Purpose and Procedure of the PPD Two-Step Process for Tuberculosis Screening

The two-step PPD testing process is specifically designed to detect the booster phenomenon in individuals with waned TB infection, preventing misinterpretation of boosted reactions as new infections during subsequent serial testing. 1

Purpose of Two-Step Testing

• The two-step PPD testing method addresses the "booster effect" phenomenon, where individuals previously infected with TB may have a diminished ability to react to tuberculin that can be restored by an initial PPD test 1
• Without two-step testing, a boosted reaction on subsequent testing might be misinterpreted as a new infection (conversion), leading to unnecessary contact investigations and preventive therapy 1
• Two-step testing is particularly important for individuals who will undergo repeated tuberculin testing, such as healthcare workers in TB screening programs 1
• The process establishes a reliable baseline for future comparison, especially in individuals who may have been infected with TB in the distant past 2

Procedure for Two-Step Testing

First Step:

• Administer the initial PPD test using the Mantoux technique (0.1 mL of PPD containing 5 tuberculin units) intradermally on the forearm 2
• Read the test result 48-72 hours after administration by measuring the induration (hardened area) in millimeters 2
• Document the result as negative or positive based on risk-stratified cutoff values (≥5 mm for high-risk, ≥10 mm for moderate-risk, ≥15 mm for low-risk individuals) 2

Second Step (for those with negative initial results):

• If the first test is negative, administer a second PPD test 1-3 weeks after the first test 1
• Read the second test result 48-72 hours after administration using the same technique 2
• The result of this second test is considered the true baseline for future comparisons 1

Interpretation of Results

• If the first test is positive: The individual is considered to have TB infection and should be evaluated for active TB and considered for treatment of latent TB infection 2
• If the first test is negative and the second test is positive: This indicates a boosted reaction from prior TB infection that had waned over time; the individual is classified as previously infected 1
• If both tests are negative: The individual is considered uninfected; any future positive test would represent a true conversion suggesting new infection 1

Clinical Applications

• Two-step testing should be performed on all newly employed healthcare workers who have an initial negative PPD test and no documented negative PPD test within the preceding 12 months 1
• It is particularly important in settings where periodic TB screening is conducted, such as healthcare facilities, correctional institutions, and nursing homes 1
• The process is valuable for establishing baseline TB status in populations with high rates of prior TB exposure or BCG vaccination 1, 2
• Two-step testing helps reduce unnecessary investigations and preventive therapy that might result from misinterpreting boosted reactions 1

Special Considerations

• The booster phenomenon is more common in older adults but can occur in any age group 1
• Boosted reactions are particularly common in individuals previously vaccinated with BCG or exposed to nontuberculous mycobacteria 1
• The boosting effect can occur years after the initial infection or BCG vaccination 1
• Repeated tuberculin skin testing with PPD will not induce a positive reaction in individuals who have no cellular immunity to tuberculin antigens 1

Common Pitfalls and Caveats

• Failure to implement two-step testing in appropriate populations can lead to misclassification of boosted reactions as new infections 1
• Improper timing between the first and second test (should be 1-3 weeks) may affect the accuracy of detecting the booster phenomenon 1
• Reading the test outside the recommended 48-72 hour window reduces accuracy of both steps 2
• Measuring erythema (redness) instead of induration (hardened area) leads to incorrect interpretation 2
• Relying on patient self-reading of results is unreliable and not recommended 2