For a two-step Tuberculin Purified Protein Derivative (PPD) test, is the second PPD placement administered 7 days from the placement or the reading of the first test?

Two-Step PPD Test Timing

The second PPD should be placed 1-3 weeks after the reading of the first test (which occurs at 48-72 hours post-placement), meaning the second placement occurs approximately 1-3 weeks after the first test was read, not from the initial placement. 1, 2

Timing Algorithm for Two-Step Testing

Standard Two-Step Protocol

• Day 0: Place first PPD test 3
• Day 2-3 (48-72 hours): Read first PPD test 3
• Day 9-24 (1-3 weeks after first reading): Place second PPD test if first test is negative 1, 2
• Day 11-26 (48-72 hours after second placement): Read second PPD test 1, 2

Critical Timing Considerations

The 1-3 week interval is measured from the first test reading, not placement, because:

• The immunologic response needs time to develop after the first tuberculin exposure 1
• Placing the second test too soon can interfere with the immunological response and lead to inaccurate results 1
• This interval allows detection of the "booster phenomenon" where waned TB immunity is restored by the initial PPD 1, 2

Clinical Context for Two-Step Testing

When Two-Step Testing Is Required

• Healthcare workers: All newly employed HCWs with initial negative PPD and no documented negative test within preceding 12 months 1, 2
• Serial testing populations: Healthcare facilities, correctional institutions, nursing homes where periodic TB screening occurs 1, 2
• High prior exposure risk: Populations with high rates of previous TB exposure or BCG vaccination 1

Purpose of Two-Step Method

The two-step process prevents misinterpretation of boosted reactions as new infections during subsequent serial testing 1, 2. Without this baseline establishment, a boosted reaction on future annual testing might be incorrectly classified as a conversion, triggering unnecessary contact investigations and preventive therapy 1.

Common Pitfalls to Avoid

• Timing error: Measuring the 1-3 week interval from placement rather than reading leads to improper timing 1, 2
• Reading window: Tests must be read at 48-72 hours; readings outside this window reduce accuracy 3, 4
• Patient self-reading: Never rely on patient self-reading of results—this is highly inaccurate 5
• Measuring erythema: Only measure induration (hardened area), not redness 3, 1

Special Populations

The booster phenomenon is particularly common in:

• Older adults 1
• Individuals with previous BCG vaccination 1, 6
• Foreign-born persons from high TB prevalence countries 1
• Those exposed to nontuberculous mycobacteria 1