How is a two‑step tuberculin skin test (TST) performed, when should it be repeated, and how are induration results interpreted for the various risk groups?

Two-Step Tuberculin Skin Test (TST): Performance, Timing, and Interpretation

How to Perform the Two-Step TST

All newly hired healthcare workers should receive a baseline two-step TST to establish a reliable baseline and prevent misinterpretation of boosted reactions as new infections during subsequent serial testing. 1

First Test Administration

• Administer 0.1 mL of PPD (5 tuberculin units) intracutaneously on the volar or dorsal forearm using the Mantoux technique on Day 0 2
• A proper injection produces a pale wheal measuring 6-10 mm in diameter 2
• Read the test at 48-72 hours (Day 2-3) by measuring only the induration (hardened area), not erythema 2

Second Test Administration (If First Test is Negative)

• Place the second PPD test 1-3 weeks after reading the first test (approximately Day 9-24) 2, 3
• Read the second test 48-72 hours after placement 2
• This timing is critical—performing the second test too soon interferes with immunological response and leads to inaccurate results 3

Purpose of Two-Step Testing

The two-step method detects the "booster phenomenon," where individuals with waned TB immunity from prior infection or BCG vaccination may have a negative initial test but a positive second test due to immune memory reactivation, not new infection 2. Without two-step testing, this boosted reaction on future serial testing would be misinterpreted as a new TB conversion, triggering unnecessary contact investigations and preventive therapy 2.

When to Repeat Serial Testing

The frequency of repeat testing depends on the healthcare setting's risk classification: 1

Low-Risk Settings

• No routine serial testing required after baseline two-step TST 1
• Only test upon unprotected exposure to M. tuberculosis 1

Medium-Risk Settings

• Serial testing at least every 12 months 1
• More frequent testing (every 8-10 weeks) during investigations of potential ongoing transmission 1

After Unprotected Exposure

• Perform initial TST as soon as possible after exposure 3
• If negative, repeat TST 8-10 weeks after the end of exposure—this is the optimal window to detect new infection 2, 3
• A negative test performed less than 8 weeks after exposure is unreliable because tuberculin sensitivity takes 8-10 weeks to develop 3

Interpretation of Induration Results by Risk Group

Measure only induration (palpable raised area), never erythema (redness): 2

≥5 mm Induration is Positive for High-Risk Individuals:

• HIV-infected persons 2
• Recent contacts of persons with infectious TB 2
• Persons with fibrotic changes on chest radiograph consistent with prior TB 2
• Immunosuppressed patients (organ transplant recipients, chronic corticosteroid use ≥15 mg/day prednisone for ≥1 month) 2

≥10 mm Induration is Positive for Moderate-Risk Individuals:

• Recent immigrants (<5 years) from high-prevalence countries 2
• Injection drug users 2
• Healthcare workers 2
• Residents and employees of high-risk congregate settings (prisons, nursing homes, homeless shelters) 2
• Mycobacteriology laboratory personnel 2
• Persons with clinical conditions that increase TB risk (diabetes, chronic renal failure, malignancies, gastrectomy) 2
• Children <4 years of age 2

≥15 mm Induration is Positive for Low-Risk Individuals:

• Persons with no known risk factors for TB 2

Critical Pitfalls to Avoid

Common Errors in Administration and Reading

• Reading outside the 48-72 hour window reduces accuracy—late readings underestimate induration 2
• Measuring erythema instead of induration leads to incorrect interpretation 2
• Improper injection technique (subcutaneous instead of intradermal) causes false-negative results 2
• Patient self-reading is unreliable and should never be accepted 2

Timing Confusion

• Do not confuse baseline two-step testing with post-exposure testing—they serve different purposes and have different timing requirements 2
• Two-step baseline testing uses a 1-3 week interval between tests 2
• Post-exposure testing uses an 8-10 week interval after exposure ends 2, 3

Interpretation Errors

• Failure to consider the patient's risk category when interpreting results leads to inappropriate clinical decisions 2
• Previous BCG vaccination can cause positive results, but this should not prevent testing in individuals who need screening 2
• The booster phenomenon is more common in older adults, foreign-born persons from high TB prevalence countries, and those with prior BCG vaccination 2, 3
• Even after negative two-step testing, foreign-born HCWs and those with BCG vaccination history remain at higher risk for delayed boosting on future tests 4

Follow-Up Actions

• All positive TST results require chest radiography and clinical evaluation to exclude active TB before initiating preventive therapy 2
• Persons with previously documented positive TST do not require repeat testing after exposure unless they develop TB symptoms 3