What induration size on a tuberculin skin test (TST) defines a positive result for different risk categories?

Tuberculin Skin Test (TST) Interpretation: Risk-Stratified Cutoffs

A positive tuberculin skin test is defined by measuring only the transverse diameter of induration (not erythema) at 48-72 hours, using risk-stratified cutoffs: ≥5 mm for highest-risk patients (HIV-infected, recent TB contacts, immunosuppressed), ≥10 mm for moderate-risk groups (immigrants from high-TB countries, healthcare workers, injection drug users), and ≥15 mm for persons with no identified TB risk factors. 1

Test Reading Technique

• The test must be read between 48-72 hours after intradermal injection when induration reaches maximum size; readings after 72 hours systematically underestimate induration and should be avoided. 2, 3

• Measure only the transverse diameter of palpable induration in millimeters—erythema (redness) is not measured or recorded. 1, 2

• The measurement should be performed in good lighting with the forearm slightly flexed, measuring perpendicular to the long axis of the forearm. 3

• The ball-point pen method reduces inter-observer variability and is recommended. 2, 3

• Trained healthcare personnel must perform the reading—patient self-reading is unacceptable and unreliable. 2, 3

Risk-Stratified Interpretation Cutoffs

≥5 mm Induration = Positive (Highest Risk)

This cutoff applies to patients at highest risk for TB disease or progression: 1

• HIV-infected persons (regardless of CD4 count) 1
• Recent close contacts of persons with active TB (within 8-10 weeks of exposure ending) 1
• Persons with fibrotic changes on chest radiograph consistent with prior TB 1
• Organ transplant recipients and other immunosuppressed patients (≥15 mg/day prednisone for ≥1 month) 1
• Persons receiving TNF-blocking agents 1
• Children younger than 5 years or children/adolescents exposed to high-risk adults 2, 3

≥10 mm Induration = Positive (Moderate Risk)

This cutoff applies to moderate-risk groups: 1

• Recent immigrants (≤5 years) from high-TB-prevalence countries 1
• Injection drug users 1
• Residents and employees of high-risk congregate settings (prisons, nursing homes, hospitals, homeless shelters) 1
• Mycobacteriology laboratory personnel 2, 3
• Persons with medical conditions increasing TB risk: silicosis, diabetes mellitus, chronic renal failure, hematologic malignancies, certain solid-organ cancers, >10% body weight loss, gastrectomy, or jejunoileal bypass 2, 3

≥15 mm Induration = Positive (Low Risk)

This cutoff applies to persons with no identified TB risk factors; however, routine testing of low-risk populations is not recommended. 1, 2

Special Considerations for Contact Investigations

• In contact investigations, all contacts with induration >5 mm should undergo further diagnostic evaluation starting with chest radiograph. 1

• For children <5 years with initial TST <5 mm and exposure <8 weeks prior, window prophylaxis is recommended after excluding active TB disease, with repeat testing at 8-10 weeks post-exposure. 1

• The window period for TST conversion is 8-10 weeks after exposure ends—a negative test obtained <8 weeks after exposure is unreliable for excluding infection. 1

BCG Vaccination Considerations

• Prior BCG vaccination does not preclude TST use—in BCG-vaccinated persons with TB risk factors, a positive reaction should be interpreted as M. tuberculosis infection. 1, 3

• Test specificity is reduced in persons vaccinated after infancy or with multiple BCG doses, but this should not prevent appropriate interpretation based on risk stratification. 1

False-Negative Results and Interfering Factors

The TST has an approximate 25% false-negative rate during initial evaluation of active TB patients, with higher rates in immunosuppressed individuals. 3

False-negative results are more common in: 1

• Infants and young children
• Early infection (<6-8 weeks after exposure)
• Persons with overwhelming illness or disseminated TB
• HIV-infected persons, especially with low CD4 counts
• Persons on immunosuppressive medications (high-dose corticosteroids, TNF inhibitors)
• After recent viral or bacterial infections
• Within 4-6 weeks after live-attenuated viral vaccines (measles, mumps, rubella, varicella, yellow fever) 3, 4

Critical Timing with Viral Infections and Vaccines

• The TST should be performed either on the same day as a live-attenuated vaccine or 4-6 weeks after vaccination to avoid vaccine-induced immunosuppression causing false-negative results. 3, 4

• After influenza or other viral infections, wait 4-6 weeks before placing TST, or perform the test on the same day as symptom onset before viral-induced immunosuppression develops. 4

TST Conversion Definition

Skin test conversion is defined as an increase in induration of ≥10 mm within a 2-year period after an initially negative TST, indicating recent M. tuberculosis infection. 1, 2, 3

However, in contact investigations using the 5 mm cutoff, any change from negative to positive is considered a change in tuberculin status requiring evaluation. 1

Critical Pitfalls to Avoid

• Never measure or record erythema alone—only induration should be documented. 2, 3

• Never use multiple-puncture tests (Tine, Heaf methods)—they are insufficiently accurate for TB screening. 3

• Never delay reading beyond 72 hours—this systematically underestimates induration size. 2, 3

• Never accept patient self-reading—trained personnel must perform all measurements. 2, 3

• Never interpret a positive TST as "negative" in BCG-vaccinated individuals when they have TB risk factors—use the same risk-stratified cutoffs. 2, 3

• Never place TST during the 1-4 week period after influenza or live viral vaccines—this represents peak viral-induced immunosuppression and will likely yield false-negative results. 4

• Never assume a negative TST during active viral illness rules out TB infection—this is a critical error that could delay necessary treatment. 4