How is the tuberculin skin test interpreted—what induration size is considered positive for different risk groups and what follow‑up steps are recommended?

Tuberculin Skin Test Interpretation

The tuberculin skin test (TST) is interpreted by measuring only the transverse diameter of induration (not erythema) between 48-72 hours after injection, 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-prevalence countries, injection drug users, healthcare workers), and ≥15 mm for persons with no TB risk factors. 1, 2

Proper Test Administration and Reading Technique

The TST must be administered by intradermal injection of 0.1 mL of 5 tuberculin units (TU) PPD into the volar or dorsal surface of the forearm, producing a discrete wheal 6-10 mm in diameter. 1

Critical timing: Tests must be read between 48-72 hours after injection when induration reaches maximum size. 1, 2 Reading after 72 hours underestimates the true induration size and should be avoided. 1, 2

Measurement technique:

• Measure only induration (palpable, raised, hardened area), never erythema (redness) alone 1, 2
• Measure the transverse diameter perpendicular to the long axis of the forearm 1, 2
• Perform measurement in good light with the forearm slightly flexed at the elbow 1, 2
• Record results in millimeters; record absence of induration as "0 mm," not "negative" 1, 2
• The ball-point pen method can decrease interobserver variability 1, 2
• Trained personnel must perform the reading—patient self-reading is unacceptable 2, 3

Risk-Stratified Interpretation Cutoffs

≥5 mm Induration (Highest Risk)

A reaction of ≥5 mm is considered positive for: 1, 2

• HIV-infected persons (regardless of CD4 count) 1, 3
• Recent close contacts of persons with active pulmonary or laryngeal TB 1, 3
• Persons with fibrotic changes on chest radiograph consistent with prior TB 1, 3
• Organ transplant recipients and other immunosuppressed patients receiving ≥15 mg/day prednisone equivalent for ≥1 month 1, 3
• Patients receiving TNF-blocking agents 1
• Children younger than 4 years of age or those exposed to high-risk adults 1

≥10 mm Induration (Moderate Risk)

A reaction of ≥10 mm is considered positive for: 1, 2

• Recent immigrants (within 5 years) from high TB prevalence countries 1
• Injection drug users 1
• Residents and employees of high-risk congregate settings: prisons, jails, nursing homes, hospitals, healthcare facilities, residential facilities for AIDS patients, homeless shelters 1
• Mycobacteriology laboratory personnel 1
• Persons with medical conditions increasing TB risk: silicosis, diabetes mellitus, chronic renal failure, hematologic disorders (leukemias, lymphomas), specific malignancies (head/neck, lung), weight loss >10% ideal body weight, gastrectomy, jejunoileal bypass 1

≥15 mm Induration (Low Risk)

A reaction of ≥15 mm is considered positive for persons with no known TB risk factors. 1, 2 Routine tuberculin testing is not recommended for low-risk populations. 1

TST Conversion

For persons with negative TST who undergo repeat testing (e.g., healthcare workers), an increase in induration of ≥10 mm within a 2-year period is considered a skin-test conversion, indicating recent infection with M. tuberculosis. 1, 2

Mandatory Follow-Up Actions for Positive Tests

For any positive TST based on the above cutoffs: 3

1. Obtain chest radiograph to exclude active pulmonary TB 3
2. Perform clinical evaluation for TB symptoms: cough, fever, night sweats, weight loss 3
3. Obtain sputum samples if symptoms present or radiograph abnormal 3
4. If active TB is excluded, treat for latent TB infection 3

Special Considerations and Test Limitations

BCG vaccination: Prior BCG vaccination does not contraindicate TST. 1, 3 A positive reaction in BCG-vaccinated persons should be interpreted as M. tuberculosis infection when the person is at increased risk for recent infection or has medical conditions increasing disease risk. 1, 3 Test specificity is decreased in persons vaccinated post-infancy or with repeat BCG vaccination. 1

False-negative results: The TST has a reported false-negative rate of 25% during initial evaluation of persons with active TB. 1, 3 False-negatives occur more frequently in: 1

• Infants and young children
• Early infection (<6-8 weeks after exposure)
• Persons with immunosuppression (HIV infection, high-dose corticosteroids, TNF inhibitors)
• Overwhelming illness or disseminated TB
• Recent viral or bacterial infections
• Recent live-attenuated viral vaccination (measles, mumps, rubella, varicella, yellow fever)

Timing with live vaccines: TST should be done either on the same day as live-attenuated viral vaccination or 4-6 weeks later to avoid false-negative results. 1

Anergy testing: Anergy testing is not recommended for routine use in HIV-infected or other immunocompromised persons due to lack of standardization, poor reproducibility, and no documented benefit. 3

Common Pitfalls to Avoid

• Do not measure or record erythema alone—only induration counts 2, 3
• Do not accept patient self-reading of results 2, 3
• Do not use multiple puncture tests (Tine, Heaf)—they are insufficiently accurate 1, 2
• Do not dismiss positive TST in BCG-vaccinated individuals when they have TB risk factors 3
• Do not delay reading beyond 72 hours, as accuracy decreases 2, 3
• Do not use anergy testing to guide treatment decisions in immunocompromised patients 3