When is a TB (tuberculosis) skin test considered positive?

When is a TB Skin Test Considered Positive?

A TB skin test is considered positive based on three risk-stratified cutoff levels: ≥5 mm induration for high-risk individuals (HIV-positive, recent TB contacts, immunosuppressed patients), ≥10 mm for moderate-risk groups (recent immigrants from high-prevalence countries, injection drug users, healthcare workers), and ≥15 mm for persons with no TB risk factors. 1, 2

Risk-Stratified Interpretation Criteria

≥5 mm Induration (High-Risk Groups)

The following individuals require only 5 mm of induration to be considered positive 1, 2:

• HIV-positive persons - This lower threshold is critical because HIV-infected patients have significantly higher false-negative rates and impaired tuberculin reactivity, particularly as CD4 counts decline 1, 3
• Recent close contacts of active TB cases - The risk of recent infection and progression to disease justifies increased sensitivity 1, 2
• Persons with fibrotic changes on chest radiograph consistent with prior TB - Radiographic evidence of old TB indicates high risk for reactivation 1, 2
• Organ transplant recipients and other immunosuppressed patients - Including those receiving ≥15 mg/day of prednisone for ≥1 month or TNF-blocking agents 1

≥10 mm Induration (Moderate-Risk Groups)

This threshold applies to 1, 2:

• Recent immigrants (within 5 years) from high TB prevalence countries 1, 2
• Injection drug users 1, 2
• Residents and employees of high-risk congregate settings - Including prisons, jails, nursing homes, hospitals, healthcare facilities, homeless shelters, and residential AIDS facilities 1, 2
• Mycobacteriology laboratory personnel 1
• Persons with medical conditions increasing TB risk - Including silicosis, diabetes mellitus, chronic renal failure, certain malignancies, gastrectomy, and jejunoileal bypass 1

≥15 mm Induration (Low-Risk Groups)

This threshold applies to persons with no identifiable TB risk factors 1, 2

Critical Testing and Reading Details

Proper Administration and Timing

• The test must be administered using the Mantoux method: 0.1 mL of 5 TU PPD injected intradermally into the volar or dorsal forearm surface 1
• Read at 48-72 hours after administration by measuring the transverse diameter of palpable induration (not erythema) in millimeters 1
• Multiple puncture tests (Tine, Heaf) and non-standard PPD strengths (1 TU, 250 TU) should never be used due to insufficient accuracy 1

Important Clinical Caveats

BCG Vaccination Does Not Change Interpretation

A positive tuberculin skin test in BCG-vaccinated persons should be interpreted as indicative of TB infection, especially in individuals from high-prevalence countries 2. Do not dismiss positive results based on BCG history.

TST Conversion Definition

Skin test conversion is defined as an increase of ≥10 mm in induration within a 2-year period, indicating recent infection requiring evaluation and treatment 2

False-Negative Results Are Common in High-Risk Populations

Be aware that TST has significant limitations 1, 3:

• Approximately 10% of HIV-negative children with culture-positive TB have negative initial TST 3
• Up to 50% of patients with severe TB (miliary TB, meningitis) may have false-negative results 3
• HIV-infected patients are significantly more likely to have false-negative tests, with likelihood increasing as CD4 counts decline 3, 4
• False-negatives also occur in early infection (<6-8 weeks post-exposure), after recent viral vaccination, with overwhelming illness, and with immunosuppressive medications 1

Window Period Considerations

A negative test obtained <8 weeks after TB exposure is unreliable for excluding infection; repeat testing at 8-10 weeks after exposure ends is mandatory 2, 5

Concurrent Illness Effects

Fever and acute illness can suppress tuberculin reactivity, potentially causing false-negative results 5. For high-priority contacts (children <5 years, HIV-infected, immunocompromised), administer TST at initial encounter regardless of fever, but plan repeat testing at 8-10 weeks 5

Mandatory Follow-Up Actions

All persons with positive tuberculin skin test results require further evaluation, beginning with chest radiography 2. High-priority contacts should undergo complete evaluation regardless of skin test results 2.

Clinical suspicion must drive diagnostic workup, not TST results alone - patients with persistent cough, abnormal chest radiograph, known TB exposure, or high-risk group membership warrant full TB evaluation (sputum microscopy, culture, chest radiography, nucleic acid amplification testing) even with negative TST 3