What are the ranges for a tuberculin (TB) skin test?

Tuberculin (TB) Skin Test Interpretation Ranges

The tuberculin skin test is interpreted based on three different cut-off points (5 mm, 10 mm, and 15 mm of induration) depending on the individual's risk factors for TB infection and progression to active disease. 1

Cut-off Points for Positive TB Skin Test Results

≥5 mm Induration (Highest Risk Groups)

• HIV-positive persons
• Recent contacts of TB cases
• Persons with fibrotic changes on chest radiograph consistent with prior TB
• Organ transplant recipients and other immunosuppressed patients (receiving ≥15 mg/day of prednisone for ≥1 month)
• Children younger than 4 years of age
• Infants, children, and adolescents exposed to adults in high-risk categories 1

≥10 mm Induration (Intermediate Risk Groups)

• Recent immigrants (within last 5 years) from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings:
  • Prisons and jails
  • Nursing homes and long-term care facilities for the elderly
  • Hospitals and healthcare facilities
  • Residential facilities for AIDS patients
  • Homeless shelters
• Mycobacteriology laboratory personnel
• Persons with clinical conditions that increase risk for TB:
  • Silicosis
  • Diabetes mellitus
  • Chronic renal failure
  • Certain hematologic disorders (leukemias and lymphomas)
  • Specific malignancies (carcinoma of head, neck, or lung)
  • Weight loss >10% of ideal body weight
  • Gastrectomy
  • Jejunoileal bypass 1

≥15 mm Induration (Low Risk Groups)

• Persons with no known risk factors for TB 1

Skin Test Conversion Definition

For individuals with negative tuberculin skin test reactions who undergo repeat testing (e.g., healthcare workers):

• An increase in reaction size of ≥10 mm within a period of 2 years should be considered a skin test conversion indicative of recent infection with M. tuberculosis 1
• For persons less than 35 years of age: ≥10 mm increase within a 2-year period is classified as a conversion 1
• For persons 35 years or older: ≥15 mm increase within a 2-year period is classified as a conversion 1

Special Considerations

Healthcare Workers (HCWs)

• In facilities with essentially no risk for TB exposure (minimal or very low-risk facilities), ≥15 mm may be appropriate for HCWs with no other risk factors
• In facilities where TB patients receive care, ≥10 mm may be appropriate for HCWs with no other risk factors 1
• A recent PPD test conversion in an HCW should generally be defined as an increase of ≥10 mm in induration size within a 2-year period 1

BCG Vaccination

• No reliable method exists to distinguish tuberculin reactions caused by BCG vaccination from those caused by natural mycobacterial infections
• A positive reaction to tuberculin in BCG-vaccinated persons should be considered to indicate TB infection when the person has risk factors for infection 1

Anergy Testing

• Anergy testing is not routinely recommended for persons who are HIV-infected or otherwise immunocompromised 1
• HIV-infected persons may have suppressed reactions to PPD skin tests due to anergy, particularly with declining CD4+ T-lymphocyte counts 1

Common Pitfalls and Caveats

1. False negatives: Can occur in immunosuppressed patients, very young or elderly patients, or those with overwhelming TB disease
2. False positives: Can occur due to cross-reactivity with non-tuberculous mycobacteria or previous BCG vaccination
3. Boosting phenomenon: Previous PPD sensitivity may wane over time but can be "boosted" by a subsequent test, potentially leading to misinterpretation as a new infection
4. Improper administration or reading: The tuberculin test must be administered intradermally (Mantoux method) and read by measuring induration (not erythema) 48-72 hours after placement 1
5. Self-reading by patients: Not reliable, as demonstrated in studies showing poor sensitivity (0.68) and specificity (0.83) compared to trained examiners 2

Remember that a positive skin test only indicates infection with M. tuberculosis, not necessarily active TB disease. Further evaluation, including chest radiography, is required for individuals with positive test results to rule out active disease before considering treatment for latent TB infection.