What indicates tuberculosis (TB) in a tuberculin skin test?

Tuberculin Skin Test Interpretation for Tuberculosis Detection

A positive tuberculin skin test indicating tuberculosis infection is determined by measuring the induration size, with specific cutoff thresholds based on patient risk factors: ≥5 mm for high-risk individuals, ≥10 mm for moderate-risk individuals, and ≥15 mm for low-risk individuals. 1

Interpretation Criteria by Risk Group

Induration ≥5 mm is considered positive in:

• HIV-positive individuals 1
• Recent contacts of TB cases 1
• Persons with fibrotic changes on chest radiographs consistent with prior TB 1
• Organ transplant recipients and other immunosuppressed patients (receiving ≥15 mg/day of prednisone for >1 month) 1
• Residents and employees of high-risk congregate settings (prisons, nursing homes, hospitals, homeless shelters) 1
• Mycobacteriology laboratory personnel 1
• Children younger than 4 years of age or those exposed to adults in high-risk categories 1

Induration ≥10 mm is considered positive in:

• Recent arrivals (<5 years) from high-prevalence countries 1
• Injection drug users 1
• Persons with clinical conditions that increase TB risk (silicosis, diabetes mellitus, chronic renal failure, certain hematologic disorders, specific malignancies, weight loss >10% of ideal body weight, gastrectomy, jejunoileal bypass) 1
• Residents and employees of high-risk congregate settings not in the highest risk category 1

Induration ≥15 mm is considered positive in:

• Persons with no known risk factors for TB 1
• Individuals at low risk for whom testing is not generally indicated 1

Special Considerations

BCG Vaccination History

• No reliable method exists to distinguish tuberculin reactions caused by BCG vaccination from those caused by natural TB infection 1
• A positive test in BCG-vaccinated persons should be considered indicative of TB infection, especially in persons from high-prevalence countries 1
• Reasons to not attribute positive results to BCG vaccination:
  • Tuberculin test conversion rates after vaccination may be less than 100% 1
  • Mean reaction size among BCG-vaccinated persons is often <10 mm 1
  • Tuberculin sensitivity tends to wane after vaccination 1

Skin Test Conversion

• For persons with negative tuberculin skin test reactions who undergo repeat testing, an increase in reaction size of ≥10 mm within 2 years should be considered a skin test conversion indicative of recent infection 1
• For individuals previously infected with nontuberculous mycobacteria or BCG-vaccinated, conversion is defined as an increase in induration by 10 mm on subsequent tests 1

Anergy Testing

• HIV-infected persons may have compromised ability to react to tuberculin tests due to cutaneous anergy 1
• Anergy testing is not recommended for routine use in persons with HIV infection or other immunocompromised states 1
• If anergy testing is performed, it should include mumps and Candida skin tests, with positivity defined as ≥5 mm induration 1

Follow-up Evaluation

For Positive Tuberculin Tests

• All persons with positive results should undergo further examination, typically starting with a chest radiograph 1
• Collection of specimens for mycobacteriologic testing (e.g., sputum) is determined case-by-case and not recommended for healthy contacts with normal chest radiographs 1
• High-priority contacts (due to special susceptibility or vulnerability) should undergo further examination regardless of skin test results 1

Window Period

• A negative test result obtained <8 weeks after exposure is considered unreliable for excluding infection 1
• Follow-up testing at 8-10 weeks after exposure ends is recommended for contacts with initial negative results 1

Common Pitfalls and Caveats

• False-negative results may occur in immunosuppressed individuals, very young children, or those with overwhelming TB disease 2
• The positive predictive value of the test varies significantly with the prevalence of TB infection in the tested population 1, 3
• Boosting phenomenon may occur with serial testing and can be misinterpreted as new infection 1
• Terminal digit preference by inexperienced readers can lead to measurement errors 4
• Environmental mycobacteria can cause cross-reactivity and false-positive results 4, 3

The tuberculin skin test remains a valuable tool for TB infection diagnosis when properly administered and interpreted according to risk-stratified guidelines 5, 3.