Tuberculin Skin Testing in BCG-Exposed Individuals

Interpretation Challenges and Guidelines

• The American Thoracic Society recommends that a positive tuberculin skin test result in BCG-exposed individuals be considered as indicating infection with M. tuberculosis, especially in high-risk contexts, due to the inability to distinguish between BCG vaccination and natural infection 1, 2
• The Centers for Disease Control and Prevention suggests that tuberculin test conversion rates after BCG vaccination may be less than 100%, and tuberculin sensitivity from BCG tends to wane over time, making it prudent to consider positive reactions as true TB infection 1, 2
• The Infectious Diseases Society of America recommends using risk-stratified interpretation thresholds for tuberculin skin test results, including ≥5 mm induration for HIV-infected persons or recent TB contacts, ≥10 mm for persons from high TB prevalence countries, and ≥15 mm for those with no known TB risk factors 5
• The size of tuberculin reaction in BCG-exposed persons does not reliably distinguish between M. tuberculosis infection and prior BCG effect, with mean reaction sizes often <10 mm 1, 2

Testing Approach and Thresholds

• For patients ≥5 years old with prior BCG exposure, an FDA-approved interferon-gamma release assay (IGRA) is preferred over tuberculin skin testing due to its specificity and lack of interference from BCG vaccination 5
• The standard Mantoux test (0.1 mL of 5-TU PPD intradermally) should be performed and read at 48-72 hours, with results interpreted using standard risk-stratified thresholds 3, 4, 6