Tuberculin skin test

Tuberculin skin test (TBST)

Procedure

• Place tuberculin intradermally
• Read it for area of induration at 48 hours
• But there's also a two-step that I have to look up

Interpretation

• ≥5mm if high risk or immunosuppressed
  • HIV positive/immune suppressed: induration ≥5mm
  • TB close contact/suggestive CXR: induration ≥5mm
• ≥10mm if medium risk or child ≥15mm if low risk (all others)
  • Immigration from TB country within past 5 years: induration ≥10mm
  • Healthcare workers/IVDU/LTC residents: induration ≥10mm
  • Children <4 years: induration ≥10mm
• ≥15mm if low risk (all others), or increase in induration by ≥10mm in last two years
• History of BCG vaccination does not change interpretation of result

False Positives

• Infection with nontuberculosis mycobacteria, including M. bovis
• Previous BCG vaccination
• Incorrect method of TST administration
• Incorrect interpretation of reaction
• Incorrect bottle of antigen used

False Negatives

• Cutaneous anergy (anergy is the inability to react to skin tests because of a weakened immune system)
• Recent TB infection (within 8-10 weeks of exposure)
• Very old TB infection (many years)
• Very young age (less than 6 months old)
• Recent live-virus vaccination (e.g., measles and smallpox)
• Overwhelming TB disease
• Some viral illnesses (e.g., measles and chicken pox)
• Incorrect method of TST administration
• Incorrect interpretation of reaction

See Also

• TST in 3D