Tuberculin skin test

From IDWiki
(Redirected from TST)

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