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
