12- Latent TB

• Latent TB

  Detected in asymptomatic patients using the purified protein derivative (PPD) skin test or interferon gamma release assay (IGRA) blood test
• PPD test

  Considered positive based on the size of the INDURATION, not the erythema
• Induration larger than 5 millimeters is considered +ve in


  • HIV-positive patients
  • Glucocorticoid users
  • Close contacts of those with active TB
  • Abnormal calcifications on chest x-ray
  • Organ transplant recipients
• Induration larger than 10 millimeters is considered +ve in


  • Recent immigrants/individuals from high-incidence settings
  • Prisoners
  • Healthcare workers
  • Close contacts of someone with TB
  • Hematologic malignancy, alcoholics, diabetes mellitus
• Induration larger than 15 millimeters is considered +ve in


  • Those with no risk factors
• If the patient has never had a PPD skin test before

  A second test is indicated within 1 to 2 weeks if the first test is negative because it may be falsely negative
• Everyone with a reactive IGRA or PPD test should have a chest x-ray to exclude active disease before starting latent Tb treatment
• Latent TB treatment

  Isoniazid continued for 9 months
• Previous BCG has no effect on these recommendations