What is the purpose and procedure of the TB (tuberculosis) skin test?

Tuberculin Skin Test (TB Skin Test): Purpose and Procedure

The tuberculin skin test (TST) is the primary diagnostic tool for detecting infection with Mycobacterium tuberculosis by measuring delayed-type hypersensitivity reaction to tuberculin purified protein derivative (PPD). 1 The test helps identify both latent TB infection and supports the diagnosis of active TB disease.

Purpose of the TB Skin Test

The TB skin test serves several critical purposes:

1. Identification of latent TB infection in asymptomatic individuals 2
2. Contact investigation for persons exposed to infectious TB cases 3
3. Screening high-risk populations including:
   • HIV-infected individuals 3
   • Close contacts of persons with TB 3
   • Healthcare workers 1
   • Immunocompromised patients 4
   • Children under 5 years of age exposed to TB 3
   • Residents of congregate settings (prisons, nursing homes) 3
   • Foreign-born persons from high TB prevalence countries 4

Procedure for TB Skin Testing

Administration (Mantoux Method)

• Injection technique: 0.1 mL of PPD (5 tuberculin units) is injected intradermally on the volar or dorsal surface of the forearm 3
• Proper administration creates a pale wheal (elevation of skin) 6-10 mm in diameter 3

Reading the Test

• Timing: Results must be read by trained personnel between 48-72 hours after injection 3
• Self-reading by patients is not acceptable 3
• Measurement: Only the induration (hardened area) is measured, not redness/erythema 3
• Documentation: The transverse diameter of induration must be recorded in millimeters 3

Interpretation of Results

Interpretation varies based on risk factors:

• ≥5 mm is positive for:

  • HIV-infected persons 4
  • Recent contacts of TB cases 4
  • Persons with fibrotic changes on chest X-ray 4
  • Immunosuppressed individuals 4

• ≥10 mm is positive for:

  • Recent immigrants from high-prevalence countries 4
  • Injection drug users (HIV-negative) 4
  • Residents of high-risk congregate settings 4
  • Persons with certain medical conditions (diabetes, silicosis, etc.) 4
  • Children under 4 years 4

• ≥15 mm is positive for persons with no known risk factors 4

Follow-up Testing and Special Considerations

Window Period

• Initial negative test results obtained less than 8 weeks after exposure are considered unreliable 3, 1
• Follow-up testing is recommended 8-10 weeks after exposure ends if initial test is negative 3, 1

Special Populations

• Children under 5: Should receive full diagnostic evaluation including chest X-ray regardless of TST result due to higher risk of progression to disease 3
• HIV-infected persons: May have false-negative results due to anergy; consider treatment for LTBI regardless of TST result if exposed to infectious TB 3, 5

Common Pitfalls

1. Boosting phenomenon: Previous TST can restore waned hypersensitivity, potentially misinterpreted as new infection 3, 1

   • More common in foreign-born individuals 3, 1
   • Two-step testing should NOT be used for contact investigations 1

2. Testing too early: May miss infections due to delayed hypersensitivity development 1

3. Improper reading technique: Only induration should be measured, not erythema 3

4. Improper interpretation: Using incorrect cut-points for different risk groups 3

Post-Test Actions

• Persons with positive TST results should undergo chest radiography to rule out active TB disease 3
• Those with positive tests and no evidence of active disease should be considered for treatment of latent TB infection 4
• Persons with symptoms consistent with TB should undergo further diagnostic evaluation regardless of TST result 3

The TB skin test remains a valuable tool for TB control efforts despite some limitations in sensitivity and specificity, particularly when interpreted in the context of clinical and epidemiological factors.