How many hours are required to read the results of a Tuberculin Skin Test (TST)?

Tuberculin Skin Test Reading Time

The tuberculin skin test (TST) should be read between 48 and 72 hours after injection, when the induration is at its maximum. 1

Proper Administration and Reading Protocol

Administration

• The TST is administered using the Mantoux method:
  • 0.1 ml of 5 tuberculin units (TU) PPD is injected intradermally into the volar or dorsal surface of the forearm 1
  • A proper injection creates a wheal (pale elevation) 6-10 mm in diameter 1
  • The site should be free of lesions, veins, and away from areas with heavy hair 1

Reading Time

• Reading must occur within the 48-72 hour window after injection 1
• Tests read after 72 hours tend to underestimate the true size of induration 1
• In some individuals (particularly the elderly and first-time recipients), the reaction may not peak until after 72 hours 1
• Research shows that readings at 72 hours are significantly larger than those at 48 hours, with an average difference of 1.7 mm 2

Reading Method

• Reading should be performed by trained personnel only - patient self-reading is not reliable 1, 3
• The reading should be done in good light with the forearm slightly flexed at the elbow 1
• The diameter of induration (not erythema/redness) should be measured transversely to the long axis of the forearm 1
• Measurement should be recorded in millimeters, not as "positive" or "negative" 1
• The ball-point pen method of Sokal can help decrease interobserver variability 1, 4

Interpretation of Results

The interpretation of TST results is risk-stratified based on the size of induration:

• ≥5 mm is considered positive in:

  • HIV-positive persons
  • Recent contacts of TB cases
  • Persons with fibrotic changes on chest X-ray consistent with prior TB
  • Organ transplant recipients and other immunosuppressed patients 1, 5

• ≥10 mm is considered positive in:

  • Recent immigrants from high-prevalence countries
  • Injection drug users
  • Residents/employees of high-risk congregate settings
  • Persons with clinical conditions that increase TB risk (diabetes, silicosis, etc.) 1, 5

• ≥15 mm is considered positive in:

  • Persons with no known risk factors for TB 1, 5

Common Pitfalls and Caveats

1. Timing errors: Reading too early or too late can lead to false results. Reading at 72 hours is optimal as negative tests at 48 hours may be false negatives 2

2. Measurement errors:

   • Measuring erythema (redness) instead of induration (hardened area)
   • Using improper technique or inadequate lighting
   • Relying on patient self-reading 3

3. False negatives can occur in:

   • Immunosuppressed patients
   • Very young or elderly patients
   • Early infection (less than 6-8 weeks after exposure)
   • Overwhelming TB disease 1, 5

4. False positives can occur due to:

   • Previous BCG vaccination (especially if given after infancy)
   • Cross-reactivity with non-tuberculous mycobacteria 1, 5

5. Improper documentation: Always record the exact measurement in millimeters, not simply as "positive" or "negative" 1

By following these guidelines for proper timing and technique of TST reading, healthcare providers can maximize the accuracy of this important diagnostic tool for tuberculosis infection.