In a 3-year-old child with close exposure to a parent recently diagnosed with pulmonary tuberculosis, a tuberculin skin test shows a 10 mm induration after 72 hours. How should this result be interpreted?

Interpretation: Positive

A 10 mm induration in this 3-year-old child who is a close contact of a parent with active pulmonary tuberculosis should be interpreted as POSITIVE. 12

Risk-Stratified TST Interpretation

The tuberculin skin test interpretation is risk-stratified based on the individual's likelihood of TB infection and progression to disease. For this child, multiple high-risk factors apply:

Why This is Positive (≥5 mm threshold applies):

• Close contact of an active TB case: This child is a household contact of a parent with pulmonary TB, placing them in the highest risk category 12
• Age < 5 years: Children under 5 years are at substantially increased risk for progression to severe TB disease (including TB meningitis and miliary TB) if infected 12
• Recent exposure: The parent was "recently diagnosed," indicating ongoing or very recent exposure

Standard TST Interpretation Thresholds 12:

• ≥5 mm = Positive for:

  • Close contacts of TB cases (this child qualifies)
  • Immunosuppressed persons
  • Those with radiographic evidence of TB

• ≥10 mm = Positive for:

  • Other at-risk populations (immigrants from high-incidence countries, healthcare workers, etc.)

• ≥15 mm = Positive for:

  • Low-risk individuals with no known risk factors

Clinical Significance

This child's 10 mm induration exceeds the 5 mm threshold applicable to close contacts, making it unequivocally positive. Research confirms that children exposed to adults with pulmonary TB have infection rates of 33.8-47.5%, with risk increasing based on intimacy of contact and bacillary load of the source case 34.

Important Caveats

BCG vaccination consideration: While BCG can cause false-positive TST results, this effect typically wanes by 3 years of age 5. More importantly, in a close contact of active TB, a positive TST should be attributed to true TB infection rather than BCG, especially when the induration is ≥10 mm 23.

Next Steps

Following positive TST interpretation:

1. Chest radiograph (PA and lateral in children <5 years) to exclude active TB disease 2
2. If chest X-ray is normal and no symptoms present: treat for latent TB infection (LTBI)
3. If any clinical or radiographic findings suggest active disease: pursue full TB workup including sputum/gastric aspirate samples 12

The answer is B. Positive.