The TST is read by measuring the diameter of the induration (not erythema) in millimeters using a ruler. The interpretation depends on the size of the induration and the child's risk factors:
â¥5 mm: Positive in children with recent close contact with TB cases, immunocompromised children, or children with abnormal chest X-rays consistent with TB. â¥10 mm: Positive in children under 5 years old, children with medical risk factors, or children from high-prevalence countries. â¥15 mm: Positive in children 5 years and older without any risk factors.
