Any tuberculin reaction more than the cut off JUST suggests TB infection even if there is ulceration. Strength of reaction does not discriminate between infection and disease. Unfortunately, many of us get carried away by the severity of the TST reaction and start ATT, which should not be done.
TST does not help in guiding the duration of ATT treatment. Once the child is infected TST is likely to stay positive for a very long time.
TST does not help in detecting a re-infection or reactivation