How can you tell if a tb shot is positive? my daughter's looks like a mosquito bite approx 15 mm

20 mm : 20 mm of induration (swelling) at 48 hrs. Is the minimum for a positive test. The site should be inspected by a health care professional experienced in TB testing. This is a serious condition that must be screened for properly. Inspection after 48 hrs is unreliable.

Depends ...: Positivity using the TB skin test (tst) depends on 1. The size of the skin reaction ( the wheal) 48-72 h after placement and 2. Pretest probability of a positive reaction based on TB exposure. An immunodeficiency patient has a "positive" test when the wheal is 5mm or more; an immigrant from a country with high TB rates- 10+mm; otherwise , > 15mm is considered positive for low risk patients.