text: why does the nruse have to chck it.? i had a tb shot today and she is coming thurs. why. 10 pints i am a registered nurse, so i'll help you here.  a nurse gives you the injection (and you should have had a "wheal" that looks like a mosquito bite).  you have 48-72 hours and then it must be "read".  all that means is that the nurse is checking for what's called induration, or redness and swelling.  if there is any swelling that you can feel, then she/he should measure this diameter in mm.  anything above 10mm indicates a positive tb test, and referral for a chest x-ray.  that basically means that you carry the tuberculosis infection in your blood due to exposure previously, or you have an active case of tb.    the nurse is allowed to check this legally, because it is like a lab result.  a doctor is not usually involved with tb skin tests unless there's a positive reaction as described above.  i administer tb tests everyday in my job, and i seldom have anything that is above a 3mm, which is negative.