"The TB skin test is performed by injecting a small amount of fluid (called tuberculin) into the skin on the lower part of the arm. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm."

In health care as a care giver we do it this way to see if we have it. I am not sure why they would not do it this way as opposed to a chest X-ray but I am going to go out on a limb and guess you could probably limit or maybe slow down the reaction with antihistamines or some other means I am not sure of where a chest x-ray would be impossible to hide evidence of TB.