When I was negative for TB, I would get a TB test (in elementary school) and it would go away by the third day and you couldn't even tell I had a test. In fact, they usually circle it so they know where it is. I don't know if they are using another form of the test, but the nurse drew lines to my son's test and didn't draw them through it at all but stopped at the edge and measured it. She claimed it was negative though it looked quite positive to me. He also got it worse later. I don't know if they are using different stuff now that causes redness, but that would be stupid since what you are looking for is redness---an immune response. How do they know what component of the shot is causing the redness. The sad thing is that many nurses with large red swollen reactions are continuing to treat patients, believing that they are negative but we don't know for sure.
I know that among all the children for years of TB tests, we all did not get any redness and you couldn't tell where the shot had been given. In fact, my boyfriend had one recently and they circled it and if they hadn't, you wouldn't have known where it was given. Apparently alot of people are "allergic" to the ppd test and get redness and swelling that resembles identically a positive result, but which nurses or their assistants feel are actually negative. And once they note the redness down, if you are positive, it will be harder to get someone to actually go ahead and give you the positive reading.
Redness is clearly a sign of an immune response. How do they know what you are responding to? One nurse claimed this redness was a reaction to the liquid the bacterium are mixed with, but it would be simple to test that by giving a second shot of just the liquid with no bacterium in it to confirm which the person is reacting to. In fact, it is a guess on the part of the nurses who were discussing it.