I just went through two Mantoux sticks for my volunteer assignment here in the hospital. I had a positive stick back in my teens, so I talked quite a bit with the nurse who was administering the tests. A positive can only, according to her, be determined by measuring induration, or swelling. Erythema, or redness, is inconsequential to the exam. This is good, since I got quite a bit of redness that took more than 6 weeks to go away entirely. I probably have a bit of a sensitivity to the carbolic acid they use to stabilize the tuberculin.
Anyway, they're looking for a distinct edge to the swollen bump. They'll then measure the diameter of the induration, and if it's wider than (what is it, 5mm?) then perhaps you're positive. Anyway, it's the swelling, the induration, and not the redness.
Oh, and when I tested positive for the tuberculin, I went in for a chest film which was negative, and I was fine.