If you're having the two-step, that first one "primes" your body's immune system for an enhanced reaction to the 2nd test. You could be negative to the first and positive with the second. Normally, you'll only need a 2-step test the first time you're tested, and after that, usually only a 1-step is needed if it's done annually.
Okay I Just remembered I got a TB test last June or July. I was wondering, do they use the same agent for 1 steps and 2 steps? Because I got the first injection today for the 2 step but I found out I only need the one step.