? about Quest UA

I have a UA coming up for school, and i have a couple question about Quest screening procedures. I have read a lot of the threads here and i think i understand how these UA work, but i want to make sure. To me, it seems that you go give your piss, and then they run an immunoassy screen on it which has a cut off of 50 ng/ml, and if you pass this (have a negative test) than Quest does not run a GC/MS test on your piss which has a cut off of 15 ng/ml. So in my thinking if i can pass a home drug test with a cut off of 50 ng/ml, then there would be no need to substitute my pea, because no home test have cut off levels of 15 ng/ml. I hope someone understand my question, and i appreciate any feed back THANK YOU