Question about dna testing on my dog?

my dog roxy is pit bull , but vet said that shes mixed with, are maybe shes full blooded i don't know. i was going to have a dna done at pet co and see if she comes out with a different breed in her. but i read on one of these question that there don't run dna on pit bulls , can anyone help me out here. i know shes a american pit bull, but i have been told shes not full blooded from pit breeders in pittsburgh pa pit rescue center, i received her from a friend that found her in a Snow storm wen she was a small baby and she couldn't keep her. and i know a worker from the pit rescue and he said her legs were to long . she don't meet all the criterias of a full blooded one.