I had a screwed up test years ago for blood clotting (I was getting ready to do a breeding and god forbid needed an emergency C -section - or any surgery - clotting is very important) I made the vet redo that specific test and send it back in. They were not happy about it because I refused to pay for it (a second time) when it came back normal. Tells me the first test was not accurate at the lab.
It does happen and you have to be on top of it and be the "advocate" for your dog. Don't be afraid to question things and demand results.