Pammy ... I got different results from an EMG done in a private neurologist's office and one done in a university clinic. Which is not to say that the first one in the office was "wrong." It showed clean, whereas the second one showed denervation and led to my diagnosed. HOWEVER, they were also done six months apart, so that could be the difference right there.
Also, the one at the university was much more extensive and complex than the first one. They tested more areas.
I have come to have a lot of respect for the private neuro who did my first test (he is treating my husband for Parkinson's, and I have concluded he's pretty darned smart) ... but I suspect that private neuros may not have the latest equipment. I'm sure they see so few cases of ALS in their practice that it would not pay them to keep upgrading their equipment every couple of years.