Sero-negative means that I don't have the anti AchR antibodies. Many seroneg MGers have another kind of antibody, the anti MuSK antibodies, but I've never been tested for those. And there are some MGers that are MuSK negative also. Testing positive for the anti-AchR antibodies is almost always a postive diagnosis for MG. It's when you are seroneg, as 20% or so of us are, that the diagnosis gets a bit trickier. That's when you really need a neuro who is an MG specialist. I have heard of MGers who are negative for all of the tests available, but still diagnosed with MG because of having the symptoms, and responding postitively to Mestinon. But finding a neuro who is expert enough in MG to make such a diagnosis is like finding a needle in a haystack.
If you want more info, google anti AchR antibodies, and see what you come up with.