All I can tell you is this, from my years of this frustration:

I am told they prefer 2 of 3 positives to officially dx MG - Antibodies (I'm negative); EMG (I had a VERY positive test in 2004); and medication response (mine was to the Tensilon injection in 1996).

My 1st EMG was negative, but I know the neuro was having trouble getting his baseline (kept shocking me and apologizing, then told the assistant "well let's just go with this - I don't want to keep shocking her" - then he got indignant when I questioned the results of the test - nimrod!).

My last EMG was negative, but my research online tells me that was probably botched too. The prescribed way to test for MG on EMG is to test more than one muscle group and to test proximal (closest to the body) muscles. I got that info off a website for medical professionals, and that goes along with the tests that came back positive for me. The muscles that showed strong positives on me were in the trapezius (neck/shoulder/back) - 76% decrement, the face - 23% decrement. The muscle closest to my hand only showed minimal decrement.

The neuro that did this last test ONLY tested the one next to my hand - that muscle is one of the last to go on me.

It is really frustrating, but if you have a neuro that is wanting to test the meds on you know, you're at least ahead of some of the experiences I've had. That first guy told me they had done all the tests (neuro-wise) on me and my problem was depression and actually called my family doc and told him I needed to be in a pysch hospital for observation!
His notes pretty much blocked me from getting anyone to take me seriously after that.

I'm older and wiser now, and have learned to research my butt off, look up med terms I come across that I don't understand, and fire a doc that has a problem with me being very interactive!

I also searched the i-net to find a doctor rating site and found that the neuro I have most recently been seeing doesn't list neuro-muscular or MG as one of his special interests. I will now be seeing one who does this Thursday. I figure none of us has time or money to spend on someone who is not really up on our diseases, and I know they call it "practicing" medicine, but I don't want my docs to use me for a guinea pig to further their education on my dime any more than necessary.

Hang in there. I don't know what med trials they want to do on you, but remember that not all MG treatments works the same on all MG'ers, and they all (except Tensilon, but it's about a 30 minute help is all, and sometimes Mestinon can give relief quickly but it's a band-aid drug and they have to get the dosage JUST right for optimum affect) work rather slowly.

I'll be sending lots of positive thoughts your way that you can start on the road to treatment and get off the roller coaster of diagnosis!