I'm seronegative and my first two SFEMG's were inconclusive. In addition, my symptoms are rather atypical. For example, I haven't noticed any eye symptoms.

My left eye opens less wide than my right, but I'm pretty sure it's always been like that. I never notice any variation. But, the ophthalneurologist who diagnosed me wrote to my regular neurologist that I have Cogan's lid twitch, and sure enough, I can see it for myself in the mirror. It looks exactly like the second picture (the animated one) here:

http://www.mrcophth.com/ptosis/myastheniagravis.html

I'm just delighted to have something observable. This is so cool.

Unfortunately, I find this study that says Cogan's lid twitch isn't sensitive or specific.

http://www.ncbi.nlm.nih.gov/pubmed/17374543

Abby (look, I found a Cogan's Lid Twitch Emoticon!)