Mike, Wow, that must be unsettling. MG can run in families, though it's more rare. Often it's CMS that tends to run in families due to the fact that it is from a genetic mutation.
Do you know what the exact number was and the range that was given? Was it done at Mayo? Was she on any drugs before/during the test?
The chance of a false positive is small, especially when done at a lab like Mayo. 3 - 4 % of those with MG, at least that they know of so far, only have modulating antibodies. I'm one of those 3 - 4%. Although I can't say for certain since I've been on a steroid for my asthma ever since they started testing.
Perhaps your sister can take photos of her face first thing in the morning and then later in the day. Try to make it the same lighting, distance from camera and head angle (looking straight ahead). She could even do an upward gaze after the first photo to see if her eyelids droop afterwards.
Ptosis can be subtle. It's not always the eyelid almost all the way down the way it's depicted in books! To check for double vision, she or someone else could hold their pointer finger in front of her eyes and slowly move it back and forth in front of her vision. Or start with it about two feet away and move it towards her face. A person will not always have double vision in all areas of their visual field. Sometimes I only have it on the sides. It depends upon which muscles are weak at the time.
That statistic of 3 - 4% is directly from the "MG experts" like Dr. Harper of the Mayo Clinic and Dr. Howard of UNC.
The modulating antibody is not thought to reflect anything but MG. There have been speculations but no study that shows it is related to anything but MG, at least that I can find. I have searched everywhere! And if her clinical exam is positive for muscle weakness that gets worse upon exertion, that's pretty telling. Have they done any EMG's?
I hope she will get answers soon. Hang in there.
Annie
http://www.mayomedicallaboratories.c...nit_code=83370
http://books.google.com/books?id=Ys4...tibody&f=false
One more thing. You should both be fully evaluated for the possibility of a thymoma, if that hasn't been done already.