Hi Whitecap
Welcome to NeuroTalk.
My understanding, as an immunologist, is that an ACHR antibody titre of 13 is in the range for Class IIA MG.
You might find the information in these two links helpful if you would like to discuss this with your doctor:
http://emedicine.medscape.com/article/1171206-workup
http://emedicine.medscape.com/article/1171206-overview
All the best.