I read an article in the 11th annual myasthenia gravis and related disorder conference where they tested seonegative MGer by adding more rapsyn ( the substance that makes ACHR receptors cluster) and they found if they added rapsyn to the test, 60% of SNMG tested positive for ACHR antibodies. They theorized that these MGers had low affinity binding and the test was not sensitive enough. (reference below). They felt SNMGers fit the profile of ACHR positive MGers. Theoretically, this would make thymectomy beneficial to the SNMGer. I can get to the book below because it is in the University where I work. I have not found a free link to it.

book:June 2008
Volume 1132 Myasthenia Gravis and Related Disorders 11th International Conference
article: Myasthenia Gravis Seronegative for Acetylcholine Receptor Antibodies
Angela Vincent, Maria Isabel Leite, Maria Elena Farrugia, Saiju Jacob, Stuart Viegas, Hiro Shiraishi, Olivier Benveniste, B. Paul Morgan, David Hilton-Jones, John Newsom-Davis*, David Beeson, Nick Willcox
Article first published online: 28 JUN 2008, pg 84-92