The MUsK is another antibody that was identified as having to do with MG about 7 years ago. After it's discovery, several MGer's previously thought to be seronegative became positive on the blood tests. My neuros, when trying to reconfirm my diagnosis, run both the ACHr and MUsK tests together. The MUsK seems to show more in people that don't have occular involvement right away, but more so in the neck, back, arms and shoulders. Generally, people positive for ACHr don't show MUsK. The MUsK form seems to respond better to certain types of therapy such as plasma exchange, according to a report from the American Academy of Neurology report from 2003. So your doctor wants it for dx and treatment purposes, I'm sure.

It's because of the discovery of the MUsk antibody that I don't believe anyone is truely seronegative - they just haven't found the rest of the antibodies connected with MG yet! I'm one of those people which is why my dx is constantly being questioned.

Hope this helps!