When would you repeat antibody testing for seronegative myasthenia gravis?

I will usually recheck in 1-2 years, but I mostly focus on acetylcholine receptor antibodies. I have never seen seroconversion from MuSK negative to MuSK positive.

Also, if initial antibody testing for Acetylcholine receptor binding/modulating and MuSK antibodies is negative by RIA assay, I recommen...