To the best of my knowledge, mestinon has no significant effect on the immune system and therefore should not change results of serological tests.

Take into account that the commercially available tests may not detect all antibodies (either in AhcR or MuSK MG).

Cell-based assays which are much more sensitive are not being used routinely. So, some patients may have false negative results, because of the relatively low sensitivity of the commercial tests.

http://www.ncbi.nlm.nih.gov/pubmed/23252902