https://www.eurekalert.org/pub_releases/2020-08/mcog-pwr081820.php

As a retired medical researcher, I like to see what is coming out on MG research too.  

In 2012,2013 two new antibodies were discovered and associated with MG beyond the ACHR and MuSK ones.   This study talks about their role in MG.  

"Those who tested positive for antibodies to LRP4, agrin or both, tended to be sicker when diagnosed and have a more severe disease course, investigators at Case Western Reserve University School of Medicine and the Medical College of Georgia at Augusta University report.

Patients in the new study had no evidence of long-established antibodies for the receptor for acetylcholine, a chemical released by neurons, which activates muscle cell receptors, or MuSK, an enzyme that supports the clustering of these receptors on the surface of muscle cells and is essential to maintain the function of acetylcholine receptors.

Rather patients recruited for the study had been termed "double negative." Now the investigators say the roughly 85% of study participants who also tested negative for LRP4 and agrin are more accurately termed "quad negative."

Despite increased disease severity, better than 80% of patients with LRP4 and agrin antibodies had their disease moderated by standard therapy that includes a drug that increases acetylcholine levels by decreasing its breakdown as well as immunosuppressive drugs that reduce the body's overall production of antibodies."

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This study took only those who were seronegative (for ACHR, MuSK) and found 15% of those diagnosed with MG didn't have the two newer antibodies, and so it would seem there are more antibodies out there that can cause MG as yet undiscovered.  

I don't know if testing for LRP4 and agrin are done, but I imagine if one has MG symptoms and has negative tests for ACHR, MuSK, the addtional tests could be done if one wanted to look for confirmation.  I think most folks get the sfemg test when ACHR is negative. 

Good Luck

Russ