LRP4 antibody tests are not readily available, so far as I am aware.

I guess each of these tests is expensive, so it is no surprise that, with a rare disease if 80% are AChR positive 10% MuSK positive and some smaller percentage maybe LRP4 positive, then the rarer the version you have the more costly the tests will get.

If you have a medical team accepting that you are seronegative MG, the only reason that an extra test would be worthwhile would be if that particular version of MG responded to different treatments.