Karen,

Welcome. You found a great place to learn about MG. Here is a link to the free MG manual that has lots of information. The following quote is from the section on antibody tests:

Assay for AChR-abs is an essential diagnostic test for MG. The most commonly performed assay measures binding to purified AChR from human skeletal muscle that is labeled with radioiodinated α-bungarotoxin.

Myasthenia Gravis: A Manual for the Health Care Provider (PDF) - page 15

This section also explain what they can do in the absence of a positive blood test.

The publication was designed for professionals, but it has a lot to offer us. I just skipped the 95 percent that I did not under stand.

-Mark-