Comparison of antigenic sources for acetylcholine receptor antibody assays in myasthenia gravis

Antibody titers to the acetylcholine receptor (AChR) from patients with myasthenia gravis, in identical serum samples, were directly compared using denervated rat, human, and baboon muscle as the source of AChR antigen for radioimmunoassay (RIA). Calculations were standardized by using binding isotherms for each antigen source and calculating the percentage of AChR sites labeled with [125I]alpha-bungarotoxin at the concentration used in the RIA. In patients with high AChR antibody titers, the antibody concentration when human muscle was used as the antigen source measured up to tenfold higher than that with denervated rat muscle. In patients who had low antibody titers with human muscle antigen, assays using rat denervated muscle AChR frequently failed to demonstrate diagnostically abnormal titers. The data explain difference among several reported series in the percentages of patients with myasthenia gravis who had elevated serum antibody concentrations. AChR antibody concentrations with baboon muscle as the antigen source were comparable to those in which human muscle was used.