False-positive acetylcholine receptor antibody results in patients without myasthenia gravis.

Acetylcholine receptor antibodies are very specific for myasthenia. During a large prospective cohort study of myasthenia, we encountered five patients, positive for acetylcholine receptor (AChR) antibodies by radioimmunoprecipitation assay (RIA), whose clinical course revealed diagnoses other than myasthenia. Two patients had transiently raised AChR antibodies associated with Guillain-Barré syndrome. Antibodies to clustered AChRs, in a live cell-based assay, were negative in all five patients, suggesting that results from the RIAs were false-positives. It is possible that the AChR antibodies detected by RIA in these cases were non-pathogenic, and directed to intracellular epitopes of the AChR.