Clinical Information

Fatigable weakness due to impaired postsynaptic transmission at the neuromuscular junction is characteristic of myasthenia gravis (MG). A clinical diagnosis should be supported by electrodiagnostic testing (ie, clinical-electrodiagnosis [EDX]). Positive autoimmune serology increases certainty of MG diagnosis but needs to be interpreted in the proper clinical-EDX context with response to anticholinesterase medications supporting the diagnosis. Most cases are autoimmune and are caused by IgG autoantibodies binding to critical postsynaptic membrane molecules (nicotinic muscle acetylcholine receptor [AChR] or its interacting proteins, such as muscle-specific kinase). Serologically, the detection of AChR binding antibody provides the best diagnostic sensitivity. However, the presence of both AChR binding and modulating activity improves diagnostic accuracy. Autoantibody detection frequency is lowest in patients with weakness confined to extraocular muscles (approximately 70% are positive for AChR binding antibodies) and highest in patients with generalized weakness due to MG (approximately 90% are positive for AChR binding antibodies). In adults with MG and AChR antibodies, approximately 20% will have thymoma and, very rarely (<1%), extrathymic cancers. Computed tomography imaging of the chest is considered the standard of care to evaluate for thymoma.

These results should only be interpreted in the appropriate clinical and electrophysiological context and are not diagnostic in isolation.

Note: Single antibody tests may be requested in the follow-up of patients with positive results previously documented in this laboratory.