22156 Acetylcholine Receptor (Muscle AChR) Binding Antibody (ARBI)

​​ACh Receptor (Muscle) Binding Ab

​This test should not be requested in patients who have recently received radioisotopes, therapeutically or diagnostically, because of potential assay interference. The specific waiting period before specimen collection will depend on the isotope administered, the dose given, and the clearance rate in the individual patient. Specimens will be screened for radioactivity prior to analysis. Radioactive specimens received in the laboratory will be held for 1 week and assayed if sufficiently decayed or canceled if radioactivity remains.

For optimal antibody detection, specimen collection is recommended prior to initiation of immunosuppressant medication.

The presence of elevated immunoglobulins due to therapeutic intervention or other disorders (ie, hypergammaglobulinemia) may lead to false-positive results.

Positive results may be found in some patients with Lambert-Eaton syndrome, paraneoplastic central nervous system, and peripheral nervous system autoimmune disorders and in healthy individuals.

The presence of alpha-bungarotoxin antibodies may interfere with this assay.

Specimens ideally should be collected prior to initiation of immunosuppressive therapies as these may reduce the sensitivity of this test.​

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.

Positive results (>0.02 nmol/L) are indicative of autoimmune myasthenia gravis (MG). These results should be interpreted in the appropriate clinical and electrophysiological context.

With a diagnosis of MG, a paraneoplastic basis should be considered with thymoma being the most commonly associated tumor with MG.

The clinical sensitivity of this assay is approximately 90% in nonimmunosuppressed patients with generalized MG. The frequency of antibody detection is lower in MG patients with weakness clinically restricted to ocular muscles (71%), and antibody titers are generally low in ocular MG (eg, 0.03-1.0 nmol/L).

Negative results do not exclude the diagnosis of MG. If clinical suspicion remains and symptoms persist or worsen consider retesting. Results may be negative in the first 12 months after symptoms of MG appear or during immunosuppressant therapy. Note: In follow up of seronegative patients with adult-acquired generalized MG, 17.4% seroconvert to positive at 12 months (ie, seronegativity rate at 12 months is 8.4%). A subset of MG patients that are persistently negative for acetylcholine receptor binding antibodies will have muscle-specific kinase (MuSK) antibodies, and therefore, it is recommended to test for MuSK antibodies in seronegative patients with high clinical suspicion of MG.

In general, there is not a close correlation between antibody titer and severity of weakness, but in individual patients, clinical improvement may be accompanied by a decrease in titer.