Acetylcholine Receptor (Muscle AChR) Binding Antibody, Serum

CPT CODE:

USEFUL FOR:

Confirming the diagnosis of MG
Distinguishing acquired disease (90% positive) from congenital disease (negative)
Detecting subclinical MG in patients with thymoma or graft-versus-hostdisease
Monitoring disease progression in MG or response to immunotherapy
An adjunct to the test for P/Q-type calcium channel binding antibodies as a diagnostic aid for LES or primary lung carcinoma

SPECIMEN REQUIRED:

Draw blood in a plain, red-top tube(s) or a serum gel tube(s). Spin down and send 2 mL of serum refrigerated.

TRANSPORT TEMPERATURE:

Refrig <14 days\Ambient <72 hours OK\Frozen OK

CLINICAL INFORMATION:

Myasthenia gravis (MG) is characterized by weakness and easyfatigability that are relieved by rest and anticholinesterase drugs.  The weakness in most cases results from an autoantibody-mediated loss of functional acetylcholine receptors in the postsynaptic membrane of skeletal muscle.  
Demonstration of muscle AChR autoantibodies in a patient's serum supports the diagnosis of acquired (autoimmune) MG, and quantitation provides a baseline for future comparisons.  
Muscle AChR antibodies are not found in congenital forms of MG and are uncommon in neurologic conditions other than acquired MG, with the exception of patients with paraneoplastic autoimmune neurological disorders, and Lambert-Eaton myasthenic syndrome (LES) with or without cancer (13% of LES patients have positive results for muscle AChR binding or striational antibodies). Patients with autoimmune liver disease are also frequently seropositive.
The assay for muscle AChR binding antibodies is considered a first-order test for the laboratory diagnosis of MG, and for detecting "subclinical MG" in recipients of D-penicillamine, in patients with thymoma without clinical evidence of MG, and in patients with graft-versus-host disease.

CLINICAL INTERPRETATION:

Values >0.02 nmol/L are consistent with a diagnosis of acquired MG, provided that clinical/electrophysiological criteria support that diagnosis.
The assay for muscle AChR binding antibodies is positive in approximately 90% of nonimmunosuppressed patients with generalized MG.
The frequency of antibody detection is lower in MG patients withweakness clinically restricted to ocular muscles (71%), and antibodytiters are generally low in ocular MG (eg, 0.03-1.0 nmol/L).
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-aquired generalized MG, 17.4% seroconvert to positive at 12 months (ie, seronegativity rate at 12 months is 8.4%). Thirty eight percent of persistently seronegative patients have muscle-specific kinase (MuSK) antibody.
Sera of nonmyasthenic subjects bind per liter 0.02 nmol or less ofmuscle AChR complexed with (125)I-labeled-alpha-bungarotoxin.
In general, there is not a close correlation between antibody titerand severity of weakness, but in individual patients, clinicalimprovement is usually accompanied by a decrease in titer.

REFERENCE VALUES:

< or =0.02 nmol/L