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Test Code:
090-31-4007

Order Name:
Muscle-Specific Kinase (MuSK) Autoantibody, Serum **

 
Useful For:

Diagnosis of autoimmune muscle-specific kinase (MuSK) myasthenia gravis

Second-order test to aid in the diagnosis of autoimmune myasthenia gravis when first-line serologic tests are negative

Establishing a quantitative baseline value for MuSK antibodies that allows comparison with future levels if weakness is worsening

 
Methodology:
Radioimmunoassay (RIA)
 
 
 
Test Code:
090-31-4007

Order Name:
Muscle-Specific Kinase (MuSK) Autoantibody, Serum **

 
Patient Preparation:
N/A
 
Collection Specimen Or Container:
Blood/ Plain blood (Red top) 6 mL, 2 tubes
 
 
Specimen Testing Type:
Serum, minimum volume 1.5 mL x 2 tubes
 
 
Sub Mission Container:
Plastic tube
 
Rejection Criteria:
Gross hemolysis Reject
Gross lipemia Reject
Gross icterus Reject
 
Specimen Stabillity:
Specimen Type Temperature Time
Serum
Refrigerated (preferred) 28 days
Frozen  28 days
Ambient  72 hours
 
 
 
Test Code:
090-31-4007

Order Name:
Muscle-Specific Kinase (MuSK) Autoantibody, Serum **

 
Method detail:
Radioimmunoassay (RIA)
 
Schedule:
N/A **Sent out to MAYO, USA
 
Turnaround Time:
Received specimen to reported within 14-21 days
(exception for official holidays)
 
Performing Location:
MAYO Laboratory
Referral Lab Services, Laboratory Department 14160-2
 
 
 
Test Code:
090-31-4007

Order Name:
Muscle-Specific Kinase (MuSK) Autoantibody, Serum **

 
 
Clinical Information:

Fatigable weakness due to impaired synaptic transmission at the neuromuscular junction is characteristic of myasthenia gravis (MG). The diagnosis is made by clinical and electromyographic criteria. Positive autoimmune serology must be interpreted in the clinical and electrophysiological context and response to anticholinesterase medication. Most cases are autoimmune and are caused by IgG autoantibodies binding to critical postsynaptic membrane molecules (nicotinic acetylcholine receptor or its interacting proteins). Autoantibody detection frequency is lowest in patients with weakness confined to extraocular muscles (71% muscle acetylcholine receptor: AChR binding). Mayo Clinic Laboratories' first-line serological evaluation detects muscle AChR antibody in 92% of nonimmunosuppressed patients with generalized weakness due to MG. Muscle-specific kinase (MuSK) antibody is detectable in more than one-third of those seronegative for muscle AChR antibody (less than 4% of all patients). Physiologically, MuSK is involved in integrating and stabilizing AChR clusters in the motor endplate. MuSK is activated when the nerve-derived proteoglycan agrin binds to its receptor, lipoprotein-related protein 4 (LRP4). Antibodies to LRP4 itself have been described in rare patients.

Six percent of nonimmunosuppressed patients with generalized MG lack demonstrable AChR or MuSK antibodies (double seronegative). Other rare autoantibodies no doubt remain to be discovered in such cases. However, as in autoimmune AChR MG and MuSK MG, testing for common organ-specific and nonorgan-specific autoantibodies is a valuable ancillary investigation in evaluating seronegative acquired generalized MG. General serological testing, coupled with family or personal history, will disclose autoimmune phenomena in 77% of those cases. These disorders may include thyroid disease, type 1 diabetes, vitiligo, premature greying, rheumatoid arthritis, or lupus. Testing may also reveal antinuclear antibodies, glutamic acid decarboxylase (GAD65) antibodies, thyroperoxidase/thyroglobulin antibodies, or gastric parietal cell antibodies. Objective improvement in strength following a therapeutic trial of plasmapheresis or intravenous immune globulin would justify consideration of long-term immunosuppression.

Females are generally affected by autoimmune MuSK MG more often than males. Onset can occur at any age (pediatric to elderly). Patients may derive limited benefit from anticholinesterase medication. The thymus is normal, and patients are generally not benefited by thymectomy. Antibody-lowering therapies are effective. Bulbar, facial, and respiratory weakness are prominent, and crises are common.

 
Reference Value:

< or =0.02 nmol/L

 
Interpretation:

A positive result, in the appropriate clinical context, confirms the diagnosis of autoimmune muscle-specific kinase myasthenia gravis.
Seropositivity justifies consideration of immunotherapy.

Cautions 
Immunosuppressant therapy is a common cause of false-seronegativity. It is, therefore, important to perform a comprehensive serological evaluation before initiating immunosuppressant therapy. 
Interpretation of a patient serological and clinical status is further complicated when characteristic signs of myasthenia gravis are obscured by a superimposed steroid-induced myopathy.

 
Clinical Reference:
www.mayocliniclabs.com (Retrieved: 21 July 2020)