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Test ID: GD65S Glutamic Acid Decarboxylase (GAD65) Antibody Assay, Serum

Reporting Name

GAD65 Ab Assay, S

Useful For

Assessing susceptibility to autoimmune (type 1, insulin-dependent) diabetes mellitus and related endocrine disorders (eg, thyroiditis and pernicious anemia)

 

Distinguishing between patients with type 1 and type 2 diabetes

 

Confirming a diagnosis of stiff-man syndrome, autoimmune encephalitis, autoimmune ataxia, brain stem encephalitis, autoimmune epilepsy, autoimmune myelopathy; titers generally greater than or equal to 20.0 nmol/L

Clinical Information

Glutamic acid decarboxylase (GAD) is a neuronal enzyme involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). Antibodies directed against the 65-kD isoform of GAD (GAD65) are encountered at high titers (≥20 nmol/L) in a variety of autoimmune neurologic disorders including stiff-person (Moersch-Woltman) syndrome, autoimmune cerebellitis, brain stem encephalitis, seizure disorders, and other myelopathies.

 

GAD65 antibody is also the major pancreatic islet antibody and an important serological marker of predisposition to type 1 diabetes. GAD65 autoantibody also serves as a marker of predisposition to other autoimmune disease that occur with type 1 diabetes, including thyroid disease (eg, thyrotoxicosis, Grave disease, Hashimoto thyroiditis, hypothyroidism), pernicious anemia, premature ovarian failure, Addison disease, (idiopathic adrenocortical failure) and vitiligo. GAD65 antibodies are found in the serum of approximately 8% of healthy subjects older than age 50, usually in low titer but often accompanied by related "thyrogastric" autoantibodies.

Interpretation

High titers (≥20.0 nmol/L) are found in classic stiff-person syndrome (93% positive) and in related autoimmune neurologic disorders (eg, acquired cerebellar ataxia, some acquired non-paraneoplastic encephalomyelopathies).

 

Diabetic patients with polyendocrine disorders also generally have glutamic acid decarboxylase (GAD65) antibody values 0.02 nmol/L or above.

 

Values in patients who have type 1 diabetes without a polyendocrine or autoimmune neurologic syndrome are usually 0.02 nmol/L or below. Low titers (0.03-19.9 nmol/L) are detectable in the serum of approximately 80% of type 1 diabetic patients. Conversely, low titers are detectable in the serum of less than 5% of patients with type 2 diabetes. Testing for autoimmune type 1 diabetes is complimented by testing for insulin, IA-2 and ZnT8 antibodies.

 

Eight percent of healthy Olmsted County residents over age 50 have low-positive values, and may be at risk for future autoimmune disease.

 

Values 0.03 nmol/L or above are consistent with susceptibility to autoimmune (type 1) diabetes and related endocrine disorders (thyroiditis and pernicious anemia).

Report Available

3 to 6 days

Day(s) Performed

Monday through Sunday

Clinical Reference

1. McKeon A, Tracy JA: GAD65 neurological autoimmunity. Muscle Nerve. 2017 Jul;56(1):15-27. doi: 10.1002/mus.25565.

2. Pittock SJ, Yoshikawa H, Ahlskog JE, et al: Glutamic acid decarboxylase autoimmunity with brainstem, extrapyramidal and spinal cord dysfunction. Mayo Clin Proc. 2006;81:1207-1214

3. McKeon A, Robinson MT, McEvoy KM, et al: Stiff-man syndrome and variants: clinical course, treatments, and outcomes. Arch Neurol. 2012 Feb;69(2):230-238

4. Steriade C, Britton J, Dale RC, et al: Acute symptomatic seizures secondary to autoimmune encephalitis and autoimmune-associated epilepsy: Conceptual definitions. Epilepsia. 2020 Jul;61(7):1341-1351

5. Bingley PJ. Clinical applications of diabetes antibody testing. J Clin Endocrinol Metab. 2010 Jan;95(1):25-33

Method Name

Radioimmunoassay (RIA)

Specimen Type

Serum


Ordering Guidance


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 1 week and assayed if sufficiently decayed or canceled if radioactivity remains.



Specimen Required


Container/Tube:

Preferred: Red top

Acceptable: Serum gel

Specimen Volume: 1.5 mL


Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 28 days
  Frozen  28 days
  Ambient  72 hours

Reference Values

≤0.02 nmol/L

Reference values apply to all ages.

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information

86341

LOINC Code Information

Test ID Test Order Name Order LOINC Value
GD65S GAD65 Ab Assay, S 94345-6

 

Result ID Test Result Name Result LOINC Value
81596 GAD65 Ab Assay, S 94345-6
Mayo Clinic Laboratories | Neurology Catalog Additional Information:

mml-Behavioral, mml-Epilepsy, mml-Movement-Disorders, mml-Demyelinating-Diseases, mml-Neuroimmunology, mml-Neuromuscular, mml-Pediatric, mml-Spinal-Cord, mml-Neuro-ophthalmology