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HelpEpic Test Code LAB22146 Glutamic Acid Decarboxylase (GAD65) Antibody Assay, Serum
Additional Codes
MML Code: GD65S
LIS Code: GADAB
NY State Approved
YesPerforming Laboratory
Mayo Clinic Laboratories in Rochester
Reporting Name
GAD65 Ab Assay, SMethod Name
Radioimmunoassay (RIA)
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 28 days | |
| Frozen | 28 days | ||
| Ambient | 72 hours |
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
Collection Container/Tube:
Preferred: Red top
Acceptable: Serum gel
Submission Container/Tube: Plastic vial
Specimen Volume: 1.5 mL
Collection Instructions: Centrifuge and aliquot serum into a plastic vial.
Specimen Type
SerumSpecimen Minimum Volume
1 mL
Reference Values
≤0.02 nmol/L
Reference values apply to all ages.
Report Available
3 to 6 daysDay(s) Performed
Monday through Sunday
CPT Code Information
86341
Reject Due To
| Gross hemolysis | Reject |
| Gross lipemia | Reject |
| Gross icterus | Reject |
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
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-kDa 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 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 years, 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 generally have glutamic acid decarboxylase 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, islet cell (IA-2), and ZnT8 antibodies.
Eight percent of healthy Olmsted County residents over age 50 years 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).
Cautions
Antibodies specific for glutamic acid decarboxylase (GAD65) account for most, but not all, antibodies detected in the islet cell antibody test (IA-2). IA-2 (a protein tyrosine kinase-like protein), insulin, and zinc transporter-8 antibodies are complementary islet cell antibodies.