Glutamic Acid Decarboxylase (GAD65) Antibody (GD65S)

Test Code: GAD65SO

Overview
Ordering
Specimen
Performing
Clinical/Interpretive
Contacts
Coding

Synonyms/Keywords

64K Antibody, Anti-GAD Antibodies, Anti-Glutamic Acid Decarboxylase Ab, Beta Cell Antibody, GAD65 Antibody Assay, Pancreatic Islet Cell Ab, Stiffman Syndrome, Glutamate Decarboxylase Antibodies (GAD65), Islet cell antibody, Stiff person, Ataxia

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

Specimen Requirements

Specimen Type	Preferred Container/Tube	Acceptable Container/Tube	Specimen Volume	Specimen Minimum Volume (allows for 1 repeat)	Pediatric Minimum Volume (no repeat)
Serum	Red Top Tube (RTT)	Serum Separator Tube (SST)	1.5 mL	1.0 mL

Collection Processing Instructions

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 Stability Information

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

Rejection Criteria

Gross hemolysis
Gross lipemia
Gross icterus

Interference

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.

Performing Laboratory Information

Performing Location	Day(s) Test Performed	Analytical Time	Methodology/Instrumentation
Mayo Clinic Laboratories	Monday through Sunday	3-6 days	Radioimmunoassay (RIA)

Reference Lab

Mayo Clinic Laboratories

Test 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 (> or =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.

Reference Range Information

< or =0.02 nmol/L

Reference values apply to all ages.

Interpretation

High titers (> or = 0.02 nmol/L) are found in classic stiff-person syndrome (93% positive) and in related autoimmune neurologic disorders (eg, acquired cerebellar ataxia, some acquired nonparaneoplastic encephalomyelopathies).

Diabetic patients with polyendocrine disorders also generally have GAD65 antibody values > or =0.02 nmol/L.

Values in patients who have type 1 diabetes without a polyendocrine or autoimmune neurologic syndrome are usually < or =0.02 nmol/L. 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 <5% of type 2 diabetic patients. Testin gfor 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 > or =0.03 nmol/L are consistent with susceptibility to autoimmune (type 1) diabetes and related endocrine disorders (thyroiditis and pernicious anemia).

Outreach CPTs

CPT	Modifier (if needed)	Quantity	Description	Comments
86341

Synonyms/Keywords

Ordering Applications

Ordering Application	Description
Centricity	GAD65 Ab Assay
Cerner	Glutamic Acid Decaroxylase (GAD65) Ab (81596)

If the ordering application you are looking for is not listed, contact your local laboratory for assistance.

Specimen Requirements

Specimen Type	Preferred Container/Tube	Acceptable Container/Tube	Specimen Volume	Specimen Minimum Volume (allows for 1 repeat)	Pediatric Minimum Volume (no repeat)
Serum	Red Top Tube (RTT)	Serum Separator Tube (SST)	1.5 mL	1.0 mL

Collection Processing