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)

Useful For

Assessing susceptibility to autoimmune (type 1, insulin-dependent) diabetes mellitus and related endocrine disorders (eg, thyroiditis and pernicious anemia). Titers generally < or = 0.02 nmol/L. A second islet cell antibody, IA-2, is more predictive for development of type 1 diabetes, but less frequent than GAD65 Ab amongst diabetic patients. Insulin autoantibodies also serve as a marker of susceptibility to type 1 diabetes.

Distinguishing between patients with type 1 and type 2 diabetes. Assays for IA-2, insulin, gastric parietal cell, thyroglobulin, and thyroid peroxidase antibodies, complement GAD65 antibody in this context. Titers generally < or = 0.02 nmol/L.

Confirming a diagnosis of stiff-man syndrome, autoimmune encephalitis, cerebellitis, brain stem encephalitis, myelitis. Titers generally > or = 0.03 nmol/L.

Confirming susceptibility to organ-specific neurological disorders (eg, myasthenia gravis, Lambert-Eaton syndrome). Titers generally < or = 0.02 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
*If GD65C is needed on CSF sample, it must be ordered as a MISC.

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.

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.

Performing Laboratory Information

Performing Location	Day(s) Test Performed	Analytical Time	Methodology/Instrumentation
Mayo Medical Laboratories	Sunday through Thursday	2 days/negative 3 days/positive	Radioimmunoassay (RIA)

Reference Lab

Mayo Clinic Laboratories

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-man 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.

Low titers are found in approximately 25% of patients with myasthenia gravis, Lambert-Eaton syndrome, and rarer autoimmune neurological disorders. Eight percent of healthy Olmsted County residents over age 50 have low positive values. These are not false positive; the antibodies are inhibited by unlabelled GAD65 antigen and are accompanied in at least 50% of cases by related organ-specific autoantibodies.

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.