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HelpTest Code GD65C Glutamic Acid Decarboxylase (GAD65) Antibody Assay, Spinal Fluid
Reporting Name
GAD65 Ab Assay, CSFUseful For
Possible use in evaluating patients with autoimmune encephalitis, stiff-person syndrome, autoimmune ataxia, autoimmune epilepsy, and other acquired central nervous system disorders affecting gabaminergic neurotransmission
Method Name
Radioimmunoassay (RIA)
Performing Laboratory
Mayo Clinic Laboratories in Rochester
Specimen Type
CSFOrdering 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 one week and assayed if sufficiently decayed or canceled if radioactivity remains.
Specimen Required
Container/Tube: Sterile vial
Specimen Volume: 1.5 mL
Collection Instructions: Submit specimen from collection vial 2.
Specimen Minimum Volume
1 mL
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| CSF | Refrigerated (preferred) | 28 days | |
| Frozen | 28 days | ||
| Ambient | 72 hours |
Reject Due To
| Gross hemolysis | Reject |
| Gross lipemia | Reject |
| Gross icterus | Reject |
Reference Values
≤0.02 nmol/L
Reference values apply to all ages.
Day(s) Performed
Monday through Sunday
CPT Code Information
86341
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| GD65C | GAD65 Ab Assay, CSF | 94359-7 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| 21702 | GAD65 Ab Assay, CSF | 94359-7 |
Clinical Information
Glutamic acid decarboxylase (GAD) is a neuronal enzyme involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). Serum antibodies directed against the 65-kDa isoform of GAD (GAD65) are detected in heightened frequency in a variety of autoimmune neurologic disorders, including autoimmune encephalitis, stiff-person (Moersch-Woltman) syndrome, autoimmune ataxia, and autoimmune epilepsy.
Interpretation
Intrathecal synthesis of glutamic acid decarboxylase 65 (GAD65) antibody has been demonstrated in patients with stiff-man syndrome, but cerebrospinal fluid (CSF) values are log orders lower than serum. We have not determined the frequency of GAD65 antibodies in CSF of patients with various diagnoses.
Cautions
Clinical utility of this test remains to be determined.
Method Description
(125)I-labeled recombinant human glutamic acid decarboxylase 65 (GAD65) is incubated with the patient sample. Anti-human IgG is then added to form an immunoprecipitate. After washing the immunoprecipitate, the amount of (125)I-labeled antigen in the immunoprecipitate is measured using a gamma-counter. The amount of gamma emission in the precipitate is proportional to the amount of GAD65-IgG in the sample. Results are reported as units of precipitated antigen (nMol) per L of patient sample.(Walikonis JE, Lennon VA. Radioimmunoassay for glutamic acid decarboxylase [GAD65] autoantibodies as a diagnostic aid for stiff-man syndrome and a correlate of susceptibility to type 1 diabetes mellitus. Mayo Clin Proc. 1998;73[12]:1161-1166; Horta ES, Lennon VA, Lachance DH, et al. Neural autoantibody clusters aid diagnosis of cancer. Clin Cancer Res. 2014;20[14]:3862-9386)