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GAD (glutamic acid decarboxylase) antibodies

Category Immunology
Test background

Glutamic acid decarboxylase (GAD) is an enzyme that converts glutamic acid to gamma-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter, reduced levels lead to anxiety and seizures. In the β islet cells of the pancreas GABA promotes the secretion of insulin.

GAD antibodies can be measured together with other markers of islet cell autoimmunity (insulin, IA2, ZNT8 antibodies) to predict the development and aid in the diagnosis of type 1 diabetes mellitus (T1DM). Two or more antibodies in combination increase the predictive value of disease development. Of these, insulin and GAD antibodies usually arise first, with GAD antibodies being the major diabetic autoantibody. Screening for diabetic autoantibodies is also used to distinguish between type 2 diabetes mellitus (T2DM) and latent autoimmune diabetes in adults (LADA) in the adult population.

NICE Guidelines for the diagnosis of T1DM in adults (NG17) recommend measuring diabetes-specific autoantibodies at diagnosis and state that the false negative rate can be reduced if quantitative tests for two different diabetes specific antibodies are measured. The laboratory performs quantitative GAD65 and IA2 in house.

Antibodies to GAD are also found in ~60 % of patients with stiff person syndrome and GAD encephalitis.

This assay is performed by ELISA.

Clinical Indications

Type 1 autoimmune diabetes
Stiff person syndrome, cerebellar ataxia, temporal lobe epilepsy.

Reference range

<5.0 AU/mL (reference range derived by manufacturer and verified in house)

Sample & container required Serum (RST rust top), CSF
Sample volume 5-10 mL blood (1 mL serum)
Turnaround time 21 days

The laboratory is not accredited for testing CSF samples