Glutamic Acid Decarboxylase (GAD) Antibodies

Blood Sciences Test

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SPECIMEN TYPES:

Serum for GAD, IA-2 and ZnT8 testing only (minimum 500 µL): Please send with your local Pathology request form to Blood Sciences Laboratory.

TRANSPORT OF SPECIMEN:

First class post at room temperature

NORMAL RANGE

GAD Positive: >/= 11 U/mL.

TARGET REPORTING TIME:

2 weeks

** Please note (March 2024) – We are currently experiencing delays in analysis due to reagent shortages.  The current expected TAT is 4-6 weeks. **

COST:

£29.04 (all three antibodies)

Test Usage

Autoantibodies recognising the islet cell antigens glutamic acid decarboxylase (GAD 65), Zinc Transporter 8 (ZnT8) and the insulinoma antigen 2 (IA-2) are present in up to 90% of newly diagnosed patients with type 1 diabetes.  The prevalence of GAD, IA-2 and ZnT8 autoantibodies is less than 1% in GCK, HNF1A and HNF4A MODY, and testing for these autoantibodies close to diagnosis gives very good discrimination of Type 1 diabetes from MODY (McDonald et al 2011 Diabet Med 28:1028-1033).  The Blood Sciences department at the Royal Devon & Exeter Hospital offers a highly sensitive, automated ELISA method for the quantitative determination of GAD, IA-2 and ZnT8 autoantibodies in serum samples.

Anti-GAD antibodies in high titre are also associated with the stiff-person syndrome (60% sensitivity), a rare neurological disease characterised by muscle rigidity and spasms.

Glutamic acid decarboxylase occurs in two isoforms (GAD 65 and GAD 67); although GAD 67 is only expressed in neuronal tissue, the major target antigen in both syndromes is GAD 65, which is the isoform used in this commercial assay.

Turnaround time

10 working days

Availability

In House

REFERRALS

We welcome referrals for GAD. If you are a new requestor please complete the following form: New requestor form

We now also have the option to use NPEx for referral requesting.

Further information about sample requirements for this test can be found here: GAD IA2 ZnT8 – further information

Specimen Labelling Procedure

Assayed By

This investigation is performed at the Royal Devon & Exeter Hospital (Wonford) (type 1 DM and stiff-person syndrome).
References

1. Brooking H, Ananieva-Jordanova R, Arnold C, Amoroso M, Powell M, Betterle C, Zanchetta R, Furmaniak J, Smith BR. A sensitive non-isotopic assay for GAD65 autoantibodies. Clin Chim Acta. 2003 May; 331(1-2):55-9.
2. Nilson E, Ekholm B, Rees Smith B, Törn C, Hillman M. Calcium addition to EDTA plasma eliminates falsely positive results in the RSR GADAb ELISA. Clin Chim Acta. 2008 Feb; 388(1-2):130-4.
3. A comparison of serum and EDTA plasma in the measurement of glutamic acid decarboxylase autoantibodies (GADA) and autoantibodies to islet antigen-2 (IA-2A) using the RSR radioimmunoassay (RIA) and enzyme linked immunosorbent assay (ELISA) kits. Rahmati K, Lernmark A, Becker C, Foltyn-Zadura A, Larsson K, Ivarsson SA, Törn C. Clin Lab. 2008; 54(7-8):227-35.
4. McDonald TJ, Colclough K, Brown R, Shields B, Shepherd M, Bingley P, Williams A, Hattersley AT, Ellard S. 2011. Islet autoantibodies can discriminate maturity-onset diabetes of the young (MODY) from Type 1 diabetes. Diabet Med 28:1028-1033.

Specimen Labelling Procedure