Glutamic Acid Decarboxylase (GAD) Autoantibody

CPT: 86341
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Test Details

Synonyms

  • Anti-GAD

Use

This test is intended for the semiquantitative determination of glutamic acid decarboxylase (GAD) antibody in human serum; it is useful as an aid in the diagnosis of type 1 diabetes mellitus (autoimmune mediated diabetes).1

Methodology

Enzyme-linked immunosorbent assay (ELISA)

Reference Interval

Pediatric and adult: 0.0−5.0 units/mL

Additional Information

This test allows for the detection of the presence of antibodies to glutamic acid decarboxylase, which provides early evidence of autoimmune disease activity; its measurement has been shown to be useful in assisting the physician in the prediction, diagnosis, and management of patients with diabetes.2-6

Glutamic acid decarboxylase (GAD65) is an enzyme that is produced primarily by pancreatic islet cells. A number of recent studies indicate that patients with insulin-dependent diabetes mellitus (IDDM) often have antibodies to GAD65 and several other islet cell antigens.7 This is consistent with the hypothesis that IDDM is an autoimmune disease and that autoantibody production is an early step in the development of IDDM.8 Autoantibodies can be detected in many cases prior to the onset of glucose intolerance. The presence of GAD65 autoantibodies can be detected in many cases prior to the onset of glucose intolerance. The presence of GAD65 autoantibodies has been shown to be a strong predictive marker for the eventual onset of IDDM. Measurement of GAD65 antibody can also be of use in distinguishing insulin-dependent from non−insulin-dependent diabetics when the clinical history is ambiguous. GAD65 autoantibodies are often markedly elevated in patients with the stiff-person syndrome (also referred to as stiff-man syndrome), a condition that is associated with fluctuating stiffness and paroxysmal spasms of the trunk and legs.9,10

Specimen Requirements

Specimen

Serum

Volume

0.5 mL

Minimum Volume

0.4 mL (Note: This volume does not allow for repeat testing.)

Container

Red-top tube or gel-barrier tube

Collection

If a red-top tube is used, transfer separated serum to a plastic transport tube. Avoid hemolysis.

Storage Instructions

Room temperature

Stability Requirements

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Nonserum sample; grossly hemolyzed or lipemic sample

Clinical Information

Special Instructions

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

Footnotes

1. Glutamic Acid Decarboxylase Autoantibody ELISA [package insert]. Star, Idaho: Kronus; 2009. P/N 174g 09/09.
2. Pihoker C, Gilliam LK, Hampe CS.Lernmark A. Autoantibodies in diabetes. Diabetes. 2005 Dec; 54(Suppl 2):S52-61. 16306341
3. Winter WE, Schatz DA. Autoimmune markers in diabetes. Clin Chem. 2011 Feb; 57(2):168-175. 21127152
4. Tuomilehto J, Zimmet P, Mackay IR, et al. Antibodies to glutamic acid decarboxylase as predictors of insulin-dependent diabetes mellitus before clinical onset of disease. Lancet. 1994 Jun; 343(8910):1383-1385. 7910881
5. Verge CF, Gianani R, Kawasaki E, et al. Prediction of type I diabetes in first-degree relatives using a combination of insulin, GAD, and ICA512bdc/IA-2 autoantibodies. Diabetes. 1996 Jul; 45(7):926-933. 8666144
6. Bingley PJ, Bonifacio E, Gale EA. Can we really predict IDDM? Diabetes. 1993 Feb; 42(2):213-220 (review). 8425658
7. Pietropaolo M, Peakman M, Pietropaolo SL, et al. Combined analysis of GAD(65) and ICA512(IA-2) autoantibodies in organ and non−organ-specific autoimmune diseases confers high specificity for insulin-dependent diabetes mellitus. J Autoimmun. 1998 Feb; 11(1):1-10. 9480718
8. Bach JF. Insulin-dependent diabetes mellitus as an autoimmune disease. Endocr Rev. 1994 Aug; 15(4):516-542 (review) 7988484
9. Stayer C, Meinck HM. Stiff-man syndrome: An overview. Neurologia. 1998 Feb; 13(2):83-88. 9578675
10. Dalakas MC, Li M, Fujii M, Jacobowitz DM. Stiff person syndrome: Quantification, specificity, and intrathecal synthesis of GAD65 antibodies. Neurology. 2001 Sep 11; 57(5):780-784. 11552003

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
143008 GAD-65 Autoantibody 56540-8 143010 GAD-65 U/mL 56540-8

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