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GAD65 Antibody (Glutamic Acid Decarboxylase)

Measures antibodies to GAD65, an enzyme in insulin-making cells and nerves; it is one of the islet antibodies used to look for autoimmune (type 1) diabetes.

AI-verified against cited sources. Awaiting expert reviewAlso called GAD65, GADA, GAD-65LOINC 56540-8

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What it is

GAD65 is an enzyme found in the insulin-making cells of the pancreas and in parts of the nervous system. In some people the immune system makes antibodies against it. This test measures how much of that antibody is in your blood. Most labs report a number, and many call a value near 5 units per milliliter or higher positive. Kits differ, so the cutoff printed on your own report is the one to use.

Why it matters

GAD65 antibody is one of the most common antibodies in autoimmune (type 1) diabetes, and one study reports it in about 80% of people at diagnosis. It is most useful for showing an autoimmune cause in someone already diagnosed with type 1 diabetes. ARUP advises against using it alone to tell type 1 from type 2 diabetes, and the ADA warns that one imperfect test can label some people with type 2 diabetes as having slow-onset autoimmune diabetes (LADA). Two or more islet antibodies in a person with normal blood sugar is called stage 1 type 1 diabetes. Very high levels, many times above the diabetes cutoff, are linked to rare nerve conditions such as stiff-person syndrome. One low positive result can also be a one-time finding, so it is usually checked again.

If it’s high

A positive result means GAD65 antibodies were found. In someone with diabetes, this can be a sign of an autoimmune cause (type 1 diabetes, including the slow-onset form called LADA), but one test does not settle the type of diabetes. In someone without diabetes, a single positive antibody is not a diagnosis. What it means depends on how many islet antibodies you have, how high they are and your age, and a first positive is usually confirmed with a second test. Very high levels can also be linked to nerve conditions, which your clinician would look into through your symptoms.

Common causes

  • A single low-level positive that later goes away (up to half of children with one antibody become negative later)
  • Type 1 diabetes, including slow-onset forms in adults (LADA)
  • Stage 1 or stage 2 type 1 diabetes found by screening, for example in relatives of people with the condition
  • Rare nerve conditions at very high levels, such as stiff-person syndrome, cerebellar ataxia, some forms of epilepsy and limbic encephalitis

What can help

  • Ask whether a second test, or the other islet antibodies (IA-2, ZnT8 and insulin), should be checked to confirm the result
  • If you have diabetes, ask whether this result changes how your diabetes is classified or followed
  • Ask for information on the signs of high blood sugar, such as strong thirst, frequent urination and weight loss, so you know when to call

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–5 U/mL. Source: ARUP Laboratories, Mayo Clinic Laboratories (Extended Catalog mirror, testcatalog.org), Journal of the Endocrine Society 2026;10:bvag087.

What can affect your result

  • Method and units: ELISA in U/mL or IU/mL and radioimmunoassay in nmol/L use different cutoffs
  • Age: islet antibodies usually first appear in early childhood, and the pattern of antibodies differs by age
  • Family history of type 1 diabetes
  • Time since diagnosis: the antibodies can fade after type 1 diabetes starts
  • Immune treatment for nerve disease can lower levels
  • Methods differ. ELISA kits report U/mL or IU/mL with a cutoff near 5, while radioimmunoassays such as Mayo's neurology test report nmol/L. Results from different methods cannot be compared directly.
  • Radioimmunoassay versions can be affected by recent radioactive isotope scans or treatments. Mayo says its test should not be requested soon after radioisotope exposure.
  • ARUP rejects plasma and grossly hemolyzed samples.
  • In people with GAD65-related nerve disease, treatment with IV immunoglobulin or other immune therapy can lower the level.

Levels change slowly. After type 1 diabetes is diagnosed, the antibodies fade in some people: in one UK study followed for a median of about 7 years, about 71% of those positive at diagnosis were still GAD65-positive.

When to talk to a clinician

Seek emergency care if you have very high blood sugar together with vomiting, belly pain, fast or deep breathing, fruity-smelling breath or confusion. These can be signs of diabetic ketoacidosis. Contact a clinician soon for new constant thirst, frequent urination or weight loss. For nerve symptoms, talk with a clinician about new muscle stiffness or spasms, unsteady walking, seizures or memory problems.

Questions you could ask

  • Why was this test ordered, and what will we do with the result?
  • Does my lab's cutoff differ from other labs, and how should I compare future results?
  • Should we repeat this test or check the other islet antibodies?
  • Does this result change how my diabetes is classified or followed?
  • Should my blood sugar or A1c be checked more often?

Questions people ask

My GAD65 antibody is positive and I don't have diabetes. What does that mean?

One positive antibody on its own is not a diagnosis. In studies of children, up to half with a single antibody became negative later. Your risk depends on how many islet antibodies you have, how high they are and your age. Your clinician may repeat the test and check the other antibodies.

Why do labs report GAD65 in different units?

Labs use different kits. Many diabetes labs report U/mL or IU/mL with a cutoff near 5. Mayo's nerve-disease test reports nmol/L, which is a different scale. Because of this, compare your result with the range on your own report, not with a result from another lab.

Can GAD65 antibody levels change over time?

They can. In people with type 1 diabetes the antibodies may fade after diagnosis. In nerve conditions, levels often fall with immune treatment, in one study by a median of 69%, although the antibody did not disappear in any of those patients.

Show the science and sources

For clinicians and researchers

GAD65 autoantibodies are one of four islet autoantibodies (with IAA, IA-2 and ZnT8) used for classification and staging. The ADA defines stage 1 type 1 diabetes as two or more islet antibodies with normal glucose, with a 5-year risk of symptomatic disease of about 44%, and stage 2 as the same antibodies plus dysglycemia, with about 75% risk at 5 years. About half of children with a single antibody revert to negative, so the ADA recommends confirming a first positive within 3 months, preferably in a lab that meets Islet Autoantibody Standardization Program performance standards. ISPAD 2024 puts the 15-year risk of stage 3 at about 15% for a single antibody. The ADA also notes that a single imperfect autoantibody test used to label LADA may misclassify some people with type 2 diabetes, and that higher-specificity tests, confirmatory antibodies and testing only people with suggestive features improve accuracy. Neurologic GAD65 disease is usually associated with far higher concentrations. In a cohort of 56 patients with neurologic symptoms, a serum ELISA cutoff of 10,000 IU/mL separated a high group (94% with stiff-person syndrome, cerebellar ataxia, epilepsy or limbic encephalitis) from a low group with mixed, less specific symptoms. Another group used a cutoff of 2,000 on a radioimmunoassay with different calibration, so these thresholds are assay-specific. Some people with type 1 diabetes and no nerve symptoms also had high values in that lab (range up to 145,400 IU/mL), so a high level alone does not establish a nerve condition. Mayo's neurology test is a radioimmunoassay with a reference of 0.02 nmol/L or less. Common kit cutoffs for diabetes work are 5.0 U/mL (RSR ELISA, Mayo/Esoterix) and 5.0 IU/mL (ARUP).

LOINC 56540-8 (primary), 13926-1, 112067-4, 94345-6, 30347-9. Specimen: serum.

Sources

  1. Glutamic Acid Decarboxylase Antibody (ARUP test 2001771), ARUP Laboratories (2026)
  2. Diabetes Autoimmune Profile (Mayo Clinic Laboratories test FDAIP), Mayo Clinic Laboratories (Extended Catalog mirror, testcatalog.org) (2026)
  3. Stiff-Person Spectrum Disorders Evaluation, including Progressive Encephalomyelitis with Rigidity and Myoclonus, Serum (Mayo Clinic Laboratories test SPPS), Mayo Clinic Laboratories (Extended Catalog mirror, testcatalog.org) (2026)
  4. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2025, American Diabetes Association, Diabetes Care 2025;48(Suppl 1):S27-S49 (2025)
  5. Staging presymptomatic type 1 diabetes: a scientific statement of JDRF, the Endocrine Society, and the American Diabetes Association, Diabetes Care 2015;38:1964-1974 (2015)
  6. ISPAD Clinical Practice Consensus Guidelines 2024: Screening, Staging, and Strategies to Preserve Beta-Cell Function in Children and Adolescents with Type 1 Diabetes, Hormone Research in Paediatrics 2024;97:529-545 (2024)
  7. Natural history of a pediatric cohort with islet autoantibody positivity: a single-center retrospective cohort study, Journal of the Endocrine Society 2026;10:bvag087 (2026)
  8. Neurologic syndromes related to anti-GAD65: Clinical and serologic response to treatment, Neurology Neuroimmunology & Neuroinflammation 2020;7:e696 (2020)
  9. The longitudinal loss of islet autoantibody responses from diagnosis of type 1 diabetes occurs progressively over follow-up and is determined by low autoantibody titres, early-onset, and genetic variants, Clinical & Experimental Immunology 2022;210:151-162 (2022)
  10. LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 56540-8), U.S. National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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