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Glutamic Acid Decarboxylase-65 Antibody
Test Code34878
CPT Codes
86341
Preferred Specimen
1 mL serum
Minimum Volume
0.5 mL
Transport Container
Transport tube
Transport Temperature
Room temperature
Specimen Stability
Room temperature: 7 days
Refrigerated: 14 days
Frozen: 60 days
Refrigerated: 14 days
Frozen: 60 days
Methodology
Enzyme Linked Immunosorbent Assay (ELISA)
Setup Schedule
Set up: Sun-Fri; Report available: 3-5 days
Limitations
GAD-65 antibodies are detected in 20% of diabetic twins who remain disease-free for long periods of time and in 8% of healthy individuals.
Reference Range
<5 IU/mL
Clinical Significance
Key clinical use and differentiators
Measurement of Glutamic acid decarboxylase (GAD65) is used to: 1. Screen for presymptomatic type 1 diabetes (T1D) in individuals with a family history of T1D or those known to have a high-risk HLA genotype, 2. Distinguish between T1D, type 2 diabetes mellitus (T2D), and atypical forms of diabetes such as latent autoimmune diabetes in adults (LADA). [1] Glutamic acid decarboxylase (GAD65) is a key protein that the immune system mistakenly attacks in people with T1D, and positivity to GAD65 antibodies represents a key marker of the disease. [1] GAD65 is the most common autoantibody found in adult-onset autoimmune diabetes. In addition to its importance in guiding the diagnosis of T1D, glutamic acid decarboxylase (GAD65) is also helpful to confirm the diagnosis of stiff-man syndrome, autoimmune encephalitis, autoimmune ataxia, brain stem encephalitis, autoimmune epilepsy, autoimmune myelopathy or pernicious anemia.
Intended Clinical Use
This test is crucial for differentiating between T1D and T2D. It is important to note that classifying diabetes type can be challenging at the time of presentation, and misdiagnosis can occur in approximately 40% of adults with new T1D. [1] Identifying individuals affected by stage 1 T1D is important for applying eligibility criteria to drugs, such as Teplizumab, which have demonstrated the ability to delay the transition of stage 1 to stage 3 T1D in clinical trials. [2,3] GAD65 has some distinctive features. It is commonly the first detected autoantibody in childhood, up until age 15 years. [4] Adult-onset T1D cases most often present with GADA. [5] It is associated with slower progression to T1D and is often found as a single positive islet autoantibody, especially in adults.
Limitations
T1D is classified into three stages, with stages 1 and 2 considered presymptomatic. Since the presence of stage 1 and 2 T1D requires at least two islet autoantibodies, relying solely on GAD65 as a screening test limits the ability to make an accurate diagnosis. Professional organizations, including the American Diabetes Association (ADA), recommend using tests for all four islet cell autoantibodies: IAA, glutamic acid decarboxylase (GAD65) Zinc Transporter 8 (ZnT8), and islet cell antigen 2 antibody (IA-2). [1]
References
1. American Diabetes Association Professional Practice C. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2025. Diabetes Care. 2025;48:S27-S49. doi: 10.2337/dc25-S002
2. Herold KC, Gitelman SE, Gottlieb PA, Knecht LA, Raymond R, Ramos EL. Teplizumab: A Disease-Modifying Therapy for Type 1 Diabetes That Preserves beta-Cell Function. Diabetes Care. 2023;46:1848-1856. doi: 10.2337/dc23-0675
3. Ramos EL, Dayan CM, Chatenoud L, Sumnik Z, Simmons KM, Szypowska A, Gitelman SE, Knecht LA, Niemoeller E, Tian W, et al. Teplizumab and beta-Cell Function in Newly Diagnosed Type 1 Diabetes. N Engl J Med. 2023;389:2151-2161. doi: 10.1056/NEJMoa2308743
4. Krischer JP, Lynch KF, Schatz DA, Ilonen J, Lernmark A, Hagopian WA, Rewers MJ, She JX, Simell OG, Toppari J, et al. The 6 year incidence of diabetes-associated autoantibodies in genetically at-risk children: the TEDDY study. Diabetologia. 2015;58:980-987. doi: 10.1007/s00125-015- 3514-y
5. Leslie RD, Evans-Molina C, Freund-Brown J, Buzzetti R, Dabelea D, Gillespie KM, Goland R, Jones AG, Kacher M, Phillips LS, et al. Adult-Onset Type 1 Diabetes: Current Understanding and Challenges. Diabetes Care. 2021;44:2449- 2456. doi: 10.2337/dc21-0770
Measurement of Glutamic acid decarboxylase (GAD65) is used to: 1. Screen for presymptomatic type 1 diabetes (T1D) in individuals with a family history of T1D or those known to have a high-risk HLA genotype, 2. Distinguish between T1D, type 2 diabetes mellitus (T2D), and atypical forms of diabetes such as latent autoimmune diabetes in adults (LADA). [1] Glutamic acid decarboxylase (GAD65) is a key protein that the immune system mistakenly attacks in people with T1D, and positivity to GAD65 antibodies represents a key marker of the disease. [1] GAD65 is the most common autoantibody found in adult-onset autoimmune diabetes. In addition to its importance in guiding the diagnosis of T1D, glutamic acid decarboxylase (GAD65) is also helpful to confirm the diagnosis of stiff-man syndrome, autoimmune encephalitis, autoimmune ataxia, brain stem encephalitis, autoimmune epilepsy, autoimmune myelopathy or pernicious anemia.
Intended Clinical Use
This test is crucial for differentiating between T1D and T2D. It is important to note that classifying diabetes type can be challenging at the time of presentation, and misdiagnosis can occur in approximately 40% of adults with new T1D. [1] Identifying individuals affected by stage 1 T1D is important for applying eligibility criteria to drugs, such as Teplizumab, which have demonstrated the ability to delay the transition of stage 1 to stage 3 T1D in clinical trials. [2,3] GAD65 has some distinctive features. It is commonly the first detected autoantibody in childhood, up until age 15 years. [4] Adult-onset T1D cases most often present with GADA. [5] It is associated with slower progression to T1D and is often found as a single positive islet autoantibody, especially in adults.
Limitations
T1D is classified into three stages, with stages 1 and 2 considered presymptomatic. Since the presence of stage 1 and 2 T1D requires at least two islet autoantibodies, relying solely on GAD65 as a screening test limits the ability to make an accurate diagnosis. Professional organizations, including the American Diabetes Association (ADA), recommend using tests for all four islet cell autoantibodies: IAA, glutamic acid decarboxylase (GAD65) Zinc Transporter 8 (ZnT8), and islet cell antigen 2 antibody (IA-2). [1]
References
1. American Diabetes Association Professional Practice C. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2025. Diabetes Care. 2025;48:S27-S49. doi: 10.2337/dc25-S002
2. Herold KC, Gitelman SE, Gottlieb PA, Knecht LA, Raymond R, Ramos EL. Teplizumab: A Disease-Modifying Therapy for Type 1 Diabetes That Preserves beta-Cell Function. Diabetes Care. 2023;46:1848-1856. doi: 10.2337/dc23-0675
3. Ramos EL, Dayan CM, Chatenoud L, Sumnik Z, Simmons KM, Szypowska A, Gitelman SE, Knecht LA, Niemoeller E, Tian W, et al. Teplizumab and beta-Cell Function in Newly Diagnosed Type 1 Diabetes. N Engl J Med. 2023;389:2151-2161. doi: 10.1056/NEJMoa2308743
4. Krischer JP, Lynch KF, Schatz DA, Ilonen J, Lernmark A, Hagopian WA, Rewers MJ, She JX, Simell OG, Toppari J, et al. The 6 year incidence of diabetes-associated autoantibodies in genetically at-risk children: the TEDDY study. Diabetologia. 2015;58:980-987. doi: 10.1007/s00125-015- 3514-y
5. Leslie RD, Evans-Molina C, Freund-Brown J, Buzzetti R, Dabelea D, Gillespie KM, Goland R, Jones AG, Kacher M, Phillips LS, et al. Adult-Onset Type 1 Diabetes: Current Understanding and Challenges. Diabetes Care. 2021;44:2449- 2456. doi: 10.2337/dc21-0770
Performing Laboratory
Quest Diagnostics Nichols Institute
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Chantilly, VA 20153
