What are the diagnostic criteria for Type 1 Diabetes Mellitus (DM) in adults?

From the Guidelines

Diagnosing Type 1 diabetes mellitus (T1DM) in adults requires specific clinical and laboratory criteria, including the presence of hyperglycemia and evidence of autoimmune beta-cell destruction, with the most recent guidelines recommending the use of autoantibodies such as GAD, ICA, IAA, and ZnT8 antibodies for diagnosis, as stated in the 2025 guidelines 1. The diagnosis of T1DM in adults is based on several key factors, including:

• Hyperglycemia, as indicated by fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms of hyperglycemia, 2-hour plasma glucose ≥200 mg/dL during an oral glucose tolerance test, or HbA1c ≥6.5% (48 mmol/mol) 1.
• The presence of autoantibodies, such as glutamic acid decarboxylase (GAD) antibodies, islet cell antibodies (ICA), insulin autoantibodies (IAA), and zinc transporter 8 (ZnT8) antibodies, which distinguish T1DM from Type 2 diabetes in adults 1.
• Low or undetectable C-peptide levels, indicating insufficient endogenous insulin production, which supports a T1DM diagnosis 1.
• Other supportive findings, including absence of obesity, rapid weight loss, ketosis or diabetic ketoacidosis, and lack of response to oral antidiabetic medications 1.
• Genetic testing for HLA-DR3 and HLA-DR4 haplotypes may provide additional supporting evidence, but is not routinely required for diagnosis 1. It is essential to note that the diagnosis of T1DM can be challenging, and misdiagnosis can occur, particularly in adults, where the presentation may be more gradual, and the disease may be mistaken for Type 2 diabetes 1. The use of the AABBCC approach, which considers age, autoimmunity, body habitus, background, control, and comorbidities, can be a useful clinical tool for distinguishing diabetes type 1. In summary, the diagnosis of T1DM in adults requires a comprehensive approach, considering both clinical and laboratory criteria, with a focus on the presence of autoantibodies and evidence of autoimmune beta-cell destruction, as recommended by the most recent guidelines 1.

From the Research

Diagnostic Criteria for Type 1 Diabetes Mellitus (DM) in Adults

The diagnostic criteria for Type 1 Diabetes Mellitus (DM) in adults are not explicitly stated in the provided studies, but some studies provide information on the diagnosis of diabetes in general.

• The American Diabetes Association emphasizes fasting plasma glucose (FPG) levels, rather than the oral glucose tolerance test (OGTT), to diagnose diabetes mellitus, with a diagnostic cutoff for FPG of 126 mg/dL (7.0 mmol/L) 2.
• A 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) or more during an OGTT or a random plasma glucose level of 200 mg/dL (11.1 mmol/L) or more is also diagnostic of diabetes 2.
• Recent clinical guidance recommends routine islet-autoantibody testing when type 1 diabetes is clinically suspected or in the context of rapid progression to insulin therapy after a diagnosis of type 2 diabetes, as a positive islet-autoantibody test will usually confirm autoimmune aetiology (type 1 diabetes) in this setting 3.
• The use of glycated haemoglobin A1c (HbA1c) has been included as a diagnostic criterion for diabetes, and it has comparable performance to FPG and is better than OGTT in classifying subjects with diabetes, particularly when laboratory methods with smaller analytical variation are used 4.

Autoantibody Testing

• Screening for autoantibodies in type 1 diabetes has positive clinical consequences, including reduction of diabetic ketoacidosis events, improved glycemic control, and positive impact on short- and long-term complications 5.
• Islet-autoantibody testing of those with apparent type 2 diabetes should not be routinely undertaken, as in this low prior-prevalence setting, the positive predictive value of a single-positive islet antibody for autoimmune aetiology diabetes will be modest 3.

Oral Glucose Tolerance Test

• The International Diabetes Federation (IDF) has recently recommended determination of 1-hour plasma glucose (1-hPG) during an oral glucose tolerance test (OGTT) to diagnose intermediate hyperglycemia (IH) and type 2 diabetes (T2DM) 6.
• Individuals with IH and T2DM exhibited higher adiposity, blood pressure, uric acid, a worse lipid and inflammatory profile, and a progressive reduction in insulin sensitivity associated with impaired β cell function 6.