What are the diagnostic criteria for type 1 diabetes?

Diagnostic Criteria for Type 1 Diabetes

Type 1 diabetes is diagnosed when a patient has hyperglycemia meeting the criteria for 'diabetic type' on two separate occasions, or on a single occasion if accompanied by classic symptoms, HbA1c ≥6.5%, or diabetic retinopathy. 1

General Diagnostic Criteria for Diabetes

The following criteria can be used to diagnose diabetes (including type 1):

• Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L) - fasting defined as no caloric intake for at least 8 hours 1
• 2-hour plasma glucose (2-h PG) ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) 1
• HbA1c ≥6.5% (48 mmol/mol) - test should be performed in a laboratory using NGSP certified method 1
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia or hyperglycemic crisis 1

In the absence of unequivocal hyperglycemia, the first three criteria should be confirmed by repeat testing 1.

Specific Criteria for Type 1 Diabetes

Type 1 diabetes is distinguished from other types by:

• Presence of one or more islet autoantibodies 1, 2
• Primary autoantibody measured should be glutamic acid decarboxylase (GAD) 2
• If GAD negative, testing for islet tyrosine phosphatase 2 (IA-2) and/or zinc transporter 8 (ZnT8) antibodies should follow 2
• In individuals not treated with insulin, antibodies against insulin (IAA) may also be useful 2

Staging of Type 1 Diabetes

Type 1 diabetes develops in three distinct stages 1:

Stage 1

• Presence of two or more islet autoantibodies
• Normoglycemia (no impaired glucose tolerance or impaired fasting glucose)
• Presymptomatic 1

Stage 2

• Presence of two or more islet autoantibodies
• Dysglycemia: impaired fasting glucose (FPG 100-125 mg/dL) and/or impaired glucose tolerance (2-h PG 140-199 mg/dL)
• HbA1c 5.7-6.4% or ≥10% increase in HbA1c
• Presymptomatic 1

Stage 3

• Symptomatic disease with overt hyperglycemia
• Diabetes by standard criteria
• Autoantibodies may become absent at this stage 1

Clinical Presentation

Classic symptoms of type 1 diabetes include:

• Polyuria (excessive urination)
• Polydipsia (excessive thirst)
• Weight loss
• Polyphagia (excessive hunger)
• Fatigue
• Blurred vision 1

These symptoms typically occur for several days to a few weeks prior to diagnosis 1.

Special Considerations

In Children and Adolescents

• For OGTT in children, a glucose load of 1.75 g/kg (maximum 75 g) should be used 1
• Incidental hyperglycemia without classic symptoms does not necessarily indicate new-onset diabetes, especially in young children with acute illness who may experience "stress hyperglycemia" 1

Autoantibody Testing

• Islet autoantibody tests should be performed only in an accredited laboratory with established quality control 2
• Multiple positive autoantibodies indicate higher risk of progression to insulin dependence 2

C-peptide Testing

• Useful in insulin-treated patients to assess residual β-cell function
• Should not be performed within 2 weeks of a hyperglycemic emergency as results may be misleading 2, 3

Common Pitfalls to Avoid

• Do not rely solely on A1C for diagnosis in conditions with altered relationship between A1C and glycemia (hemoglobinopathies, anemia) 1, 2
• Point-of-care A1C assays should not be used for diagnosis unless FDA-cleared specifically for diagnostic purposes 1, 2
• Be aware that 5-10% of adult-onset type 1 diabetes may be autoantibody negative 2
• The metabolic state of untreated children with type 1 diabetes can deteriorate rapidly, so a definitive diagnosis should be made immediately to avoid delays in treatment 1
• Screening for type 1 diabetes in asymptomatic children with autoantibody panels is currently recommended only in research settings or for first-degree family members of a person with type 1 diabetes 1, 3