What are the diagnostic criteria for type 1 vs type 2 diabetes?

Diagnostic Criteria for Type 1 vs Type 2 Diabetes

The key distinction between type 1 and type 2 diabetes is that type 1 is primarily an autoimmune disease characterized by beta-cell destruction leading to absolute insulin deficiency, while type 2 results from progressive insulin secretion defects on the background of insulin resistance. 1

Diagnostic Criteria for Both Types

Both types of diabetes are diagnosed using the same glycemic criteria:

• A1C ≥6.5% (using NGSP-certified method standardized to DCCT assay)
• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) after at least 8 hours fasting
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during 75g OGTT
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia 1, 2

Unless there are unequivocal symptoms with hyperglycemia or hyperglycemic crisis, diagnosis requires two abnormal test results from the same sample or in two separate test samples.

Differentiating Type 1 from Type 2 Diabetes

Type 1 Diabetes Characteristics

• Autoimmunity: Presence of islet autoantibodies is the hallmark of type 1 diabetes 1, 3

  • Primary antibody to test: Glutamic acid decarboxylase (GAD)
  • Additional antibodies if GAD negative: Islet antigen 2 (IA-2) and zinc transporter 8 (ZnT8)
  • Insulin autoantibodies (if not already on insulin therapy)
  • 5-10% of type 1 diabetes patients may be antibody-negative 1

• C-peptide levels: Values <200 pmol/L (<0.6 ng/mL) strongly suggest type 1 diabetes 1, 2

• Clinical presentation:

  • Often presents with acute symptoms (polyuria, polydipsia, weight loss)
  • Approximately one-third of children present with diabetic ketoacidosis (DKA)
  • Adult-onset may have more variable presentation 1

• Risk factors:

  • Family history of type 1 diabetes or autoimmune diseases
  • Personal history of other autoimmune conditions 2

Type 2 Diabetes Characteristics

• Insulin resistance: Often associated with obesity and metabolic syndrome
• C-peptide levels: Values >600 pmol/L (>1.8 ng/mL) suggest type 2 diabetes 1, 2
• Clinical presentation:
  • Often more gradual onset of symptoms
  • May be diagnosed incidentally during routine screening
  • Rarely presents with DKA (though more common in certain ethnic minorities) 1
• Risk factors:
  • Obesity (BMI ≥25 kg/m²)
  • Family history of type 2 diabetes
  • Older age at onset (though can occur at any age) 1

AABBCC Approach for Differentiation

A useful clinical tool for distinguishing diabetes type is the AABBCC approach 1:

• Age: Consider type 1 diabetes in individuals <35 years old
• Autoimmunity: Personal or family history of autoimmune disease
• Body habitus: BMI <25 kg/m² suggests type 1 diabetes
• Background: Family history of type 1 diabetes
• Control: Inability to achieve glycemic goals on non-insulin therapies
• Comorbidities: Presence of other autoimmune conditions

Important Considerations

• The traditional paradigm that type 1 diabetes occurs only in children and type 2 only in adults is no longer accurate - both can occur at any age 1

• Obesity does not rule out type 1 diabetes 2

• In cases of diagnostic uncertainty:

  • Test for islet autoantibodies, starting with GAD
  • Measure C-peptide (with concurrent glucose) after at least 3 years of diabetes duration
  • Consider features like age of onset, BMI, and response to oral medications 1, 2

• A1C may be normal at diagnosis in type 1 diabetes, particularly when detected through screening 4

• Marked discordance between A1C and plasma glucose levels should raise suspicion of hemoglobinopathies or other conditions affecting A1C reliability 1

Pitfalls to Avoid

• Relying solely on age or BMI for classification - 95% of patients overlap in both parameters regardless of antibody status 5

• Assuming all patients with DKA have type 1 diabetes - some type 2 diabetes patients, particularly ethnic minorities, can present with DKA 1

• Missing type 1 diabetes in adults due to more gradual presentation than in children

• Using point-of-care A1C tests for diagnostic purposes despite NGSP certification 2

• Failing to consider other forms of diabetes (MODY, pancreatic diabetes, etc.) when presentation is atypical

By systematically evaluating autoantibody status, C-peptide levels, and clinical characteristics, clinicians can more accurately differentiate between type 1 and type 2 diabetes, leading to appropriate treatment strategies and improved outcomes.