What are the diagnostic criteria for type 1 diabetes?

How to Diagnose Type 1 Diabetes

In patients with classic symptoms of hyperglycemia (polyuria, polydipsia, weight loss), a single random plasma glucose ≥200 mg/dL (11.1 mmol/L) is sufficient to diagnose diabetes immediately, and you should start insulin therapy without delay. 1

Symptomatic Patients (Most Common Presentation)

For patients presenting with classic symptoms—polyuria, polydipsia, weight loss, polyphagia, fatigue, or blurred vision—the diagnosis is straightforward:

• Measure random plasma glucose immediately 1
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) confirms diabetes 1
• No repeat testing is required when classic symptoms are present 1
• Begin insulin therapy immediately—delays in treatment must be avoided as metabolic deterioration can be rapid 1

Critical pitfall: Approximately one-third of children present with diabetic ketoacidosis, so assess for DKA immediately in all newly diagnosed patients 1. The metabolic state can deteriorate within hours to days, making immediate diagnosis and treatment essential 1.

Asymptomatic or Unclear Presentations

When the diagnosis is less obvious, use a stepwise approach:

Step 1: Demonstrate Hyperglycemia (Any ONE of the following)

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) after 8 hours of no caloric intake 1, 2
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during 75-g OGTT (or 1.75 g/kg up to 75 g maximum in children) 1, 2
• HbA1c ≥6.5% (48 mmol/mol) using NGSP-certified laboratory method 1, 2

Confirmation requirement: Two abnormal test results on separate days are required unless there is unequivocal hyperglycemia with classic symptoms 1, 2. You can repeat the same test or use two different tests (e.g., FPG and HbA1c) 1.

Step 2: Confirm Autoimmune Beta-Cell Destruction

After demonstrating hyperglycemia, test for islet autoantibodies to confirm type 1 diabetes: 2, 3, 4

• Start with GAD (glutamic acid decarboxylase) antibodies as the primary test 2, 3, 4
• If GAD is negative, proceed to IA-2 (islet tyrosine phosphatase 2) and/or ZnT8 (zinc transporter 8) antibodies 2, 3, 4
• In insulin-naïve patients, also consider IAA (insulin autoantibodies) 4
• Presence of ≥2 autoantibodies strongly confirms type 1 diabetes 2, 3

Important: Autoantibody testing must be performed in an accredited laboratory with established quality control 3, 4. Be aware that 5-10% of adult-onset type 1 diabetes may be autoantibody negative, so clinical judgment remains essential 4.

Special Diagnostic Considerations

When NOT to Use HbA1c

Do not rely on HbA1c for diagnosis in these situations: 2, 3, 4

• Hemoglobinopathies (sickle cell disease, thalassemia)
• Conditions with increased red blood cell turnover
• Pregnancy (second and third trimesters)
• Recent blood loss or transfusion
• Hemodialysis
• Erythropoietin therapy

In these conditions, use plasma glucose criteria only 1.

Point-of-Care Testing Limitations

• Glucose meters are useful for screening but diagnosis must be confirmed with venous plasma glucose on a laboratory analyzer 1
• Point-of-care HbA1c assays should not be used for diagnosis unless FDA-cleared specifically for diagnostic purposes 2, 4

Distinguishing from Stress Hyperglycemia

In young children with acute illness, incidental hyperglycemia may represent "stress hyperglycemia" rather than new-onset diabetes 1. If autoantibodies are positive, the diagnosis is type 1 diabetes; if negative, consult pediatric endocrinology 1.

Staging System for Type 1 Diabetes

The American Diabetes Association recognizes three stages: 1, 2, 3

• Stage 1: ≥2 autoantibodies + normoglycemia (presymptomatic)
• Stage 2: ≥2 autoantibodies + dysglycemia (FPG 100-125 mg/dL or 2-h PG 140-199 mg/dL or HbA1c 5.7-6.4%) (presymptomatic)
• Stage 3: Symptomatic diabetes with overt hyperglycemia (clinical diagnosis)

Screening Recommendations

Routine screening of asymptomatic children with autoantibody panels is NOT recommended in general practice 1, 3. Screening is currently recommended only in research settings or for first-degree relatives of patients with type 1 diabetes 1, 2, 3.

Additional Testing After Diagnosis

Once type 1 diabetes is confirmed, screen for associated autoimmune conditions: 2

• Thyroid antibodies (antithyroid peroxidase and antithyroglobulin)
• Celiac disease screening (IgA tissue transglutaminase antibodies)

C-Peptide Testing

C-peptide is useful in insulin-treated patients to assess residual beta-cell function 3, 4, but has important limitations:

• Do NOT perform within 2 weeks of a hyperglycemic emergency as results will be misleading 3, 4
• Multiple positive autoantibodies indicate higher risk of progression to complete insulin dependence 3, 4