What laboratory tests are indicated for diagnosing and monitoring a patient with suspected or known type 1 diabetes?

Laboratory Testing for Type 1 Diabetes

For diagnosing type 1 diabetes, measure fasting plasma glucose ≥126 mg/dL or HbA1c ≥6.5% (confirmed on repeat testing), and when phenotypic features overlap with type 2 diabetes, order a standardized islet autoantibody panel starting with GAD-65 antibodies, followed by IA-2, ZnT8, and IAA (if not yet on insulin). 1, 2, 3

Diagnostic Testing for Hyperglycemia

Initial Glucose-Based Diagnosis

• Fasting plasma glucose ≥7.0 mmol/L (≥126 mg/dL) measured in venous plasma establishes diabetes diagnosis 1
• Random plasma glucose ≥11.1 mmol/L (≥200 mg/dL) with classic symptoms (polyuria, polydipsia, weight loss) or hyperglycemic crisis confirms diabetes without need for repeat testing 1
• 2-hour post-load glucose ≥11.1 mmol/L (≥200 mg/dL) during 75g oral glucose tolerance test is diagnostic 1
• HbA1c ≥6.5% (≥48 mmol/mol) using NGSP-certified laboratory methods diagnoses diabetes 1

Confirmation Requirements

• Repeat the same test on a different day when a single abnormal result is obtained in asymptomatic patients 1
• Two different tests (e.g., fasting glucose and HbA1c) both above diagnostic thresholds from the same or different samples confirm diagnosis 1
• No repeat testing is required when unequivocal hyperglycemia (>200 mg/dL) occurs with classic symptoms or hyperglycemic crisis 1

Critical Sample Handling

• Plasma glucose samples must be centrifuged and separated immediately after collection to prevent preanalytic variability from glycolysis at room temperature 1

Autoantibody Testing to Classify Diabetes Type

When to Order Autoantibody Testing

Order standardized islet autoantibody panels when adults present with phenotypic features that overlap between type 1 and type 2 diabetes, specifically: 2, 4, 3

• Age <35 years at diagnosis
• Unintentional weight loss despite diabetes diagnosis
• Ketoacidosis or ketosis presentation
• Short time (<3 years) to insulin requirement
• Lean body habitus (BMI <25 kg/m²)

The Four-Antibody Panel

Start with GAD-65 antibodies as the first-line test because it is positive in 70-80% of type 1 diabetes cases 2, 3, 5

If GAD-65 is negative but clinical suspicion remains, proceed to: 2, 3

• IA-2 antibodies (positive in 50-60% of type 1 diabetes)
• ZnT8 antibodies (positive in ~50% of type 1 diabetes, increases diagnostic sensitivity when added to GAD and IA-2)
• Insulin autoantibodies (IAA) only if the patient has not yet started insulin therapy, as exogenous insulin renders this test unreliable 2, 3

Interpreting Autoantibody Results

• Two or more positive autoantibodies confirm autoimmune type 1 diabetes with 70% risk of insulin dependence within 10 years 2, 4
• Single positive autoantibody carries lower predictive value (15% risk within 10 years) and may occur in 1-2% of healthy individuals 2, 4
• All antibodies negative does not exclude type 1 diabetes—5-10% of true type 1 diabetes patients are antibody-negative, especially in non-White populations 2, 4

Laboratory Quality Requirements

All autoantibody testing must be performed only in accredited laboratories with established quality-control programs and participation in proficiency testing 2, 3

C-Peptide Testing for Beta-Cell Function

When to Measure C-Peptide

• Perform C-peptide testing when the patient is already on insulin therapy and you need to assess residual beta-cell function 2, 4
• Obtain a random (non-fasting) sample within 5 hours of eating with concurrent glucose measurement 2, 4
• Avoid testing within 2 weeks of a hyperglycemic emergency (DKA) 4

Interpreting C-Peptide Levels

• <200 pmol/L (<0.6 ng/mL) indicates type 1 diabetes 2, 4
• 200-600 pmol/L (0.6-1.8 ng/mL) is indeterminate and may represent type 1 diabetes, MODY, or insulin-treated type 2 diabetes 2, 4
• >600 pmol/L (>1.8 ng/mL) suggests type 2 diabetes 2, 4

Monitoring Tests for Established Type 1 Diabetes

HbA1c Monitoring

• Measure HbA1c every 3 months until individualized glycemic targets are achieved, then at least every 6 months 1
• Target HbA1c <7% (<53 mmol/mol) for most nonpregnant adults with diabetes 1
• Use only NGSP-certified methods performed in CLIA-certified laboratories 1

Screening for Autoimmune Comorbidities

• Screen for celiac disease with tissue transglutaminase antibodies (tTG) and serum IgA levels in patients with type 1 diabetes 2
• Screen for thyroid disease with anti-thyroid antibodies and TSH at diagnosis and yearly after age 12 years, as 20% of children with type 1 diabetes have anti-thyroid antibodies at onset 5

Common Pitfalls to Avoid

• Do not delay insulin therapy in GAD-positive patients with preserved C-peptide—autoimmune destruction is inevitable and early insulin initiation preserves quality of life 2
• Do not assume negative antibodies exclude type 1 diabetes in young, lean patients with acute onset—treat based on clinical phenotype 2, 4
• Do not order IAA testing after any insulin exposure, including human insulin, as it produces false-positive results 2, 3
• Do not repeat autoantibody testing to monitor disease activity in established diabetes—it has no clinical role outside research 2, 4
• Do not use point-of-care HbA1c devices for diagnosis, only for monitoring 1