What laboratory tests are used to diagnose type 1 diabetes?

Laboratory Tests for Diagnosing Type 1 Diabetes

The diagnosis of type 1 diabetes requires specific laboratory tests including measurement of plasma glucose, pancreatic autoantibodies, and assessment of beta-cell function through C-peptide levels. 1

Core Diagnostic Tests for Type 1 Diabetes

Plasma Glucose Measurements

• Plasma blood glucose rather than HbA1c should be used to diagnose the acute onset of type 1 diabetes in individuals with symptoms of hyperglycemia 1
• Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L), with fasting defined as no caloric intake for at least 8 hours 1, 2
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) using 75g of anhydrous glucose 1, 2
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in patients with classic symptoms of hyperglycemia or hyperglycemic crisis 1, 2

Autoantibody Testing

• Measurement of pancreatic autoantibodies is essential for confirming the autoimmune nature of type 1 diabetes 1, 2
• Key autoantibodies to test include:
  • Islet cell autoantibodies (ICA) 1
  • Glutamic acid decarboxylase autoantibodies (GAD65) 1
  • Insulin autoantibodies 1
  • Tyrosine phosphatase autoantibodies (IA-2 and IA-2β) 1
  • Zinc transporter 8 autoantibodies (ZnT8) 1
• The presence of one or more of these autoimmune markers defines type 1 diabetes 1

Beta-Cell Function Assessment

• C-peptide levels should be measured to assess beta-cell function 2
• Low or undetectable C-peptide levels are characteristic of type 1 diabetes 2

Confirmation and Special Considerations

• In the absence of unequivocal hyperglycemia, diagnosis requires two abnormal test results from the same sample or in two separate test samples 1
• If using two separate test samples, the second test should be performed without delay 1
• Approximately one-third of type 1 diabetes patients present with diabetic ketoacidosis (DKA) at diagnosis 1, 2
• Testing for ketones in blood or urine should be performed when type 1 diabetes is suspected, especially in the presence of hyperglycemia 1

Screening for Type 1 Diabetes Risk

• Screening for type 1 diabetes risk with a panel of autoantibodies is currently recommended only in research settings or for first-degree family members of individuals with type 1 diabetes 1
• Persistence of two or more autoantibodies predicts clinical diabetes and may serve as an indication for intervention in clinical trial settings 1

Limitations and Considerations

• HbA1c testing alone is not recommended for diagnosing type 1 diabetes, particularly in its acute onset 1
• In conditions affecting red blood cell turnover (sickle cell disease, pregnancy, hemodialysis, recent blood loss or transfusion, erythropoietin therapy), only plasma glucose criteria should be used 1, 2
• The onset of type 1 diabetes may be more variable in adults, who may not present with the classic symptoms seen in children 1

Diagnostic Algorithm for Type 1 Diabetes

1. Measure plasma glucose (fasting, random, or OGTT) in patients with symptoms suggestive of diabetes 1
2. If glucose criteria for diabetes are met, test for autoantibodies to confirm type 1 diabetes 1, 2
3. Measure C-peptide levels to assess beta-cell function 2
4. Test for ketones in blood or urine, particularly if hyperglycemia is present 1
5. Confirm diagnosis with repeat testing if initial presentation is not unequivocal hyperglycemia or DKA 1