What are the recommended laboratory tests to diagnose Type 1 Diabetes Mellitus (T1DM)?

From the Guidelines

To diagnose type 1 diabetes, the most recommended laboratory tests include measuring islet autoantibodies, such as glutamic acid decarboxylase (GAD) antibodies, and assessing C-peptide levels to determine endogenous insulin production, as suggested by the most recent guidelines 1. When diagnosing type 1 diabetes, it is crucial to consider the clinical presentation and laboratory results together. The primary steps involve:

• Measuring islet autoantibodies, with GAD being the primary antibody measured, followed by islet tyrosine phosphatase 2 (IA-2) and/or zinc transporter 8 (ZnT8) if GAD is negative 1.
• Assessing C-peptide levels, which should be measured prior to insulin discontinuation to exclude severe insulin deficiency, and considering the circumstances of testing, such as the timing in relation to eating or insulin administration 1. Key laboratory tests for diagnosing diabetes in general include:
• Fasting blood glucose
• Random blood glucose with classic symptoms of hyperglycemia
• Hemoglobin A1C test
• Oral glucose tolerance test However, for specifically identifying type 1 diabetes, autoantibody testing and C-peptide measurement are critical, as they help distinguish it from type 2 diabetes, which is essential for proper treatment, given that type 1 diabetes requires immediate insulin therapy due to the autoimmune destruction of insulin-producing beta cells in the pancreas 1.

From the Research

Diagnostic Tests for Type 1 Diabetes

The following are some of the recommended labs to diagnose type 1 diabetes:

• Fasting Plasma Glucose (FPG) 2, 3
• Oral Glucose Tolerance Test (OGTT) 2, 3, 4, 5
• Glycated Hemoglobin (HbA1c) 2, 3
• 1-hour glucose test 4, 5, 6
• 2-hour glucose test 2, 3, 5, 6

Comparison of Diagnostic Tests

The studies compared the diagnostic accuracy of different tests, including:

• HbA1c vs FPG vs OGTT 2
• FPG vs 2h PG vs HbA1c 3
• 1-hour OGTT vs 2-hour OGTT 4, 5
• The results showed that HbA1c and OGTT had higher diagnostic sensitivity than FPG 2
• The 1-hour glucose test was found to be a good predictor of type 1 diabetes 4, 5

Clinical Applications

The studies suggested that:

• The OGTT is a useful tool for diagnosing prediabetes and type 2 diabetes 5
• The 1-hour glucose test can be used for diagnostic surveillance and prediction of type 1 diabetes 4, 5
• The 2-hour glucose test is still a valid diagnostic tool for diabetes 3
• HbA1c can be used as a screening tool for newly diagnosed diabetes and pre-diabetes 2