From the Guidelines
The primary lab tests used to diagnose suspected type 1 diabetes include fasting blood glucose, hemoglobin A1C, and autoantibody tests, with a fasting blood glucose level of 126 mg/dL (7.0 mmol/L) or higher, or a hemoglobin A1C level of 6.5% or higher, being diagnostic of diabetes, as recommended by the most recent guidelines 1. To diagnose suspected type 1 diabetes, several lab tests are utilized.
- Fasting blood glucose level: a level of 126 mg/dL (7.0 mmol/L) or higher is diagnostic of diabetes, as stated in the guidelines 1.
- Hemoglobin A1C test: a level of 6.5% or higher indicates diabetes, according to the guidelines 1.
- Autoantibody tests: these tests are crucial for confirming type 1 diabetes and include tests for islet cell antibodies (ICA), insulin autoantibodies (IAA), glutamic acid decarboxylase antibodies (GADA), and zinc transporter 8 antibodies (ZnT8A), as recommended by the guidelines 1. The presence of these autoantibodies indicates the autoimmune process characteristic of type 1 diabetes.
- C-peptide levels are also measured to assess the body's insulin production, with low levels suggesting type 1 diabetes. These tests help distinguish type 1 from type 2 diabetes, which is essential for determining appropriate treatment, as type 1 diabetes requires immediate insulin therapy due to the body's inability to produce sufficient insulin. The guidelines also recommend that glucose should be measured in venous plasma when used for screening of high-risk individuals, and plasma glucose should be measured in an accredited laboratory when used for diagnosis of or screening for diabetes 1. In addition, the guidelines suggest that blood for fasting plasma glucose analysis should be drawn in the morning after the subject has fasted overnight (at least 8 h), and a tube containing a rapidly effective glycolytic inhibitor should be used for collecting the sample to minimize glycolysis 1.
From the Research
Lab Tests for Suspected Type 1 Diabetes
The following lab tests are used to diagnose suspected type 1 diabetes:
- Fasting plasma glucose (FPG) 2, 3, 4, 5, 6
- Oral glucose tolerance test (OGTT) 2, 3, 4, 5, 6
- Glycated haemoglobin A1c (HbA1c) 2, 4, 5, 6
Characteristics of Each Test
- FPG: measures fasting plasma glucose levels, with thresholds of ≥7.0mmol/l for diabetes diagnosis 4
- OGTT: measures plasma glucose levels after an oral glucose load, with thresholds of ≥11.1mmol/l for diabetes diagnosis at 2 hours 3
- HbA1c: measures average blood glucose levels over the past 2-3 months, with thresholds of ≥48mmol/mol (≥6.5%) for diabetes diagnosis 4
Comparison of Tests
- HbA1c has comparable performance to FPG and is better than OGTT in classifying subjects with diabetes, particularly when laboratory methods with smaller analytical variation are used 2
- The use of the average of the results of repeat laboratory tests can improve the consistency of disease classification 2
- FPG and HbA1c performed equally in terms of diagnosing new diabetes cases in individuals with tuberculosis, but HbA1c may be a more reliable test in individuals with transient hyperglycaemia 4
- OGTT is more sensitive than FPG and HbA1c in detecting impaired glucose tolerance and type 2 diabetes in patients with acute coronary syndromes 6