What are the diagnostic criteria for new diabetes?

Diagnostic Criteria for New Diabetes

Diabetes is diagnosed when A1C ≥6.5%, fasting plasma glucose ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test, or random plasma glucose ≥200 mg/dL in a patient with classic hyperglycemic symptoms. 1

Four Diagnostic Pathways

Any one of the following establishes the diagnosis:

• A1C ≥6.5% (48 mmol/mol) performed in a laboratory using an NGSP-certified method standardized to the DCCT assay 1
• Fasting plasma glucose ≥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 a 75-g oral glucose tolerance test 1, 2
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia or hyperglycemic crisis, results must be confirmed by repeat testing. 1 This is critical because:

• The same test should be repeated on a different day, or a different test can be performed without delay using a new blood sample 1
• If two different tests (such as A1C and fasting glucose) are both above diagnostic thresholds, this confirms the diagnosis 1
• For example, if A1C is 7.0% and a repeat result is 6.8%, the diagnosis is confirmed 1
• Only patients with classic symptoms of hyperglycemia or hyperglycemic crisis can be diagnosed with a single test result 1

When NOT to Use A1C

Use only plasma glucose criteria in conditions with altered red blood cell turnover: 1

• Sickle cell disease 1
• Pregnancy (second and third trimesters) 1
• Hemodialysis 1
• Recent blood loss or transfusion 1
• Erythropoietin therapy 1
• Glucose-6-phosphate dehydrogenase deficiency 1
• Hemoglobin variants that interfere with A1C measurement 1

Important Caveats About Test Selection

The three tests do not identify the same individuals as diabetic. 1 This creates practical challenges:

• A1C has lower sensitivity at the 6.5% cut point compared to 2-hour plasma glucose 1
• The concordance between fasting glucose and 2-hour glucose is imperfect, as is concordance between A1C and either glucose-based test 1, 2
• Marked discrepancies between measured A1C and plasma glucose levels should prompt consideration that the A1C assay may not be reliable for that individual 1

Practical Advantages of Each Test

A1C offers greater convenience: 1

• No fasting required 1, 2
• Greater preanalytical stability 1, 2
• Fewer day-to-day perturbations during stress, changes in nutrition, or illness 1

Glucose testing has limitations: 1, 3

• Fasting glucose has 12-15% day-to-day variability 1, 3
• Poor preanalytical stability—plasma must be separated immediately or samples kept on ice to prevent glycolysis 1, 3, 2
• Requires fasting for 8 hours 1, 2

Special Population Considerations

For children and adolescents, the evidence base for A1C diagnostic criteria is limited to adult populations, making it unclear whether the same 6.5% cut point should be used 1

For acute-onset type 1 diabetes in individuals with symptoms of hyperglycemia, plasma glucose should be used instead of A1C 2