What are the diagnostic criteria for diabetes?

Diagnostic Criteria for Diabetes

Diabetes is diagnosed when any one of the following criteria is met: A1C ≥6.5%, fasting plasma glucose ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL during an oral glucose tolerance test, or random plasma glucose ≥200 mg/dL with classic symptoms of hyperglycemia. 1

Primary Diagnostic Criteria

• A1C ≥6.5% (48 mmol/mol): The test should be performed in a laboratory using a method that is NGSP-certified and standardized to the Diabetes Control and Complications Trial (DCCT) assay 2

• Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L): Fasting is defined as no caloric intake for at least 8 hours 2

• 2-hour Plasma Glucose ≥200 mg/dL (11.1 mmol/L) during an Oral Glucose Tolerance Test (OGTT): The test should be performed using a glucose load containing the equivalent of 75g anhydrous glucose dissolved in water 2

• Random Plasma Glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis 2

Confirmation Requirements

• In the absence of unequivocal hyperglycemia with acute metabolic decompensation, the diagnosis requires confirmation by repeat testing 2

• Confirmation can be done by:

  • Repeating the same test on a different day 2
  • Using a different test (e.g., if A1C was initially used, FPG can be used for confirmation) 2
  • Measuring two different tests in samples obtained on the same day 2

• If two different tests are both above the diagnostic threshold, this confirms the diagnosis 2

• If results from two different tests are discordant, the test with results above the diagnostic threshold should be repeated 2

• No confirmation is needed when a patient presents with classic symptoms of hyperglycemia and a random plasma glucose ≥200 mg/dL 2

Prediabetes Criteria

• A1C: 5.7-6.4% (39-47 mmol/mol) 1, 3

• Fasting Plasma Glucose: 100-125 mg/dL (5.6-6.9 mmol/L) - Impaired Fasting Glucose (IFG) 1, 3

• 2-hour Plasma Glucose: 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT - Impaired Glucose Tolerance (IGT) 1, 3

Special Considerations for A1C Testing

• A1C should not be used for diagnosis in conditions affecting red blood cell turnover, including: 2, 1

  • Hemoglobinopathies
  • Pregnancy (second and third trimesters)
  • Glucose-6-phosphate dehydrogenase deficiency
  • HIV treated with certain drugs
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy
  • Iron-deficient anemia

• In these conditions, only plasma glucose criteria should be used for diagnosis 2, 1

• Marked discordance between measured A1C and plasma glucose levels should raise the possibility of hemoglobin variants interfering with the assay 2

Test Advantages and Limitations

• A1C advantages: Does not require fasting, reflects longer-term glycemia, less day-to-day variability 4

• FPG limitations: Requires 8-hour fast, has high diurnal variation 1, 5

• OGTT limitations: Requires fasting, has low reproducibility, requires proper preparation with adequate carbohydrate intake (at least 150g daily) for 3 days preceding the test 1, 6

• Combined use of FPG and A1C may improve sensitivity for detecting diabetes compared to either test alone 7, 6

Ethnic Considerations

• A1C levels may vary with race/ethnicity independently of glycemia 2

• African Americans may have higher A1C levels than non-Hispanic whites with similar glucose levels 2

• Sensitivity of diagnostic criteria may vary between ethnic groups, with potentially lower sensitivity among Europeans compared to South Asians and Chinese populations 6