What are the diagnostic criteria for diabetes?

Diagnostic Criteria for Diabetes

Diabetes is diagnosed when any one of the following criteria is met and confirmed with repeat testing: A1C ≥6.5%, fasting plasma glucose (FPG) ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL during OGTT, or random plasma glucose ≥200 mg/dL with classic symptoms of hyperglycemia. 1

Primary Diagnostic Tests

The American Diabetes Association (ADA) recognizes four methods for diagnosing diabetes:

1. A1C ≥6.5% (48 mmol/mol)

   • Test must be performed in a laboratory using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay
   • Point-of-care A1C testing is not recommended for diagnostic purposes

2. Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L)

   • Fasting defined as no caloric intake for at least 8 hours

3. 2-hour Plasma Glucose (2-h PG) ≥200 mg/dL (11.1 mmol/L) during OGTT

   • Test performed using 75g anhydrous glucose dissolved in water
   • Administered as described by the World Health Organization (WHO)

4. Random Plasma Glucose ≥200 mg/dL (11.1 mmol/L)

   • Only applicable in patients with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis 1

Confirmation Requirements

• In the absence of unequivocal hyperglycemia (classic symptoms or hyperglycemic crisis), diagnosis requires two abnormal test results:

  • Either two abnormal results from the same test on different days
  • Or abnormal results from two different tests obtained at the same time 1

• For example, if A1C is 7.0% and a repeat result is 6.8%, the diagnosis of diabetes is confirmed 1

Special Considerations for A1C Testing

A1C testing should not be used for diagnosis in certain conditions where the relationship between A1C and glycemia is altered:

• Hemoglobinopathies and hemoglobin variants
• Pregnancy (second and third trimesters)
• Glucose-6-phosphate dehydrogenase deficiency
• HIV infection
• Hemodialysis
• Recent blood loss, transfusion, or erythropoietin therapy
• Iron-deficient anemia 1

In these situations, only plasma glucose criteria should be used for diagnosis.

Prediabetes Diagnostic Criteria

Prediabetes is diagnosed when test results fall within these ranges:

• A1C: 5.7-6.4% (39-47 mmol/mol)
• FPG: 100-125 mg/dL (5.6-6.9 mmol/L) - Impaired Fasting Glucose (IFG)
• 2-h PG during OGTT: 140-199 mg/dL (7.8-11.0 mmol/L) - Impaired Glucose Tolerance (IGT) 1

Practical Diagnostic Approach

1. Initial Testing:

   • A1C testing is convenient (no fasting required) and has less day-to-day variability
   • FPG requires proper fasting and prompt sample processing to avoid glycolysis
   • OGTT requires proper preparation (consumption of mixed diet with at least 150g carbohydrates for 3 days prior)

2. Discordant Results:

   • When results from different tests are discordant, repeat the test that showed an abnormal result
   • Consider the possibility of A1C assay interference if marked discrepancies exist between A1C and glucose measurements 1

3. Borderline Results:

   • For results near diagnostic thresholds, educate patients about hyperglycemic symptoms and repeat testing in 3-6 months 1

Common Pitfalls to Avoid

• Relying solely on A1C for diagnosis: While convenient, A1C may miss some cases of diabetes that would be detected by glucose-based criteria 2, 3

• Using non-standardized A1C assays: Only NGSP-certified methods should be used for diagnosis 1

• Failing to confirm abnormal results: Unless unequivocal hyperglycemia is present, diagnosis requires confirmation with repeat testing 1

• Ignoring preanalytical variables: Glucose samples must be processed promptly to prevent falsely low results due to glycolysis 1

• Using inappropriate diagnostic criteria in special populations: In pregnancy, hemoglobinopathies, and other conditions affecting red blood cell turnover, only glucose criteria should be used 1