What are the diagnostic criteria for diabetes?

Diagnostic Criteria for Diabetes

Diabetes mellitus is diagnosed when any one of the following criteria is met on two separate occasions: 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 symptoms of hyperglycemia. 1

Diagnostic Tests and Criteria

Confirmed Diagnostic Criteria

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

1. Hemoglobin A1C ≥ 6.5%

   • Must be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay
   • Not recommended using point-of-care A1C assays for diagnosis

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 ≥ 200 mg/dL (11.1 mmol/L) during OGTT

   • Test performed using 75g anhydrous glucose dissolved in water

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

   • Only applicable in patients with classic symptoms of hyperglycemia or hyperglycemic crisis 2

Confirmation Requirements

• In the absence of unequivocal hyperglycemia, results should be confirmed by repeat testing 2
• Diagnosis requires either:
  • Two abnormal results from the same test on different days, or
  • Abnormal results from two different tests obtained at the same time 1

Special Considerations

When A1C Testing Should Not Be Used

A1C testing should be avoided for diagnosis in conditions where the relationship between A1C and glycemia is altered:

• Hemoglobinopathies
• Pregnancy
• Recent blood loss or transfusion
• Erythropoietin therapy
• Hemolysis
• Iron-deficient anemia
• Hemodialysis

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

Discordant Test Results

• If two different tests are available and results are discordant (one above diagnostic threshold, one below), the test whose result is above the diagnostic cut point should be repeated
• The diagnosis is made based on the confirmed test 2
• When marked discrepancies exist between A1C and glucose measurements, consider the possibility of A1C assay interference 1

Prediabetes Criteria

Prediabetes is diagnosed when test results fall within these ranges:

• A1C: 5.7-6.4%
• FPG: 100-125 mg/dL (Impaired Fasting Glucose)
• 2-h PG during OGTT: 140-199 mg/dL (Impaired Glucose Tolerance) 1, 2

Screening Recommendations

• Testing should be considered in all adults who are overweight (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) with additional risk factors:
  • Physical inactivity
  • First-degree relative with diabetes
  • High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
  • History of gestational diabetes or delivery of baby >9 lbs
  • Hypertension (≥140/90 mmHg or on therapy)
  • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
  • Polycystic ovary syndrome
  • History of cardiovascular disease 2
• Testing should begin at age 45 years for all individuals, particularly those who are overweight or obese
• If results are normal, testing should be repeated at minimum 3-year intervals 2

Common Pitfalls to Avoid

1. Failure to confirm abnormal results with repeat testing unless unequivocal hyperglycemia is present 1
2. Improper sample handling - Glucose samples must be processed promptly to prevent falsely low results due to glycolysis 1
3. Using A1C for diagnosis in contraindicated conditions - In hemoglobinopathies, pregnancy, or conditions with altered red cell turnover, use only glucose-based criteria 2
4. Missing borderline cases - Patients with test results near diagnostic thresholds should be followed closely with repeat testing in 3-6 months 2, 1
5. Relying solely on FPG - Using only FPG criteria may lead to underdiagnosis, as some patients may have normal fasting but abnormal post-load glucose levels 1

By following these diagnostic criteria and being aware of special considerations, clinicians can accurately diagnose diabetes and initiate appropriate management to reduce morbidity and mortality associated with the disease.