Diagnostic Criteria for Diabetes

Diagnostic Criteria

• The American Diabetes Association recommends diagnosing diabetes using 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.
• The American Diabetes Association suggests that A1C tests should be performed in a laboratory using a method certified by NGSP and standardized to the DCCT assay, with a threshold of ≥ 6.5% (≥ 48 mmol/mol) for diagnosis 1, 2.
• Fasting plasma glucose ≥ 126 mg/dL (≥ 7.0 mmol/L) is a diagnostic criterion, with fasting defined as no caloric intake for at least 8 hours 1, 3.
• A 2-hour plasma glucose ≥ 200 mg/dL (≥ 11.1 mmol/L) during an oral glucose tolerance test is a diagnostic criterion, using a glucose load containing the equivalent of 75 g of anhydrous glucose dissolved in water 1, 4.
• 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 is a diagnostic criterion 1, 5.

Prediabetes

• The American Diabetes Association defines prediabetes as A1C: 5.7-6.4% (39-47 mmol/mol) 6.
• Fasting plasma glucose: 100-125 mg/dL (5.6-6.9 mmol/L) is considered impaired fasting glucose (IFG) 3.
• 2-hour plasma glucose during an oral glucose tolerance test: 140-199 mg/dL (7.8-11.0 mmol/L) is considered impaired glucose tolerance (IGT) 6.

Special Considerations

• The A1C test should not be used in conditions that affect red blood cell turnover, such as hemoglobinopathies, pregnancy (second and third trimesters), glucose-6-phosphate dehydrogenase deficiency, HIV, hemodialysis, recent blood loss or transfusion, or erythropoietin therapy 1, 4, 7, 8.
• Fasting plasma glucose has disadvantages, including requiring an 8-hour fast and having high diurnal variation 6.
• The oral glucose tolerance test is considered a first-line diagnostic test but has limitations, including low reproducibility and requiring an 8-hour fast and a 75g glucose load 5, 6.

Classification of Diabetes

• Diabetes is classified into several clinical categories, including type 1 diabetes, type 2 diabetes, gestational diabetes mellitus, and specific types of diabetes due to other causes 3, 4.

Practical Considerations

• A1C tests should be performed using a method certified by NGSP and standardized to the DCCT assay 8, 9.
• A marked discrepancy between measured A1C and plasma glucose levels should raise the possibility of interference in the A1C assay due to hemoglobin variants 8.
• In conditions associated with an altered relationship between A1C and glucose, only plasma glucose criteria should be used to diagnose diabetes 8, 7.
• Individuals should consume a mixed diet with at least 150g of carbohydrates in the 3 days preceding an oral glucose tolerance test to avoid falsely elevated glucose levels 6.

Diabetes Diagnostic and Prediabetes Criteria (American Diabetes Association)

Diagnostic Thresholds for Diabetes

• The ADA defines diabetes when any one of the following is present:

  • HbA1c ≥ 6.5 % (≥ 48 mmol/mol) measured in an NGSP‑certified laboratory standardized to the DCCT assay; this reflects chronic glucose exposure over the prior 2–3 months. [10][11]
  • Fasting Plasma Glucose ≥ 126 mg/dL (≥ 7.0 mmol/L) after at least 8 hours of fasting; this cut‑point aligns with a 2‑hour OGTT value of 200 mg/dL and shows equal prevalence of diabetic retinopathy. [12][10]11
  • 2‑hour Oral Glucose Tolerance Test ≥ 200 mg/dL (≥ 11.1 mmol/L) following a 75‑g glucose load; this level marks the point where retinopathy prevalence rises sharply. [12][10]11
  • Random Plasma Glucose ≥ 200 mg/dL (≥ 11.1 mmol/L) in individuals with classic hyperglycemic symptoms (polyuria, polydipsia, unexplained weight loss) or in a hyperglycemic crisis. [10][11]

• These thresholds were derived from epidemiologic analyses showing a marked increase in microvascular complications—particularly diabetic retinopathy—once glucose values exceed the listed cut‑points. [12][10]

Confirmation of Diagnosis

• When hyperglycemia is not unequivocal (i.e., no hyperglycemic crisis with clear symptoms), two abnormal test results are required to confirm diabetes. The two results may be:
  
  • The same test performed on separate days, or
  • Two different tests (e.g., HbA1c and fasting plasma glucose) obtained from the same blood sample. [10][11]

Prediabetes (Intermediate Hyperglycemia)

• The ADA classifies prediabetes using the following lower thresholds:

  • HbA1c 5.7–6.4 % (39–47 mmol/mol). [13][10]11
  • Fasting Plasma Glucose 100–125 mg/dL (5.6–6.9 mmol/L), termed Impaired Fasting Glucose (IFG). [13][10]11
  • 2‑hour OGTT 140–199 mg/dL (7.8–11.0 mmol/L), termed Impaired Glucose Tolerance (IGT). [12][13]10

• The risk of diabetic complications increases continuously across these ranges, becoming disproportionately higher toward the upper limits. [13][10]

Test Discordance and Ethnic Considerations

• The three diagnostic tests (FPG, 2‑hour OGTT, HbA1c) assess different aspects of glucose metabolism and do not identify identical populations; each test captures a partially overlapping group of individuals. [10][11]
• The 2‑hour OGTT identifies more individuals with diabetes than either FPG or HbA1c when applied to the same cohort. [10][11]
• HbA1c values can vary by race/ethnicity independent of actual glycemia; for example, African‑American individuals may exhibit higher HbA1c levels than non‑Hispanic White individuals at comparable glucose concentrations. 14