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, 2, 3

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 1
• Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L): Fasting is defined as no caloric intake for at least 8 hours 1
• 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 75 g anhydrous glucose dissolved in water 1
• 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 1, 2

Confirmation Requirements

In the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnosis requires:

• Two abnormal test results from the same sample or in two separate test samples 1, 3
• If using two different tests (such as A1C and FPG) and both are above diagnostic thresholds, the diagnosis is confirmed 1
• If results are discordant from two different tests, the test with results above the diagnostic threshold should be repeated 1
• No confirmation is needed when a patient presents with classic symptoms of hyperglycemia or hyperglycemic crisis with a random plasma glucose ≥200 mg/dL 1

Prediabetes Criteria

• A1C: 5.7-6.4% (39-47 mmol/mol) 2
• Fasting Plasma Glucose: 100-125 mg/dL (5.6-6.9 mmol/L) - Impaired Fasting Glucose (IFG) 2, 3
• 2-hour Plasma Glucose: 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT - Impaired Glucose Tolerance (IGT) 2, 3

Special Considerations for A1C Testing

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

• Hemoglobinopathies (such as sickle cell trait) 1
• Pregnancy (second and third trimesters) 1, 2
• Glucose-6-phosphate dehydrogenase deficiency 1, 2
• HIV treated with certain drugs 1
• Hemodialysis 1, 2
• Recent blood loss or transfusion 1, 2
• Erythropoietin therapy 1, 2
• Iron-deficient anemia 1

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

Test Selection and Interpretation

• A1C has advantages including greater convenience (fasting not required), greater preanalytical stability, and less day-to-day perturbations during stress and illness 1
• Point-of-care A1C assays are not recommended for diagnostic purposes due to potential variability and lack of mandated proficiency testing 1, 2
• A marked discrepancy between measured A1C and plasma glucose levels should raise the possibility of hemoglobin variants interfering with the assay 2, 3
• The concordance between FPG, 2-h PG, and A1C tests is imperfect; they do not necessarily detect diabetes in the same individuals 1, 3

Clinical Pearls

• In rapidly evolving diabetes, such as the development of type 1 diabetes in some children, A1C may not be significantly elevated despite frank diabetes 1
• Analyses of NHANES data indicate that A1C at the designated cut point identifies one-third fewer cases of undiagnosed diabetes than FPG, but the greater practicality of A1C may increase the number of diagnoses made 1
• When test results are near the margins of the diagnostic threshold, the healthcare professional should follow the patient closely and repeat the test in 3-6 months 1
• The diagnosis of diabetes should be made as early as possible to prevent complications related to chronic hyperglycemia 3