What are the diagnostic criteria for Type 2 Diabetes in adults?

Diagnostic Criteria for Type 2 Diabetes

Type 2 diabetes is diagnosed when any one of the following four criteria is met: fasting plasma glucose ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test, hemoglobin A1C ≥6.5%, or random plasma glucose ≥200 mg/dL in a patient with classic hyperglycemic symptoms. 1

Primary Diagnostic Thresholds

The American Diabetes Association establishes four distinct pathways to diagnose type 2 diabetes in adults: 2, 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): Measured during a 75-g oral glucose tolerance test (OGTT) performed according to World Health Organization standards 1
• Hemoglobin A1C ≥6.5%: 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 1
• Random Plasma Glucose ≥200 mg/dL (11.1 mmol/L): Only valid when accompanied by classic symptoms of hyperglycemia (polydipsia, polyuria, unexplained weight loss) or hyperglycemic crisis 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnosis requires confirmation with repeat testing. 1 The confirmation process follows these rules: 1

• If the same test is repeated on a separate day and both results exceed the diagnostic threshold, diabetes is confirmed
• If two different tests (e.g., A1C and FPG) both exceed their respective diagnostic thresholds from the same or different samples, diabetes is confirmed
• If results from two different tests are discordant, repeat the test that exceeded the diagnostic threshold
• No confirmation is needed when a patient presents with classic hyperglycemic symptoms or hyperglycemic crisis and random plasma glucose ≥200 mg/dL 1

Test Selection in Clinical Practice

Fasting plasma glucose is the preferred screening and diagnostic test due to ease of administration, convenience, patient acceptability, lower cost, and greater reproducibility compared to OGTT. 1, 2 While A1C and 2-hour OGTT are equally acceptable alternatives, FPG offers practical advantages in routine clinical settings. 2, 1

The concordance between these three tests is imperfect—they do not necessarily detect diabetes in the same individuals. 1 This is clinically important because some patients may meet criteria on one test but not another.

Critical Limitations of A1C Testing

A1C should not be used for diagnosis in conditions affecting red blood cell turnover. 1 Specifically avoid A1C in: 2, 1

• Hemoglobinopathies (sickle cell disease, thalassemia)
• Hemolytic anemias
• Pregnancy (second and third trimesters)
• Recent blood loss or transfusion
• Hemodialysis
• Erythropoietin therapy

In these conditions, use only plasma glucose criteria (FPG or OGTT). 2, 1 Marked discrepancies between measured A1C and plasma glucose levels should prompt consideration that hemoglobin variants may be interfering with the assay. 2, 1

Prediabetes Criteria

Prediabetes is defined by any of the following: 2, 1

• Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 2
• Impaired Glucose Tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during 75-g OGTT 2
• A1C 5.7-6.4% (39-47 mmol/mol) 2

Individuals with A1C between 6.0-6.5% have a 5-year risk of developing diabetes between 25-50%, with a relative risk 20 times higher compared to A1C of 5.0%. 2 Patients with prediabetes should be tested annually. 2

Common Pitfalls to Avoid

Do not use point-of-care capillary blood glucose testing as a stand-alone diagnostic test—it performs poorly for diagnosis. 1 While point-of-care A1C may be used in CLIA-certified settings meeting quality standards, caution is advised when using it for diagnosis rather than monitoring. 1

Do not assume normal A1C excludes diabetes. 2 Among patients with newly diagnosed type 2 diabetes in the low diabetic range (FPG 126-140 mg/dL), approximately 60% have normal A1C levels, indicating A1C is less sensitive for detecting lower levels of hyperglycemia. 2

Remember that type 2 diabetes accounts for 90-95% of all diabetes cases, 2, 1 but misdiagnosis can occur—up to 40% of adults with new type 1 diabetes are initially misdiagnosed as having type 2 diabetes. 1 Consider additional testing if clinical presentation is atypical.