What are the diagnostic criteria for Type 2 Diabetes Mellitus (T2DM)?

Diagnostic Criteria for Type 2 Diabetes Mellitus

The diagnosis of Type 2 Diabetes Mellitus (T2DM) is established when any one of the following criteria is met: fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test, HbA1c ≥6.5%, or random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia. 1

Primary Diagnostic Criteria

• Fasting Plasma Glucose (FPG): ≥126 mg/dL (7.0 mmol/L), with fasting defined as no caloric intake for at least 8 hours 2, 1
• 2-hour Plasma Glucose (2-h PG): ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test (OGTT) performed according to World Health Organization standards 2, 1
• Hemoglobin A1C: ≥6.5% (48 mmol/mol), 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 2, 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 2, 1

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
• If using two different tests (e.g., FPG and HbA1c) and both are above diagnostic thresholds, the diagnosis is confirmed 1
• If results from two different tests are discordant, 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 and a random plasma glucose ≥200 mg/dL (11.1 mmol/L) 2, 1

Prediabetes Criteria

• Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 1
• Impaired Glucose Tolerance (IGT): 2-hour PG 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT 1
• HbA1c: 5.7-6.4% (39-46 mmol/mol) 1

Special Considerations

• To minimize glycolysis in blood samples for glucose testing, the sample tube should be placed immediately in an ice-water slurry, and plasma should be separated from cells within 30 minutes 2
• If immediate processing is not possible, use a tube containing a rapidly effective glycolysis inhibitor, such as citrate buffer 2
• HbA1c should not be used for diagnosis in conditions affecting red blood cell turnover, including hemoglobinopathies, hemolytic anemias, pregnancy, recent blood loss or transfusion, hemodialysis, and erythropoietin therapy 1
• Point-of-care A1C assays may be used for monitoring but are not recommended for diagnosis 1

Distinguishing Between Type 1 and Type 2 Diabetes

• T2DM accounts for 90-95% of all diabetes cases 1, 3
• When clinical presentation is unclear, islet autoantibody testing can help differentiate between T1DM and T2DM 4
• C-peptide measurement can assess endogenous insulin production capacity, with lower levels typically indicating T1DM and higher levels suggesting T2DM 4

Screening Recommendations

• The American Diabetes Association recommends screening for adults aged 45 years and older 1
• Earlier screening is recommended for overweight or obese adults (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) with one or more additional risk factors 1
• Women with prior gestational diabetes should be screened regularly 1

Clinical Pearls

• The diagnosis of T2DM should be made as early as possible to prevent complications related to chronic hyperglycemia 1
• Even undiagnosed patients are at increased risk of developing macrovascular and microvascular complications 1
• The concordance between FPG, 2-h PG, and HbA1c tests is imperfect; they do not necessarily detect diabetes in the same individuals 1
• Misdiagnosis of diabetes type can occur in up to 40% of adults with new type 1 diabetes who are initially misdiagnosed as having type 2 diabetes 1