Diagnostic Criteria for Type 2 Diabetes
Type 2 diabetes is diagnosed when any one of the following 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, 2
Primary Diagnostic Thresholds
Any single criterion below establishes the diagnosis:
Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L): Requires no caloric intake for at least 8 hours prior to testing 1, 2, 3
2-Hour Plasma Glucose ≥200 mg/dL (11.1 mmol/L): Measured during a 75-g oral glucose tolerance test (OGTT) performed according to WHO standards 1, 2, 3
Hemoglobin A1C ≥6.5% (48 mmol/mol): 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, 2, 4
Random Plasma Glucose ≥200 mg/dL (11.1 mmol/L): Only diagnostic when accompanied by classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis 1, 2, 5
Confirmation Requirements
Two abnormal test results are required for diagnosis in asymptomatic patients or those without acute metabolic decompensation. 2, 5
If using the same test twice, obtain results from two separate samples on different days 2, 5
If using two different tests (e.g., FPG and A1C) and both exceed diagnostic thresholds, the diagnosis is immediately confirmed 2, 5
If two different tests yield discordant results (one positive, one negative), repeat the test that was above the diagnostic threshold 2, 5
No confirmation testing is needed when random plasma glucose ≥200 mg/dL occurs with classic hyperglycemic symptoms or hyperglycemic crisis 2, 5
Critical Limitations of A1C Testing
A1C should not be used for diagnosis in the following conditions that affect red blood cell turnover 2, 4, 5:
- Hemoglobinopathies (sickle cell disease, thalassemia) 2, 4
- Hemolytic anemias 2, 4
- Pregnancy (second and third trimesters) 4, 5
- Recent blood loss or transfusion 2, 4
- Hemodialysis 2, 4
- Erythropoietin therapy 2, 4
- Glucose-6-phosphate dehydrogenase deficiency 4
In these situations, use only plasma glucose criteria (FPG or OGTT) for diagnosis. 2, 4, 5
Prediabetes Criteria
Prediabetes identifies individuals at high risk for progression to diabetes and cardiovascular disease 1:
Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 1, 4, 5
Impaired Glucose Tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT 1, 4, 5
Special Testing Considerations
OGTT Preparation
Consume at least 150 g of carbohydrates daily for 3 days prior to testing to avoid falsely elevated glucose levels 1, 4, 5
Point-of-Care A1C Testing
While convenient for monitoring glycemic control, exercise caution when using point-of-care A1C for diagnosis; laboratory-based testing is preferred 2
Test Concordance
The concordance between FPG, 2-hour PG, and A1C is imperfect—these tests do not necessarily identify diabetes in the same individuals 2, 5. Over 30% of people with diabetes have a nondiabetic fasting glucose but abnormal 2-hour glucose values 6
Clinical Context
Type 2 diabetes accounts for 90-95% of all diabetes cases and affects approximately 1 in 6 adults in the United States 1, 2, 3. The disease frequently goes undiagnosed for years because hyperglycemia develops gradually 1. Even undiagnosed patients are at increased risk for macrovascular and microvascular complications, making early detection critical. 1, 2
Common Pitfalls to Avoid
Do not rely on symptoms alone: Type 2 diabetes is often asymptomatic in early stages, and patients may not notice classic symptoms until hyperglycemia is severe 1
Do not use A1C in conditions affecting red blood cell turnover: This leads to inaccurate results and potential misdiagnosis 2, 4
Do not skip confirmation testing in asymptomatic patients: A single abnormal test (except random glucose with symptoms) requires confirmation 2, 5
Do not assume all diabetes is type 2: Up to 40% of adults with new type 1 diabetes are initially misdiagnosed as having type 2 diabetes 2, 5