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-gram oral glucose tolerance test, hemoglobin A1C ≥6.5% (using a NGSP-certified laboratory method), or random plasma glucose ≥200 mg/dL in a patient with classic hyperglycemic symptoms. 1, 2
Primary Diagnostic Thresholds
The four accepted diagnostic pathways are:
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) during a 75-gram oral glucose tolerance test (OGTT), using anhydrous glucose dissolved in water 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, 3
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, 3
Confirmation Requirements
In the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnosis requires confirmation with repeat testing. 1, 2 The confirmation algorithm works as follows:
Two abnormal results from the same sample or two separate samples are required for diagnosis when the patient does not have classic hyperglycemic symptoms 1
If two different tests are both above diagnostic thresholds, the diagnosis is immediately confirmed without need for additional testing 1
If results are discordant between two different tests, repeat the test that exceeded the diagnostic threshold to confirm the diagnosis 1
No confirmation is needed when random plasma glucose ≥200 mg/dL occurs with classic hyperglycemic symptoms or hyperglycemic crisis - a single test suffices 1, 2
Prediabetes Categories
Prediabetes is defined by glucose levels above normal but below diabetic thresholds:
Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 1, 2
Impaired Glucose Tolerance (IGT): 2-hour PG 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT 1, 2
Critical Limitations of A1C Testing
A1C should not be used for diagnosis in conditions affecting red blood cell turnover; use only plasma glucose criteria in these situations. 1, 2 Specific contraindications include:
- Hemoglobinopathies and hemolytic anemias 1, 2
- Pregnancy (second and third trimesters) 2
- Recent blood loss or transfusion 1, 2
- Hemodialysis 1, 2
- Erythropoietin therapy 1, 2
- Glucose-6-phosphate dehydrogenase deficiency 2
- HIV infection 2
Marked discordance between measured A1C and plasma glucose levels should raise suspicion for hemoglobin variants interfering with the assay. 1, 2
Test Selection Considerations
FPG is the preferred test due to ease of administration, convenience, patient acceptability, and lower cost compared to OGTT. 4 However, important caveats exist:
- FPG has high diurnal variation as a disadvantage 2
- The concordance between FPG, 2-hour PG, and A1C is imperfect - they do not necessarily detect diabetes in the same individuals 1
- OGTT, while considered a first-line diagnostic test, has low reproducibility 2
Point-of-Care Testing
Point-of-care A1C assays may be used in CLIA-certified settings meeting quality standards, but caution is advised when using them for diagnosis rather than monitoring. 1 These settings must meet personnel requirements and participate in approved proficiency testing programs. 1
Common Pitfalls to Avoid
- Do not rely on A1C alone in populations with high prevalence of hemoglobinopathies - always have plasma glucose testing available 1, 2
- Do not use capillary blood glucose testing as a stand-alone diagnostic test - it is useful for education but performs poorly for diagnosis 4
- Ensure patients consume at least 150g of carbohydrates in the 3 days preceding OGTT to avoid falsely elevated glucose levels 2
- Remember that up to 40% of adults with new type 1 diabetes are misdiagnosed as having type 2 diabetes - consider additional testing if clinical presentation is atypical 1