Glucose Tolerance Test Cut-offs for Diagnosing Diabetes and Impaired Glucose Tolerance
The diagnostic cut-offs for diabetes using the oral glucose tolerance test (OGTT) are: fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) or 2-hour post-load glucose ≥200 mg/dL (11.1 mmol/L), while impaired glucose tolerance is defined as 2-hour post-load glucose between 140-199 mg/dL (7.8-11.0 mmol/L). 1
Diagnostic Criteria for Diabetes Mellitus
Diabetes Diagnosis
The American Diabetes Association (ADA) defines diabetes using any of these criteria:
- Fasting Plasma Glucose (FPG): ≥126 mg/dL (7.0 mmol/L)
- 2-hour Plasma Glucose during OGTT: ≥200 mg/dL (11.1 mmol/L)
- A1C: ≥6.5% (48 mmol/mol)
- Random Plasma Glucose: ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia 1
In the absence of unequivocal hyperglycemia, diagnosis requires two abnormal test results from the same sample or from two separate testing occasions 1.
Impaired Glucose Tolerance (Prediabetes)
Prediabetes is categorized as:
- Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L)
- Impaired Glucose Tolerance (IGT): 2-hour PG during 75-g OGTT 140-199 mg/dL (7.8-11.0 mmol/L)
- A1C: 5.7-6.4% (39-47 mmol/mol) 1
OGTT Administration Protocol
The OGTT should be performed as follows:
- Morning test after 8-14 hour overnight fast
- 75g anhydrous glucose dissolved in water (250-300 mL)
- Blood samples taken at baseline (fasting) and 2 hours after glucose consumption
- Patient should consume a mixed diet with at least 150g carbohydrates for 3 days prior to testing 1
Special Considerations
Different Blood Sample Types
When using samples other than venous plasma, different cut-offs apply:
| Diagnosis | Venous plasma | Venous blood | Capillary blood |
|---|---|---|---|
| IFG | 6.1 (110) | 5.0 (90) | 5.6 (101) |
| IGT | 7.8 (140) | 6.5 (117) | 7.2 (130) |
| Diabetes | 7.0 (126) | 5.8 (104) | 6.5 (117) |
| Diabetes (2h) | 11.1 (200) | 9.4 (169) | 10.3 (185) |
Values shown as mmol/L (mg/dL) 1
Gestational Diabetes Mellitus (GDM)
For diagnosing GDM, two approaches exist:
One-step approach (IADPSG) - 75g OGTT with diagnosis made when any of these values are exceeded:
- Fasting: ≥92 mg/dL (5.1 mmol/L)
- 1 hour: ≥180 mg/dL (10.0 mmol/L)
- 2 hour: ≥153 mg/dL (8.5 mmol/L) 1
Two-step approach:
- 50g glucose load (non-fasting) with 1-hour threshold of ≥140 mg/dL (7.8 mmol/L)
- If positive, proceed to 100g OGTT with diagnosis made when at least two values are met:
| Time | Carpenter/Coustan | NDDG |
|---|---|---|
| Fasting | 95 mg/dL (5.3 mmol/L) | 105 mg/dL (5.8 mmol/L) |
| 1 hour | 180 mg/dL (10.0 mmol/L) | 190 mg/dL (10.6 mmol/L) |
| 2 hour | 155 mg/dL (8.6 mmol/L) | 165 mg/dL (9.2 mmol/L) |
| 3 hour | 140 mg/dL (7.8 mmol/L) | 145 mg/dL (8.0 mmol/L) |
Clinical Implications
- Risk for diabetes is continuous, extending below the lower limit of prediabetes ranges and becoming disproportionately greater at higher ends 1
- Individuals with IFG or IGT have significantly increased risk of developing diabetes (10-15% of US adults have one of these conditions) 2
- Patients with prediabetes should be counseled on lifestyle modification with 5-7% weight loss and at least 150 minutes of physical activity per week 2
Common Pitfalls
- Improper fasting: Ensure true 8-14 hour fast before testing
- Inadequate carbohydrate intake: Patients should consume adequate carbohydrates (≥150g) for 3 days before testing
- Sample processing delays: Glycolysis can falsely lower glucose values if samples aren't processed promptly
- Misinterpreting single values: Confirmation testing is required in the absence of classic symptoms
- Ignoring interfering factors: Recent illness, medications, physical activity, and stress can affect results 1, 3
When diagnosing diabetes or prediabetes, it's crucial to consider the patient's overall clinical picture and confirm abnormal results with repeat testing to ensure accurate diagnosis and appropriate management.