What is the procedure and interpretation for a 2-hour glucose tolerance test (GTT)?

2-Hour Glucose Tolerance Test: Procedure and Interpretation

Procedure

The 2-hour oral glucose tolerance test (OGTT) requires an 8-hour overnight fast, followed by administration of 75 grams of anhydrous glucose dissolved in water, with blood glucose measurement at baseline (fasting) and 2 hours post-load. 1

Key Procedural Details:

• Timing: Perform the test in the morning after an 8-14 hour fast (no caloric intake) 1
• Glucose load: 75 grams of anhydrous glucose dissolved in 250-300 mL of water, consumed over 5 minutes 1
• Blood sampling: Draw venous plasma at fasting (0 hours) and at 120 minutes after starting glucose consumption 1
• Patient preparation: The patient should remain seated and avoid smoking during the test 1

Important Caveats:

• The OGTT has poor reproducibility and requires patient compliance with fasting requirements 1
• Do not use in conditions with increased red blood cell turnover (sickle cell disease, pregnancy second/third trimesters, recent blood loss/transfusion, hemodialysis, erythropoietin therapy) - use plasma glucose criteria only 1
• The test is considered the reference standard for diagnosing impaired glucose tolerance, which cannot be detected by fasting glucose alone 1

Interpretation

Diagnostic Thresholds for Diabetes:

A 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) is diagnostic for diabetes mellitus. 1

• Normal: 2-hour glucose <140 mg/dL (7.8 mmol/L) 1
• Impaired Glucose Tolerance (Prediabetes): 2-hour glucose 140-199 mg/dL (7.8-11.0 mmol/L) 1
• Diabetes: 2-hour glucose ≥200 mg/dL (11.1 mmol/L) 1

Confirmation Requirements:

• In the absence of unequivocal hyperglycemia (symptoms plus random glucose ≥200 mg/dL), abnormal results should be confirmed by repeat testing on a separate day 1
• Confirmation can use the same test repeated or a different diagnostic test (e.g., fasting glucose or A1C) 1

Clinical Significance

Prognostic Value:

The 2-hour glucose is a superior predictor of cardiovascular mortality and all-cause mortality compared to fasting glucose alone. 2

• The 2-hour glucose value adds significant predictive information beyond fasting glucose for cardiovascular disease mortality (P<0.001) 2
• The largest number of excess deaths occurs in subjects with impaired glucose tolerance but normal fasting glucose - a population that would be missed by fasting glucose screening alone 2
• Patients with impaired glucose tolerance have 10-15% prevalence in U.S. adults and significant risk of progression to diabetes 3

Gestational Diabetes Screening:

For pregnant women at 24-28 weeks gestation, different thresholds apply 1:

• One-step strategy (IADPSG): 75-gram OGTT with diagnosis if any value is met or 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 strategy: 100-gram OGTT with diagnosis requiring at least two abnormal values:

  • 2-hour: ≥155 mg/dL (8.6 mmol/L) 1

Management Implications:

Patients diagnosed with impaired glucose tolerance should receive intensive lifestyle counseling targeting 5-7% body weight loss and at least 150 minutes weekly of moderate physical activity. 3

• Lifestyle interventions are more effective than pharmacologic therapy (metformin or acarbose) for preventing progression to diabetes 3
• These patients require at least annual monitoring for diabetes development 1