How is a glucose tolerance test (GTT) result interpreted in an adult patient with potential risk factors for diabetes mellitus?

Interpretation of Glucose Tolerance Test in Adults

A 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) after a 75g oral glucose load indicates diabetes, while values between 140-199 mg/dL (7.8-11.0 mmol/L) indicate prediabetes (impaired glucose tolerance), and values <140 mg/dL (<7.8 mmol/L) are normal. 1

Diagnostic Thresholds for the 75g OGTT

The interpretation is based on the 2-hour plasma glucose value after administering 75g of anhydrous glucose dissolved in water: 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, 2
• Diabetes: 2-hour glucose ≥200 mg/dL (11.1 mmol/L) 1, 3

The fasting value also provides diagnostic information: 1

• Normal fasting: <100 mg/dL (5.6 mmol/L) 1
• Impaired Fasting Glucose: 100-125 mg/dL (5.6-6.9 mmol/L) 1
• Diabetes: Fasting ≥126 mg/dL (7.0 mmol/L) 1

Confirmation Requirements

Any abnormal result requires confirmation on a separate day unless the patient presents with unequivocal hyperglycemia (random glucose ≥200 mg/dL with classic symptoms of polyuria, polydipsia, and unexplained weight loss). 1, 3

Confirmation can be achieved through: 2, 3

• Repeating the same test (another OGTT) on a different day
• Using a different diagnostic test: fasting plasma glucose or HbA1c
• Two abnormal results from different tests on the same day (e.g., abnormal OGTT plus abnormal fasting glucose or HbA1c) 3

This confirmation requirement exists because day-to-day variability in glucose measurements is substantial (12-15%), and results can be affected by recent physical activity, acute illness, or stress. 2

Proper Test Administration

For accurate interpretation, the test must be performed correctly: 1, 4

• Fasting requirement: No caloric intake for at least 8 hours before the test 1
• Carbohydrate loading: Patient should consume at least 150g of carbohydrates per day for the 3 days preceding the test 2
• Patient positioning: Patient should remain seated and not smoke throughout the test 1
• Glucose load: 75g of anhydrous glucose dissolved in water 1
• Sample timing: Blood drawn at baseline (fasting) and exactly 2 hours after glucose administration 1, 3
• Sample type: Venous plasma glucose (not whole blood or capillary samples) 1, 4

Clinical Context and Risk Stratification

Prediabetes is not a disease itself but a risk factor—approximately 10% of people with impaired glucose tolerance progress to diabetes annually. 2 The risk is continuous across the glycemic spectrum and becomes disproportionately higher at the upper end of the prediabetes range. 1, 2

For patients with confirmed prediabetes, annual screening is recommended to monitor progression. 2 Intensive lifestyle modification (targeting 5-10% weight loss and regular exercise) has been shown to be particularly effective in individuals with impaired glucose tolerance. 1, 2

Important Caveats

The OGTT is more sensitive than fasting glucose alone for detecting diabetes and prediabetes, but it is less reproducible and more cumbersome to perform. 1 This is why fasting plasma glucose is generally preferred for routine screening, though the OGTT remains valuable when: 1

• Fasting glucose is borderline or equivocal
• Clinical suspicion for diabetes remains high despite normal fasting glucose
• Evaluating patients with risk factors who have normal fasting values

The 1-hour glucose value during the OGTT is not used for diabetes diagnosis in non-pregnant adults, though values ≥180 mg/dL may provide additional prognostic information about cardiovascular risk. 5

A critical pitfall: The OGTT should not be used during acute illness, stress, or in conditions with increased red blood cell turnover, as these invalidate glucose measurements. 2, 3