Oral Glucose Tolerance Test (OGTT)

How it's done

• Patient preparation: unrestricted diet with at least 150 g carbohydrate/day for 3 days beforehand, normal activity, then a 10-16 hour fast.
• Test performed in the morning (7-9 am), patient seated, ambulatory (not on bed rest, not acutely ill/hospitalized), no smoking during the test.
• Discontinue medications that affect glucose tolerance beforehand if possible (thiazides, corticosteroids, oral contraceptives, etc.).
• Fasting venous plasma glucose is drawn first.
• Glucose load: 75 g anhydrous glucose dissolved in water for non-pregnant adults (WHO standard); for children, 1.75 g/kg up to a 75 g max.
• Venous plasma glucose measured fasting and again at 2 hours post-load (standard diagnostic OGTT). For gestational diabetes, a 100 g, 3-hour test with measurements at fasting, 1, 2, and 3 hours is often used.

Diagnostic interpretation (ADA criteria, non-pregnant adults)

• Fasting: ≥126 mg/dL = diabetes
• 2-hour post-75g load:
  • <140 mg/dL = normal
  • 140-199 mg/dL = impaired glucose tolerance (prediabetes)
  • ≥200 mg/dL = diabetes

Limitations

• About 20% of OGTTs are "nondiagnostic" (only one abnormal sample).
• Poor reproducibility due to many confounders: posture, anxiety, caffeine, smoking, activity, illness, age, prior carbohydrate intake, medications, sample handling.
• Many guidelines recommend repeating on a separate day unless grossly abnormal.
• Should use NGSP-certified lab methods, not point-of-care glucometers, for diagnosis.

Gestational diabetes

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Goldman-Cecil Medicine, International Edition

Oral Glucose Tolerance Test (OGTT)

What it is

Standard protocol (non-pregnant adults)

• Preparation: unrestricted diet with at least 150 g carbohydrate/day and normal activity for 3 days beforehand; discontinue drugs known to affect glucose tolerance if possible.
• Fast: 10-16 hours overnight, ambulatory (not bedridden/hospitalized/acutely ill patients).
• Timing: test starts between 7-9 am; patient remains seated, no smoking.
• Glucose load: 75 g anhydrous glucose in water for adults (children: 1.75 g/kg up to 75 g max).
• Sampling: venous plasma glucose measured fasting and at 2 hours post-load.

Diagnostic criteria (ADA, any one of the following is diagnostic)

1. HbA1c ≥ 6.5% (≥48 mmol/mol)
2. FPG ≥ 126 mg/dL (≥7.0 mmol/L)
3. 2-hour plasma glucose ≥ 200 mg/dL (≥11.1 mmol/L) during OGTT
4. Random plasma glucose ≥ 200 mg/dL with classic hyperglycemic symptoms/crisis

• Normal: <140 mg/dL (<7.8 mmol/L)
• Impaired glucose tolerance (prediabetes): 140-199 mg/dL (7.8-11.0 mmol/L)
• Diabetes: ≥200 mg/dL (≥11.1 mmol/L)

Factors that can distort results (non-diabetic influences)

• Patient prep: fasting duration, prior carbohydrate intake, medications (thiazides, oral contraceptives, corticosteroids), trauma, intercurrent illness, age, activity level, weight
• During the test: posture, anxiety, caffeine, smoking, activity, time of day, sample handling
• Glucose administration: form (anhydrous vs. monohydrate) and quantity ingested (Box 47.4, Tietz Textbook of Laboratory Medicine)

Gestational diabetes variant

• Two-step approach: 1-hour, 50-g glucose challenge test first (screening); if the result exceeds threshold (≥135-140 mg/dL), proceed to a confirmatory 3-hour, 100-g OGTT.
• Diagnostic thresholds for the 100-g/3-hour OGTT (ACOG, per Creasy & Resnik's Maternal-Fetal Medicine, p. gestational diabetes chapter):

Other clinical uses