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3-Hour Oral Glucose Tolerance Test (100-g OGTT)

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The 3-hour 100-gram oral glucose tolerance test (OGTT) is used to diagnose gestational diabetes mellitus (GDM). It is conducted after:

Diagnosis of GDM requires ≥ 2 values at or above the thresholds.

Fasting glucose ≥ 126 mg/dL (7.0 mmol/L) indicates overt diabetes.

Values shown use the Carpenter–Coustan thresholds, derived from plasma-corrected O’Sullivan glucose oxidase measurements.

Units Fasting 1 Hour 2 Hour 3 Hour
mg/dL 95 180 155 140
mmol/L 5.3 10 8.6 7.8
Causes of High Fasting Glucose
  • Insulin resistance (pregnancy, obesity, PCOS)
  • Preexisting type 2 diabetes
  • Gestational diabetes
  • Glucocorticoid therapy
  • Cushing syndrome
  • Increased nocturnal hepatic glucose output
  • Liver disease reducing insulin clearance
Causes of High 1-Hour Glucose
  • Impaired first-phase insulin response
  • Rapid gastric emptying
  • High carbohydrate intake night before
  • Elevated counterregulatory hormones (HPL, cortisol, GH)
  • Obesity-related insulin resistance
Causes of High 2-Hour Glucose
  • Impaired glucose tolerance (IGT)
  • Gestational diabetes
  • Reduced insulin sensitivity (late pregnancy)
  • Inadequate second-phase insulin secretion
  • Medications:
    • Glucocorticoids
    • Beta-agonists
Causes of High 3-Hour Glucose
  • Delayed or insufficient insulin response
  • Advanced GDM
  • Reduced insulin sensitivity
  • Placental hormone effects (HPL, estrogen, progesterone)
  • β-cell dysfunction
Causes of Low OGTT Values (Hypoglycemia)
  • Dumping syndrome (rapid gastric emptying)
  • Post-bariatric reactive hypoglycemia
  • Insulinoma (rare)
  • Overfasting
  • Strenuous exercise before testing
  • Glucose dose ingestion errors

References

  1. O’Sullivan JB, Mahan CM. Criteria for OGTT in pregnancy. Diabetes. 1964;13:278–85. PMID: 14166677
  2. Carpenter MW, Coustan DR. Criteria for screening GDM. Am J Obstet Gynecol. 1982;144:768–73. PMID: 7148898
  3. Sacks DA et al. Validity of conversion of O’Sullivan criteria. Am J Obstet Gynecol. 1989;161:638–41. PMID: 2782345
  4. NIH Consensus: Diagnosing GDM. NIH Consens State Sci Statements. 2013;29:1–31. PMID: 23748438
  5. ACOG Practice Bulletin: Gestational Diabetes Mellitus. Obstet Gynecol. 2013;122:406–16.