How is oral glucose tolerance test (OGTT) performed in the diagnosis for gestational diabetes mellitus (GDM)?

Updated: Apr 29, 2020
  • Author: Thomas R Moore, MD; Chief Editor: George T Griffing, MD  more...
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Patients undergoing oral glucose tolerance testing for gestational diabetes should undertake carbohydrate loading for 3 days preceding the test (>150 g carbohydrates) and an overnight fast of 8–14 hours the night before. The patient should remain seated during the test, and should not smoke. Two or more glucose values, as listed in the table below, must be met or exceeded for the diagnosis of gestational diabetes.

Table 2. Plasma Glucose Criteria for Gestational Diabetes (Open Table in a new window)


100 g Glucose Load,

mg/dL (mmol/L)


95 (5.3)

1 hour

180 (10.0)

2 hours

155 (8.6)

3 hours

140 (7.8)

Patients with a single abnormal value on a 3-hour OGTT are likely to exhibit some degree of glucose intolerance. Left untreated, these patients are at higher risk for fetal macrosomia and neonatal morbidity. Consequently, patients with a single abnormal value should receive dietary and physical activity counseling. If the abnormal value on the OGTT was obtained before 26 weeks' gestation, a repeat OGTT should be performed approximately 4 weeks later.

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