The 1-hour 50-gram glucose challenge test (GCT) is an initial screening test for gestational diabetes. Blood glucose is measured exactly 1 hour after ingestion of a 50-g glucose load. Fasting is not required.
| Units | Fasting | 1 Hour |
|---|---|---|
| mg/dL | Not required | 130 or 140 |
| mmol/L | Not required | 7.2 or 7.8 |
Interpretation
A value ≥ 140 mg/dL (7.8 mmol/L) identifies approximately 80% of gestational diabetes cases.
A value ≥ 130 mg/dL (7.2 mmol/L) identifies approximately 90% of cases.
California Diabetes & Pregnancy Program (Sweet Success)
- 140–179 mg/dL → Perform 3-hour 100-g OGTT within 1 week.
- ≥180 mg/dL → Check a fasting glucose immediately.
- If fasting ≥95 mg/dL AND 1-hour ≥180 mg/dL → Two abnormal values → diagnose GDM (3-hr OGTT not required).
When a GCT is not needed
- Fasting glucose ≥126 mg/dL (7.0 mmol/L)
- Random glucose ≥200 mg/dL (11.1 mmol/L)
Causes of high 1-hour GCT values
- Gestational diabetes mellitus (GDM)
- Insulin resistance (obesity, PCOS, family history)
- High-glycemic meal before testing
- Rapid absorption of oral glucose
- Reduced maternal insulin secretion
- Cushing syndrome
- Medications:
- Systemic steroids
- Beta-agonists (e.g., terbutaline)
Causes of low 1-hour GCT values
- Reactive hypoglycemia
- High insulin sensitivity
- Strenuous exercise before testing
- Malabsorption / post-bariatric surgery
- Delayed gastric emptying
- Low carbohydrate intake before test
References
- California Diabetes and Pregnancy Program (Sweet Success). Guidelines for Care. 2002.
- American Diabetes Association. Diagnosis and classification of diabetes mellitus. Diabetes Care. 2009;32(Suppl 1):S62-S67. PMID: 19118289.