The oral glucose tolerance test (OGTT) measures how well the body processes glucose (sugar) over a defined period. It is the most sensitive test for diagnosing gestational diabetes and, in non-pregnant adults, is used to confirm prediabetes or type 2 diabetes when fasting glucose is borderline.
Many patients encounter the OGTT for the first time when they are pregnant and their routine blood glucose screening comes back borderline. Others are referred after a high fasting glucose or HbA1c. This guide explains what the test involves, how to prepare and how to interpret the results.
How the OGTT works
You fast for 8–10 hours before the test — typically from midnight. On arrival at the lab, a fasting blood glucose is drawn. You then drink a glucose solution (usually 75 g of glucose in 250–300 ml of water) within 5 minutes. Blood is drawn again at 1 hour and 2 hours.
The test takes approximately 2 hours, during which you should rest — physical activity affects glucose uptake and can produce misleading results. Some people feel nauseous from the glucose drink; this is normal and usually passes.
Interpreting results: non-pregnant adults
| Category | Fasting glucose | 2-hour glucose |
|---|---|---|
| Normal | Below 5.6 mmol/L (100 mg/dL) | Below 7.8 mmol/L (140 mg/dL) |
| Impaired fasting glucose (prediabetes) | 5.6–6.9 mmol/L | — |
| Impaired glucose tolerance (prediabetes) | — | 7.8–11.0 mmol/L |
| Type 2 diabetes | 7.0+ mmol/L | 11.1+ mmol/L |
Interpreting results: gestational diabetes
Different diagnostic thresholds are used for gestational diabetes, and there is some variation between countries and guidelines. The WHO and most Middle East guidelines use the following cut-offs for the 75 g OGTT in pregnancy:
| Timepoint | Threshold for gestational diabetes |
|---|---|
| Fasting | 5.1 mmol/L (92 mg/dL) |
| 1 hour | 10.0 mmol/L (180 mg/dL) |
| 2 hours | 8.5 mmol/L (153 mg/dL) |
A diagnosis of prediabetes or gestational diabetes: what next?
Prediabetes is reversible in many cases. Losing 5–7% of body weight, increasing moderate physical activity to 150 minutes per week and reducing refined carbohydrate intake can reduce progression to type 2 diabetes by more than 50%.
Gestational diabetes requires dietary management, close monitoring of blood glucose, and sometimes insulin or metformin during pregnancy. It usually resolves after delivery, but increases the lifetime risk of type 2 diabetes — so a follow-up OGTT at 6 weeks postpartum is recommended.
Who should have an OGTT?
- All pregnant women at 24–28 weeks (or earlier if high-risk)
- Adults with a borderline fasting glucose (5.6–6.9 mmol/L) or HbA1c
- Adults with risk factors — family history, obesity, PCOS, previous gestational diabetes
- Anyone with symptoms of diabetes — thirst, frequent urination, unexplained weight loss