The oral glucose tolerance test (OGTT) is one of the most important investigations used for diagnosing diabetes mellitus and prediabetes. If you are about to take this test, it is essential to know how to prepare correctly to obtain accurate results.
In this article we explain clearly and simply everything you need to know about OGTT: when it is recommended, how you prepare, how it is performed, and how to interpret the results.
What is OGTT
OGTT (oral glucose tolerance test) is an analysis that measures how the body processes glucose (sugar) after ingestion of a standard amount.
The test helps diagnose:
- diabetes mellitus
- prediabetes (impaired glucose tolerance)
- gestational diabetes (during pregnancy)
When is OGTT recommended
Your doctor may recommend this test in the following situations:
- slightly elevated blood glucose
- suspected diabetes
- risk factors (obesity, family history)
- metabolic syndrome
- pregnancy (screening between weeks 24–28)
Preparation for OGTT (very important)
For the result to be correct, preparation is essential.
3 days before the test:
- maintain normal eating
- consume at least 150g carbohydrates/day
- avoid low-carb or keto diets
8–12 hours before:
- DO NOT eat (fasting)
- you can drink water
On the morning of the test:
- DO NOT consume coffee
- DO NOT smoke
- DO NOT do intense physical activity
Other recommendations:
- avoid stress
- inform your doctor if you are taking medications (some may influence the result)
🧪 How OGTT is performed
The test is usually performed in the morning and lasts approximately 2 hours.
The steps are:
- Blood glucose is collected on an empty stomach (baseline)
- You will drink a solution with 75g anhydrous glucose dissolved in a glass of cold water within a maximum of 5 minutes.
- Blood glucose is collected at 1 hour and/or 2 hours
During the test:
- you must rest, stay calm
- you are not allowed to eat
- you are not allowed to do physical activity
OGTT values (interpretation outside pregnancy)
At 2 hours after glucose administration:
- Below 140 mg/dl → normal
- 140–199 mg/dl → prediabetes (impaired glucose tolerance)
- ≥ 200 mg/dl → diabetes mellitus
Correct interpretation must be made by your doctor, in clinical context.
Common mistakes before OGTT
These mistakes can lead to falsely altered results:
- low-carbohydrate diet before the test
- coffee consumption in the morning
- smoking before the test
- intense physical activity
- stress or lack of sleep
- certain medications
OGTT during pregnancy
OGTT is mandatory in screening for gestational diabetes.
It is usually performed between weeks 24 and 28 of pregnancy.
The interpretation values are different compared to the general population, and early diagnosis is essential for the health of the mother and fetus.
OGTT values (interpretation during pregnancy)
Before glucose administration, on an empty stomach:
- Below 92 mg/dl → normal
- ≥ 92→ gestational diabetes
At 1 hour after glucose administration:
- Below 180 → normal
- ≥ 180 mg/dl → gestational diabetes
At 2 hours after glucose administration:
- Below 153 mg/dl → normal
- ≥ 153 mg/dl → gestational diabetes
What you do after OGTT
After completing the test:
- you can eat normally
- it is recommended to have a snack with you
- discuss the results with your diabetologist
If the values are abnormal, it may be necessary:
- blood glucose monitoring
- lifestyle modification
- completion of additional tests
- specific treatment
❓ Frequently Asked Questions (FAQ)
Are you allowed to drink water before OGTT?
Yes, water is allowed.
Are you allowed coffee before OGTT?
No, coffee can influence the results.
How long does OGTT take?
Approximately 2 hours.
Is OGTT dangerous?
No, it is a safe test, but it may cause mild nausea in some people. Therefore, it is recommended that the glucose solution be prepared with cold water.
Can you take OGTT if you have a cold?
Preferably not, as infections can influence the results.
🩺 When you should see a doctor
Consult a diabetologist if:
- you have abnormal values
- you have suggestive symptoms (excessive thirst, frequent urination, fatigue)
- you have risk factors for diabetes
📞 Schedule your evaluation
If you have taken OGTT or are about to take this test, correct interpretation is essential.
👉 Schedule a diabetology consultation and a personalized plan for prevention or treatment.
