The OGTT — a blood test done over 2 to 3 hours after drinking a measured glucose solution. It reveals how your body processes sugar dynamically, making it the gold standard for diagnosing gestational diabetes and detecting hidden glucose abnormalities.
Written & Reviewed by
MBBS (Col) · MD (Col) · SLMC No. 27329 · Medical Director, Sineth Hospitals
🩺 Medically Reviewed
📅 Updated: April 2026
🕐 6 min read
🔬 Overview
What is the OGTT?
The Oral Glucose Tolerance Test (OGTT) measures how effectively your body processes glucose over time. After fasting overnight, you drink a measured glucose solution. Blood sugar levels are then checked at specific intervals to see how well your body clears the sugar load from your bloodstream.
Unlike a single fasting or random blood sugar test — which captures glucose at one moment — the OGTT shows how your body responds dynamically to a glucose challenge. This makes it particularly valuable in borderline or special clinical situations where simpler tests may miss disease.
Fast overnight (8–12 hours)
Fasting blood sample taken
Drink 75g glucose solution within 5 minutes
Blood taken at 1 hour (pregnancy) and 2 hours
Why OGTT is different from other blood sugar testsA fasting blood sugar (FBS) only tells you your glucose level before eating. A PPBS tells you how you responded to one specific meal. The OGTT uses a standardised glucose load — 75 grams — so results can be compared against validated diagnostic thresholds, making it the most accurate test for detecting early or hidden glucose abnormalities.
⭐ Why It Matters
Why is OGTT Important — and When is it the Gold Standard?
OGTT is considered the gold standard diagnostic test in specific situations where simpler tests may miss disease. In these situations, fasting glucose alone can appear normal while OGTT reveals hidden abnormalities after the glucose load.
Primary Diagnostic Uses
Diagnose gestational diabetes (GDM) — most important use in Sri Lanka
Detect prediabetes (impaired glucose tolerance)
Diagnose diabetes mellitus in borderline or unclear cases
Evaluate high-risk individuals with normal fasting glucose
Special Clinical Situations
Your doctor may also request OGTT in the following situations:
Features of insulin resistance (e.g., PCOS)
Unexplained early neuropathy or recurrent infections possibly related to glucose
Borderline FBS (100–125 mg/dL) or borderline HbA1c
Complications suggestive of diabetes but with inconclusive routine tests
When fasting sugar looks normal but disease is presentPost-meal glucose abnormality is often the earliest defect in developing diabetes. A patient may have a perfectly normal fasting blood sugar but show a significantly elevated glucose level 2 hours after the glucose load. These early cases are caught by OGTT but missed entirely by FBS alone.
📋 When It Is Recommended
When Should You Do an OGTT?
General Population
OGTT is recommended for non-pregnant adults in the following situations:
- 1Fasting blood sugar in the borderline range: 100–125 mg/dL
- 2HbA1c in the borderline range — not clearly normal, not yet diagnostic for diabetes
- 3Symptoms suggestive of diabetes (thirst, frequent urination, fatigue) but routine labs remain unclear
Pregnancy — Gestational Diabetes Screening (Sri Lanka)
In Sri Lanka, OGTT is widely used as the main screening and diagnostic test for gestational diabetes (GDM). Sri Lankan practice commonly follows universal screening — meaning all pregnant women are tested — rather than selective screening, due to the high prevalence of diabetes in the population.
All pregnant women: tested at 24–28 weeks
Earlier testing (first antenatal visit) if high-risk
High-risk factors that warrant earlier testing include:
Previous gestational diabetes in a prior pregnancy
Previous baby weighing ≥ 3.5–4 kg at birth
Obesity (BMI ≥ 30)
Strong family history of diabetes
What Counts as a "Significant" Family History?
Not all diabetes in relatives carries the same level of risk. A strong family history relevant to your risk includes:
- 🔴First-degree relatives — parents or siblings — with diabetes, especially if diagnosed before age 50, if multiple family members are affected, or if associated with obesity or metabolic syndrome
A single elderly relative is less significantA grandparent or distantly related relative diagnosed with diabetes late in life (after age 60) represents a lower level of inherited risk compared to a parent or sibling diagnosed early, or multiple relatives across generations.
🧪 How the Test Works
How is the OGTT Done?
The standard OGTT uses a 75g glucose load and is the same for both adults and pregnant women. The procedure is straightforward — the key is following each step correctly to ensure an accurate result.
- 1Overnight fast (8–12 hours) — No food. Plain water is allowed. Arrive at the laboratory in the morning.
- 2Fasting blood sample — Blood is drawn before you drink anything. This is your baseline fasting glucose level.
- 3Drink the glucose solution — 75 grams of glucose dissolved in 250–300 mL of water. You must finish it within 5 minutes. Drinking it too slowly affects the accuracy of the test.
- 41-hour blood sample (pregnancy only) — Blood is taken exactly 1 hour after finishing the drink. This extra timepoint is used in pregnancy for GDM diagnosis.
- 52-hour blood sample — Blood is taken at exactly 2 hours after finishing the drink. This is the main diagnostic reading for non-pregnant adults.
About the glucose drinkThe glucose solution contains exactly 75 grams of glucose in 250–300 mL of water. It is sweet and should be consumed in one sitting — within 5 minutes. If you drink it too slowly or incompletely, the glucose absorption pattern changes and the test becomes unreliable.
Can You Leave the Laboratory During the Test?
No — you must stay seated in the laboratory throughout
There are three important reasons:
• Physical activity — even walking to the car — alters your blood glucose level and produces false results
• The timing of blood samples must be precise; leaving makes accurate timing impossible
• If you experience nausea or vomiting after the drink, the laboratory team needs to be present to manage this and advise you
There are three important reasons:
• Physical activity — even walking to the car — alters your blood glucose level and produces false results
• The timing of blood samples must be precise; leaving makes accurate timing impossible
• If you experience nausea or vomiting after the drink, the laboratory team needs to be present to manage this and advise you
What if You Vomit the Glucose Drink?
If you vomit after drinking the glucose solution, the test is invalid and must be repeated on another day — with full preparation done again from scratch, including the 3-day normal diet and the overnight fast. The laboratory team will advise you on when to reschedule.
📝 How to Prepare
How to Prepare for the OGTT
Correct preparation is essential. Poor preparation is one of the most common causes of inaccurate or invalid OGTT results.
3 Days Before the Test
Eat your normal diet — at least 150g of carbohydrates per day
Do not follow a low-carb diet, fast, or restrict food intake
Why normal carbohydrate intake mattersIf you have been eating a low-carbohydrate diet in the days before the test, your body becomes less adapted to processing a sudden glucose load. This can produce a falsely elevated 2-hour result, leading to a misdiagnosis of prediabetes or diabetes.
8–12 Hours Before the Test (Overnight)
No food at all after your last meal
Plain water is allowed
No smoking and no alcohol
No tea, coffee, or any other drinks
Medications
Certain medications affect blood glucose levels and can interfere with the test. Common examples include:
Steroids (corticosteroids)
Diuretics (water tablets)
Beta-blockers
Some anti-psychotic medications
Best approach for medications
• Continue all essential medications (for blood pressure, heart disease, epilepsy, etc.) — do not stop them on your own
• If you are taking medications that affect blood sugar and they cannot be safely stopped, the test may need to be postponed or the results interpreted with caution
• Inform the laboratory and your doctor in advance — bring a written list of all your medications on the day of the test
• Continue all essential medications (for blood pressure, heart disease, epilepsy, etc.) — do not stop them on your own
• If you are taking medications that affect blood sugar and they cannot be safely stopped, the test may need to be postponed or the results interpreted with caution
• Inform the laboratory and your doctor in advance — bring a written list of all your medications on the day of the test
On the Day of the Test
- 🌅Come to the laboratory in the morning — this makes it easier to complete the test and manage the fasting period
- 🏃Avoid all physical exercise before and during the test — even a short walk affects your blood glucose
- 🪑Remain seated and relaxed throughout — rest quietly between blood draws
- 🚭Do not smoke, eat, or drink anything other than plain water during the test
📊 Reference Values
OGTT Reference Values & Interpretation
Non-Pregnant Adults — 2-Hour Value
| Category | 2-Hour Glucose (mg/dL) |
|---|---|
| Normal | < 140 mg/dL |
| Prediabetes (Impaired Glucose Tolerance) | 140 – 199 mg/dL |
| Diabetes Mellitus | ≥ 200 mg/dL |
Pregnancy — 75g OGTT for Gestational Diabetes (GDM)
In pregnancy, gestational diabetes is diagnosed if any one of the following values is met or exceeded:
| Timepoint | GDM Threshold (mg/dL) |
|---|---|
| Fasting (before glucose drink) | ≥ 92 mg/dL |
| 1 Hour after glucose drink | ≥ 180 mg/dL |
| 2 Hours after glucose drink | ≥ 153 mg/dL |
Any one value is enough for a GDM diagnosisUnlike the non-pregnant adult interpretation — where the 2-hour value alone determines the category — in pregnancy, meeting or exceeding any single threshold (fasting, 1-hour, or 2-hour) is sufficient to diagnose gestational diabetes. This is why all three timepoints are checked.
Your doctor interprets results in the full clinical contextThese are diagnostic thresholds, not absolute verdicts. Your doctor will consider your full clinical picture — medications, symptoms, other investigations, and risk factors — before making a diagnosis or recommending treatment. Do not adjust treatment based on numbers alone without consulting your doctor.
❌ Common Mistakes
Common Mistakes to Avoid
-
Not fasting properly — Eating or drinking anything other than water within 8–12 hours of the test invalidates the fasting sample. Schedule the test early in the morning so the overnight fast is easier to manage.
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Eating a low-carbohydrate diet in the days before — Restricting carbohydrates before the test produces a falsely abnormal result. Eat your normal everyday diet for at least 3 days beforehand, including at least 150g of carbohydrates per day.
-
Walking or exercising during the test — Physical activity, even mild, lowers blood glucose and produces false results. Remain seated throughout the test period.
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Drinking the glucose solution too slowly — The 75g glucose drink must be finished within 5 minutes. Drinking it slowly changes the rate of glucose absorption and affects accuracy.
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Smoking, drinking tea or coffee before or during the test — These affect blood glucose levels and must be completely avoided on the morning of the test.
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Not informing the lab about medications — Some drugs significantly affect glucose levels. If the laboratory and doctor are not informed in advance, the test may need to be repeated. Always bring a medication list.
🧪 Key Takeaways — OGTT Test
- OGTT measures how your body processes a 75g glucose load over 2–3 hours — it is a dynamic test, unlike a single fasting or random blood sugar
- It is the gold standard for diagnosing gestational diabetes, detecting prediabetes, and evaluating high-risk individuals with normal fasting glucose
- In Sri Lanka, OGTT is used for universal pregnancy screening at 24–28 weeks — or earlier for high-risk women
- For GDM diagnosis in pregnancy: any one value meeting the threshold (fasting ≥92, 1h ≥180, 2h ≥153 mg/dL) is diagnostic
- For non-pregnant adults: 2-hour value <140 is normal; 140–199 is prediabetes; ≥200 is diabetes
- Eat a normal diet (150g carbs/day) for 3 days before the test — do not fast or restrict food intake beforehand
- Drink the glucose solution within 5 minutes; remain seated and do not leave the laboratory during the test
- Inform your doctor and laboratory about all medications in advance
❓ FAQs
Frequently Asked Questions
The OGTT itself is not painful. The only discomfort is mild, from the blood sampling — typically 2 to 3 needle pricks over the course of the test. The glucose drink is sweet and most people tolerate it without difficulty, though some may feel mild nausea — especially during pregnancy.
Yes, small amounts of plain water are allowed both during the fasting period and while waiting between blood draws. However, avoid all other drinks — tea, coffee, juice, milk, or soft drinks — as these affect blood glucose levels and will invalidate the test.
No. You must remain seated in the laboratory throughout the entire test. Even mild physical activity — such as walking to the car or around the building — lowers blood glucose and produces false results. Leaving also makes precise timing of blood samples impossible, which further compromises accuracy.
If you vomit after drinking the glucose solution, the test becomes invalid. It must be repeated on another day — with full preparation done again from the beginning, including 3 days of normal diet and an overnight fast. Inform the laboratory team immediately if you feel nauseous, as they may be able to help manage symptoms before they progress to vomiting.
Yes — in specific situations. OGTT is superior for:
• Detecting early or borderline diabetes that HbA1c may miss
• Diagnosing gestational diabetes — HbA1c is not recommended for GDM screening in most guidelines
• Identifying impaired glucose tolerance (prediabetes) — where post-load glucose is abnormal but fasting and HbA1c appear normal
HbA1c reflects average glucose over 3 months and is convenient for monitoring, but it is less sensitive for early detection.
• Detecting early or borderline diabetes that HbA1c may miss
• Diagnosing gestational diabetes — HbA1c is not recommended for GDM screening in most guidelines
• Identifying impaired glucose tolerance (prediabetes) — where post-load glucose is abnormal but fasting and HbA1c appear normal
HbA1c reflects average glucose over 3 months and is convenient for monitoring, but it is less sensitive for early detection.
The OGTT takes approximately 2 to 3 hours from start to finish — including the fasting blood draw, drinking the glucose solution, waiting, and having the final blood sample taken. In pregnancy, where a 1-hour sample is also taken, plan for the full 3-hour window. Come prepared with something to read or listen to and expect to remain at the laboratory throughout.
Do not stop your medications on your own. The practical approach is:
• Continue all essential medications — blood pressure, heart disease, thyroid, anti-epileptics, etc.
• For medications that affect blood sugar (steroids, diuretics, beta-blockers): your doctor may need to adjust the timing of the test or interpret results with this in mind — do not stop them without medical advice
• Inform the laboratory in advance and bring a written list of all your medications on the day of the test so the team can plan accordingly
• Continue all essential medications — blood pressure, heart disease, thyroid, anti-epileptics, etc.
• For medications that affect blood sugar (steroids, diuretics, beta-blockers): your doctor may need to adjust the timing of the test or interpret results with this in mind — do not stop them without medical advice
• Inform the laboratory in advance and bring a written list of all your medications on the day of the test so the team can plan accordingly