Pregnancy screening
It's the main test for gestational diabetes, usually done between 24 and 28 weeks of pregnancy — covered in detail in the pregnancy section below.
The oral glucose tolerance test (OGTT) measures how well your body handles sugar. After a fasting blood sample, you drink a measured glucose solution, and your blood sugar is checked again — usually 2 hours later. It's the most sensitive test for diabetes and the main test for gestational diabetes in pregnancy. A 2-hour result of 200 mg/dL or higher, confirmed by a doctor, indicates diabetes.
Cure.Care may earn a commission from some links on this page, at no extra cost to you. This never influences our editorial content or medical review. Learn more.
The oral glucose tolerance test (OGTT) is a blood test that measures how your body handles a known amount of sugar. After a fasting sample, you drink a glucose solution, and your blood sugar is measured again — usually 2 hours later. By watching how quickly your body clears the sugar, the OGTT reveals problems that a single fasting test can miss. It's also called the GTT or glucose challenge test.
Think of the OGTT as a "stress test" for your blood sugar. The glucose drink deliberately challenges your body, and the timed samples show how well your insulin responds. A healthy body clears the sugar quickly; if blood sugar stays high after 2 hours, it points to prediabetes or diabetes.
The OGTT is one of the main blood tests used to identify diabetes. To see how it sits alongside the others, read our guide to how diabetes is diagnosed.
How your body processes a measured glucose load over time.
Around 2 hours — fasting sample, glucose drink, then a 2-hour sample.
GTT, oral GTT, glucose challenge test, sugar tolerance test.
When would a doctor choose this test over a simpler one? Next: why and when the OGTT is done.
The OGTT is used when a more sensitive test is needed — to confirm borderline results, to screen for gestational diabetes in pregnancy, and to detect early diabetes that a fasting test might miss. Because it shows how your body handles a sugar load over time, it can reveal problems before fasting sugar rises. It's not usually the first test, but it's often the most revealing.
A fasting test or HbA1c is often done first because they're simpler. The OGTT comes in when a clearer answer is needed — it's more sensitive, but also takes longer. Here's when doctors reach for it.
It's the main test for gestational diabetes, usually done between 24 and 28 weeks of pregnancy — covered in detail in the pregnancy section below.
When a fasting test or HbA1c is borderline or unclear, the OGTT gives a more definitive answer about whether someone has prediabetes or diabetes.
As the most sensitive test, it can detect prediabetes and early diabetes that fasting sugar alone might miss.
The OGTT is one of four main tests doctors use — see how they compare in our diabetes diagnosis guide, or check your overall risk with the diabetes risk calculator.
What counts as a normal OGTT result? Next: the OGTT normal range.
For a standard (non-pregnant) OGTT, a 2-hour blood sugar below 140 mg/dL (7.8 mmol/L) is normal. A 2-hour value of 140–199 mg/dL (7.8–11.0 mmol/L) indicates prediabetes, and 200 mg/dL (11.1 mmol/L) or higher indicates diabetes. The fasting sample is read against the usual fasting ranges. These cut-offs are set by the WHO and ADA, and a doctor confirms the result.
The OGTT reads two key numbers — the fasting sample and the 2-hour sample after the glucose drink. The table below shows both in mg/dL (used in India) and mmol/L. Note: pregnancy uses different, lower thresholds — see the pregnancy section below.
| Category | Fasting (mg/dL) | 2-hour (mg/dL) | 2-hour (mmol/L) |
|---|---|---|---|
| Normal | Below 100 | Below 140 | Below 7.8 |
| Prediabetes | 100 – 125 | 140 – 199 | 7.8 – 11.0 |
| Diabetes | 126 or higher | 200 or higher | 11.1 or higher |
Prediabetes on the 2-hour OGTT is called impaired glucose tolerance (IGT). Values shown are for a non-pregnant adult 75g OGTT. Conversion: mmol/L = mg/dL ÷ 18.
mg/dL vs mmol/L: India and the US report blood sugar in mg/dL; the UK and many countries use mmol/L. Divide mg/dL by 18 to convert (e.g. 200 ÷ 18 = 11.1 mmol/L). See our blood sugar levels chart for every test and unit.
The 2-hour value is the key number. The OGTT's diagnostic strength is the reading 2 hours after the glucose drink. A single result is interpreted by your doctor alongside your symptoms and other tests — and diabetes is usually confirmed rather than diagnosed on one OGTT alone.
In pregnancy, the OGTT works differently and uses its own thresholds. Next: the OGTT in pregnancy and gestational diabetes.
In pregnancy, the OGTT is the main test for gestational diabetes (GDM) — high blood sugar that develops during pregnancy. It's usually done between 24 and 28 weeks. Pregnancy uses lower cut-offs than the standard test, because even mildly raised sugar can affect mother and baby. The exact values and number of blood samples depend on which protocol your clinic follows.
Gestational diabetes is common and very manageable, especially when caught early. Because pregnancy changes how the body handles sugar, the OGTT is timed and interpreted differently than usual.
Usually between 24 and 28 weeks of pregnancy. Those at higher risk may be tested earlier, sometimes at the first antenatal visit.
Untreated GDM can raise the risk of a large baby, delivery complications and later diabetes. Detecting it early allows safe, effective management.
Pregnancy uses lower thresholds than the standard OGTT, so milder rises in blood sugar are picked up and treated.
A single 75g glucose drink with blood taken fasting, at 1 hour and at 2 hours. One raised value is enough to diagnose GDM. Common in India and many countries.
A 50g non-fasting screen first; if raised, a longer 100g OGTT with samples at fasting, 1, 2 and 3 hours confirms GDM. Common in the US.
The exact GDM cut-off numbers depend on the protocol your clinic and country follow, and they're lower than the standard OGTT values. Rather than self-checking against a single number, ask your doctor which test you're having and what your results mean. A GDM diagnosis is made and explained by your maternity team.
If you're diagnosed with gestational diabetes, it's usually well controlled with diet, activity and monitoring — and sometimes medication. Most women go on to have healthy pregnancies. Your doctor and dietitian will guide a plan, and our Indian diet guidance can help with everyday food choices.
Whether for pregnancy or a standard test, good preparation matters. Next: how to prepare for the OGTT.
To prepare for an OGTT, fast for 8–12 hours beforehand, drinking only water. Importantly, eat normally — including your usual carbohydrates — in the days before, as cutting carbs can skew the result. Plan to stay at the lab for the full 2 hours or more, keep taking regular medicines unless your doctor says otherwise, and tell the lab what you take.
The OGTT has one preparation step people often miss: you should eat your normal diet, including carbohydrates, in the days leading up to it. Crash-dieting or going low-carb beforehand can make the result inaccurate.
Why eat normal carbs first? The OGTT measures how your body responds to sugar. If you've eaten very few carbs in the days before, your body may handle the glucose drink unusually and produce a misleadingly high result. Eating your normal diet gives an accurate picture.
If you take diabetes medicines or insulin, fasting plus a glucose drink needs careful planning — ask your doctor how to prepare safely and whether to adjust any medicine on the test day. Never change a prescribed medicine on your own. Tell staff if you feel unwell during the test.
Ready for test day? Next: what happens during the OGTT procedure.
The OGTT takes about 2 hours. First, a fasting blood sample is taken. You then drink a measured glucose solution, and your blood sugar is checked again — usually at 2 hours, and sometimes at 1 hour too. You stay at the lab between samples. The pregnancy version may involve more draws over a longer period, depending on the protocol.
Unlike a simple blood test, the OGTT involves more than one sample over time — that's what lets it measure how your body handles sugar. Here's the typical flow for a standard 75g test.
After your overnight fast, a first blood sample is taken to measure your fasting blood sugar — the baseline.
You drink a measured sugary solution (usually 75g of glucose) within about 5 minutes. It's very sweet — some people find it a little unpleasant.
You rest at the lab — no eating, walking around or smoking, as activity changes blood sugar. Bring something to keep you occupied.
A second blood sample 2 hours after the drink shows how well your body cleared the sugar — the key diagnostic reading. Then you can eat.
How it feels. The sweet drink makes some people feel mildly nauseous, sweaty or light-headed for a while — this usually passes. The blood draws are quick, with only minor soreness. Tell the staff if you feel faint or unwell, or if you vomit, as the test may need rescheduling.
Once your samples are processed, what do the numbers mean? Next: understanding your OGTT results.
Your OGTT result is read mainly from the 2-hour sample. Below 140 mg/dL is normal, 140–199 mg/dL indicates prediabetes (impaired glucose tolerance), and 200 mg/dL or higher indicates diabetes. The fasting sample is checked too. A doctor interprets the result alongside your symptoms and other tests — and in pregnancy, different cut-offs apply.
The block below summarises what each 2-hour result range means for a standard (non-pregnant) OGTT. As always, the right next step depends on your doctor's view, not the number alone.
Your body cleared the glucose well. Keep up healthy habits and routine checks based on your age and risk.
Higher than normal but not diabetes. Often reversible with diet, activity and weight changes. Your doctor will advise on follow-up.
Meets the diabetes threshold, usually confirmed with a second test. Your doctor plans treatment and checks the type.
Standard non-pregnant 75g OGTT, 2-hour value. Pregnancy uses lower, protocol-specific cut-offs.
A high 2-hour result means your body was slow to clear the sugar — pointing to prediabetes or diabetes. Because the OGTT is sensitive, it can detect this earlier than a fasting test. A doctor confirms diabetes with a repeat or second test, then plans next steps. Catching it early makes management far more effective. You can compare your numbers on our blood sugar levels chart.
Occasionally blood sugar drops too low after the glucose drink, sometimes called reactive hypoglycaemia. It can cause shakiness, sweating, hunger or dizziness, and should be discussed with your doctor, who can advise on what it means for you.
Seek urgent help for severe low blood sugar — confusion, seizures or fainting. Very low blood sugar is a medical emergency. During the test, tell staff straight away if you feel seriously unwell.
How does the OGTT compare to the other diabetes tests? Next: OGTT vs FBS, HbA1c and PPBS.
The OGTT is the most sensitive diabetes test and the main one for pregnancy, but it's also the most time-consuming. The FBS measures fasting sugar at one moment, HbA1c shows your 2–3 month average, and PPBS measures sugar after a meal. The OGTT's strength is catching early or borderline cases the others might miss.
Each test has a job. Seeing them side by side shows why the OGTT is reserved for situations where a sensitive, detailed answer matters most — like pregnancy or unclear results.
| Feature | OGTT | FBS | HbA1c | PPBS |
|---|---|---|---|---|
| What it measures | Response to a sugar load | Fasting blood sugar | 3-month average | Sugar 2h after a meal |
| Fasting needed | Yes, then drink | Yes (8–12h) | No | No (eat first) |
| Diabetes cut-off | ≥200 mg/dL (2h) | ≥126 mg/dL | ≥6.5% | ≥200 mg/dL* |
| Time taken | ~2 hours | A few minutes | A few minutes | ~2 hours |
| Best for | Pregnancy & early/borderline | Simple, low-cost check | Long-term control | After-meal spikes |
* A random/PPBS value of ≥200 mg/dL suggests diabetes when symptoms are present. The OGTT's 2-hour reading is the most sensitive single value.
Still have questions about the glucose tolerance test? The FAQ below answers the most common ones.
Clear, evidence-based answers to common questions about the OGTT.
For a standard (non-pregnant) OGTT, a 2-hour blood sugar below 140 mg/dL (7.8 mmol/L) is normal. A 2-hour value of 140 to 199 mg/dL indicates prediabetes, and 200 mg/dL or higher indicates diabetes. Pregnancy uses different, lower cut-offs.
A standard OGTT takes about 2 hours. A fasting blood sample is taken first, you drink a glucose solution, and a second sample is taken 2 hours later. You stay at the lab in between. The pregnancy version may take longer if extra samples are needed.
Yes. You usually fast for 8 to 12 hours before the OGTT, drinking only water. Importantly, you should also eat normally — including your usual carbohydrates — in the days before, as cutting carbs can make the result inaccurate.
The OGTT is the main test for gestational diabetes, usually done between 24 and 28 weeks. Pregnancy uses lower cut-offs because even mildly raised blood sugar can affect mother and baby. The exact values depend on the testing protocol your clinic follows.
The glucose solution is very sweet, a bit like flat, syrupy lemonade or cola. Some people find it unpleasant and feel mildly nauseous, sweaty or light-headed for a while afterward. This usually passes. Tell staff if you feel unwell or vomit, as the test may need rescheduling.
The fasting test measures your blood sugar at a single fasting moment, while the OGTT also measures how your body handles a glucose drink over 2 hours. This makes the OGTT more sensitive — it can detect early or borderline diabetes that a fasting test might miss. Compare them in our FBS test guide.
A 2-hour OGTT value of 200 mg/dL or higher suggests diabetes, but it's usually confirmed with a repeat or second test, unless clear symptoms accompany a very high reading. A doctor interprets the result and decides what to confirm.
Yes, the OGTT is generally safe. The main effects are temporary nausea or light-headedness from the sweet drink and minor soreness from the blood draws. If you take diabetes medication, ask your doctor how to prepare safely, and tell staff if you feel seriously unwell during the test.
Standard OGTT 2-hour thresholds (normal <140 mg/dL, prediabetes 140–199 mg/dL, diabetes ≥200 mg/dL) follow the WHO and ADA criteria above. Gestational diabetes uses different, protocol-specific cut-offs. These are general guidance, not a diagnosis. Confirm against the latest published guidance before publishing.
This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Only a qualified doctor can interpret your test results and diagnose diabetes or gestational diabetes. Read our full Medical Disclaimer and Editorial Policy.