Lab result explainedMetabolic panelLast updated November 11, 2026

Glucose tolerance test (OGTT): normal range, 2-hour and 3-hour results, and pregnancy cut-offs

Oral glucose tolerance test (OGTT) · Oral glucose tolerance test (fasting glucose plus timed samples after 75 g or 100 g of glucose) · reported in mg/dL (plasma glucose; divide by 18 for mmol/L)

The oral glucose tolerance test (OGTT) measures how well your body clears a measured dose of sugar from the blood. After an overnight fast, a baseline blood sample is drawn, you drink a solution containing 75 grams of glucose (100 grams for the 3-hour pregnancy version), and blood is drawn again at set times, usually 2 hours. In a non-pregnant adult, a 2-hour glucose under 140 mg/dL is normal, 140 to 199 is impaired glucose tolerance (a form of prediabetes), and 200 mg/dL or more is diabetes. It is the most sensitive of the standard tests for picking up early diabetes, because it catches people whose fasting glucose and A1C are still normal but whose insulin response to a meal is already failing.

In pregnancy, the OGTT is the diagnostic test for gestational diabetes. Most US practices use it as the second step after a positive 1-hour screen: a fasting draw, 100 g of glucose, and draws at 1, 2 and 3 hours, with gestational diabetes diagnosed when two or more values reach 95, 180, 155 and 140 mg/dL (Carpenter-Coustan criteria). Some practices use a single-step 75 g 2-hour test with lower thresholds of 92, 180 and 153 mg/dL, where one abnormal value is enough. The same test, with the non-pregnant cut-offs, is repeated 4 to 12 weeks after delivery to check that glucose handling has returned to normal.

Because the result depends on what you ate in the days before, how long you fasted, whether you sat still, and whether you were ill or on steroids, preparation matters, and 'how to pass' questions have a straightforward answer: eat normally (at least 150 g of carbohydrate a day) for three days, fast 8 to 14 hours overnight with water only, do not smoke or exercise that morning, and sit quietly during the test. Those steps give an accurate result rather than a flattering one, which is the point, since finding and treating prediabetes or gestational diabetes early is what changes outcomes.

OGTT at a glance

  • What it is: fasting glucose followed by timed blood glucose measurements after drinking 75 g of glucose (100 g for the 3-hour pregnancy test); it shows how well insulin handles a sugar load.
  • Normal (non-pregnant adults): fasting under 100 mg/dL and 2-hour under 140 mg/dL.
  • Prediabetes: 2-hour 140–199 mg/dL (impaired glucose tolerance) or fasting 100–125 (impaired fasting glucose). Diabetes: 2-hour 200 or more, or fasting 126 or more, confirmed by a repeat test unless symptoms are obvious.
  • Pregnancy, 3-hour 100 g test (Carpenter-Coustan): fasting 95, 1-hour 180, 2-hour 155, 3-hour 140 mg/dL; two or more values at or above these diagnose gestational diabetes.
  • Pregnancy, one-step 75 g test (IADPSG/ADA): fasting 92, 1-hour 180, 2-hour 153 mg/dL; one value is enough.
  • Preparation: at least 150 g of carbohydrate a day for 3 days, then an 8–14 hour overnight fast; sit quietly, no smoking, no exercise. Low-carbohydrate dieting beforehand falsely raises the result.

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What OGTT measures

A fasting glucose tells you what your liver does overnight; the OGTT tells you what happens after a meal. Seventy-five grams of glucose in 250 to 300 mL of water, drunk within 5 minutes, is absorbed over the following hour and pushes blood glucose up. In a person with normal insulin secretion and sensitivity, glucose peaks around 120 to 160 mg/dL at 30 to 60 minutes and is back below 140 by 2 hours as muscle and liver take it up. In prediabetes the first-phase insulin response is blunted and the tissues are resistant, so the peak is higher and the 2-hour value stays between 140 and 199. In diabetes the 2-hour value remains at 200 or above. Samples are collected in fluoride (gray-top) tubes so red cells do not consume glucose before analysis; a sample left standing in a plain tube loses 5 to 7 mg/dL an hour.

The 1-hour value on the 75 g test is increasingly reported too. A 1-hour glucose of 155 mg/dL or more in someone with a normal 2-hour value identifies people at substantially higher risk of progressing to type 2 diabetes and cardiovascular disease, and the International Diabetes Federation proposed in 2024 that 155 be used to define intermediate hyperglycemia and 209 mg/dL to define diabetes. Not every lab reports the 1-hour draw outside pregnancy, but if yours does, it carries real information. In children the glucose dose is 1.75 g per kg of body weight up to the adult 75 g, with the same cut-offs.

Reproducibility is the OGTT's known weakness. Day-to-day variation in the 2-hour value is about 15 to 20 percent, and roughly half of people classified as impaired glucose tolerance on one test fall into a different category on repeat, which is why a diagnosis of diabetes requires either two abnormal results (from the same sample or a second test) or one abnormal result with unmistakable symptoms. Prior carbohydrate restriction, bed rest, acute illness, recent surgery, steroids and even a short night's sleep all push the 2-hour value up; vigorous exercise the day before and walking during the test push it down. The protocol exists to standardize these.

Normal range for OGTT

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

GroupTypical range (mg/dL (plasma glucose; divide by 18 for mmol/L))Notes
Non-pregnant adults and children: normalFasting below 100 mg/dL; 2-hour below 140 mg/dLADA criteria. Some labs use a fasting cut-off of 110 (WHO). Labs differ slightly in reporting.
Impaired fasting glucose (prediabetes)Fasting 100–125 mg/dLWHO uses 110–125. Reflects overnight liver glucose output.
Impaired glucose tolerance (prediabetes)2-hour 140–199 mg/dLAbout a third of people with impaired glucose tolerance have a normal fasting glucose and A1C; progression to diabetes is roughly 5–10 percent a year without lifestyle change.
DiabetesFasting 126 or more, or 2-hour 200 mg/dL or moreConfirm with a repeat test or a second abnormal measure (A1C 6.5 percent or more) unless glucose is unequivocally high with symptoms.
1-hour value on the 75 g test (non-pregnant)Below 155 normal; 155–208 intermediate hyperglycemia; 209 or more diabetesInternational Diabetes Federation 2024 position; not yet adopted by all guidelines.
Pregnancy, 3-hour 100 g test (Carpenter-Coustan)Fasting 95; 1-hour 180; 2-hour 155; 3-hour 140 mg/dLGestational diabetes if two or more values are at or above these. ACOG allows one abnormal value to be treated as positive in some settings.
Pregnancy, 3-hour 100 g test (National Diabetes Data Group)Fasting 105; 1-hour 190; 2-hour 165; 3-hour 145 mg/dLOlder, higher thresholds still used by some practices; diagnose fewer people.
Pregnancy, one-step 75 g 2-hour test (IADPSG/ADA)Fasting 92; 1-hour 180; 2-hour 153 mg/dLOne value at or above any threshold is diagnostic. Identifies about twice as many people as the two-step method.
Hypoglycemia during the testBelow 70 mg/dL at any point; below 54 is clinically significantA dip into the 60s at 2–4 hours occurs in up to 10 percent of healthy people on extended tests and is not diagnostic by itself.

High glucose tolerance test (abnormal OGTT)

A 2-hour value of 140 to 199 mg/dL means impaired glucose tolerance, the form of prediabetes that most reliably predicts both type 2 diabetes and cardiovascular disease. About 38 percent of US adults have prediabetes by some measure and eight in ten do not know it. Without intervention, roughly 5 to 10 percent of people with impaired glucose tolerance progress to diabetes each year, but this group also has the most to gain: in the Diabetes Prevention Program, 7 percent weight loss with 150 minutes a week of activity cut progression by 58 percent over three years, and metformin by 31 percent. A fasting value of 100 to 125 with a normal 2-hour value is impaired fasting glucose, which carries a somewhat lower risk.

A 2-hour value of 200 mg/dL or more, or a fasting value of 126 or more, meets the definition of diabetes. Unless you have classic symptoms (thirst, frequent urination, weight loss) or a glucose above about 300, the diagnosis is confirmed with a second test, which may be a repeat OGTT, a fasting glucose or an A1C. Once confirmed, the questions become which type (type 2 is by far the most common in adults, but adult-onset type 1 and secondary causes such as pancreatic disease, steroids and Cushing's syndrome are checked for when the picture does not fit) and whether complications are already present, since diabetes has often been present for several years by the time an OGTT is abnormal.

In pregnancy, meeting the thresholds diagnoses gestational diabetes, which affects about 6 to 9 percent of pregnancies by the two-step method and 15 to 18 percent by the one-step method. Treatment with a meal plan, glucose monitoring, activity and insulin or metformin when needed reduces large-for-gestational-age babies by about half, shoulder dystocia by about 60 percent and preeclampsia by about a third in randomized trials. One abnormal value on the 3-hour test, short of the two needed for diagnosis, still carries a higher risk of a large baby, and some clinicians manage it with the same dietary advice and glucose checks.

Common causes

  1. Type 2 diabetes and prediabetes (insulin resistance)

    Excess visceral fat, inactivity, age and genetics reduce insulin sensitivity; the pancreas compensates for years and then falls behind, which first shows as a high 2-hour value. The most common reason for an abnormal OGTT outside pregnancy.

  2. Gestational diabetes

    Placental hormones (human placental lactogen, progesterone, cortisol) make tissues insulin resistant from mid-pregnancy; those whose pancreas cannot compensate meet the pregnancy thresholds, which are deliberately lower because fetal growth responds to glucose levels below the non-pregnant diabetes range.

  3. Polycystic ovary syndrome

    Insulin resistance is central to PCOS; 30–40 percent of women with PCOS have impaired glucose tolerance and up to 10 percent have diabetes by age 40, so guidelines recommend an OGTT at diagnosis and every 1–3 years.

  4. Medications

    Glucocorticoids (prednisone, dexamethasone), thiazide diuretics, atypical antipsychotics (olanzapine, clozapine), niacin, tacrolimus and cyclosporine, some HIV medicines and, to a small degree, statins and beta-blockers all raise post-load glucose.

  5. Carbohydrate restriction before the test

    Three or more days of low carbohydrate intake (under about 100–150 g a day) reduces insulin secretion and glucose uptake in response to a sudden load, raising the 2-hour value by 20–40 mg/dL in some studies; this is the classic cause of a false-positive OGTT.

  6. Pancreatic disease

    Chronic pancreatitis, cystic fibrosis (annual OGTT screening from age 10 is standard), hemochromatosis, pancreatic surgery and pancreatic cancer destroy insulin-producing cells. New diabetes after age 50 with weight loss is a recognized early sign of pancreatic cancer.

  7. Hormonal disorders

    Cushing's syndrome, acromegaly, hyperthyroidism, pheochromocytoma and glucagonoma each raise glucose through counter-regulatory hormones; the OGTT is also used in acromegaly to check whether growth hormone suppresses normally.

  8. Acute illness, surgery, stress and poor sleep

    Infection, injury and surgery raise cortisol and catecholamines; a single night of 4 hours' sleep reduces insulin sensitivity by about 20–25 percent. The test should be postponed until at least 2 weeks after an acute illness.

  9. Bed rest and physical inactivity

    Even a few days of immobility reduces muscle glucose uptake and raises post-load glucose; conversely, walking during the test lowers it, which is why you are asked to sit.

  10. Rapid gastric emptying

    After gastric surgery (sleeve gastrectomy, bypass) glucose is absorbed very fast, giving a 1-hour spike above 200 followed by a low value at 2–3 hours; the OGTT is often misleading in these patients.

Symptoms that can go with it

  • Usually none with prediabetes or early diabetes, which is why screening is needed
  • Increased thirst, frequent urination (including at night) and unexplained weight loss when glucose is well above 200
  • Fatigue, blurred vision and slow-healing cuts or recurrent skin and genital yeast infections
  • Tingling or numbness in the feet if diabetes has been present for some years
  • Darkened, velvety skin on the neck or armpits (acanthosis nigricans), a sign of insulin resistance
  • In pregnancy, usually nothing; occasionally a baby measuring large for dates or excess amniotic fluid

Low glucose during a glucose tolerance test

Blood glucose can fall below the starting value during the later hours of a glucose tolerance test, and people often leave the lab shaky, sweaty, hungry or lightheaded. This happens because the large glucose load triggers a brisk insulin surge that outlasts the glucose, so glucose overshoots downward at 2 to 4 hours. On extended 3- to 5-hour tests, up to 10 percent of healthy people dip below 70 mg/dL at some point without any disease, which is why the OGTT is no longer recommended for diagnosing reactive hypoglycemia; what matters is whether symptoms occur with a documented low glucose after ordinary meals (a mixed-meal test is used for that).

A low value is clinically meaningful in a few settings. After gastric bypass or sleeve surgery, a 1-hour spike followed by a 2- to 3-hour value under 55 mg/dL with symptoms is late dumping syndrome (post-bariatric hypoglycemia). In early type 2 diabetes, delayed insulin release can produce a high 1- to 2-hour value and then a low at 3 to 4 hours. A flat curve, where glucose barely rises at all, is sometimes seen with malabsorption, celiac disease or very slow gastric emptying. Insulinoma is not diagnosed with an OGTT; it requires a supervised fast. A glucose under 54 mg/dL with symptoms at any point in the test should be reported to the person who ordered it.

Common causes

  1. Reactive (post-load) dip in healthy people

    A vigorous first-phase insulin response overshoots the glucose load, bringing glucose into the 60s at 2–4 hours; common in lean, insulin-sensitive people and not a disease.

  2. Late dumping after bariatric or gastric surgery

    Very rapid delivery of glucose to the small intestine causes an exaggerated GLP-1 and insulin response; glucose peaks over 200 at 30–60 minutes and falls below 55 at 2–3 hours with sweating, confusion and palpitations.

  3. Early type 2 diabetes with delayed insulin peak

    Insulin release is slow but eventually excessive, so a high 2-hour value can be followed by a low 3- to 5-hour value; the high early values, not the late low, are the diagnostic finding.

  4. Prolonged fasting, alcohol or heavy exercise before the test

    A fast beyond 14–16 hours, alcohol the night before (which blocks liver glucose production) or a hard workout that morning depletes glycogen and increases the chance of a low value.

  5. Medications

    Insulin, sulfonylureas (glipizide, glyburide) and, less often, high-dose aspirin, quinine or beta-blockers (which also mask warning symptoms) can cause low values; tell the lab what you take.

  6. Malabsorption, celiac disease or delayed gastric emptying

    Produce a flat curve rather than a true low: glucose fails to rise because it is not absorbed in the usual time frame. Gastroparesis in long-standing diabetes can do the same.

  7. Adrenal insufficiency or hypopituitarism

    Lack of cortisol or growth hormone impairs the counter-regulatory response, so glucose falls further and recovers more slowly after an insulin surge.

Symptoms that can go with it

  • Shakiness, sweating, palpitations and anxiety 2 to 4 hours after the drink
  • Hunger, nausea and a feeling of weakness
  • Lightheadedness, difficulty concentrating or blurred vision
  • Headache and fatigue for the rest of the day
  • Confusion, slurred speech or fainting with values under about 50 mg/dL, which needs immediate attention

Read it alongside

  • Fasting glucose — the first sample of the test; 100–125 is impaired fasting glucose and 126 or more is diabetes on its own.
  • A1C — three-month average glucose; 5.7–6.4 percent is prediabetes and 6.5 or more diabetes. A normal A1C with an abnormal 2-hour value is common in early disease. Not used to diagnose gestational diabetes.
  • Fasting insulin and HOMA-IR, or C-peptide — show whether a high glucose reflects insulin resistance (high insulin) or failing insulin production (low insulin or C-peptide), which separates type 2 from type 1 or pancreatic diabetes.
  • 1-hour glucose challenge test — the 50 g non-fasting screen that usually precedes the 3-hour test in pregnancy.
  • Lipid panel, blood pressure and urine albumin-to-creatinine ratio — impaired glucose tolerance travels with the rest of the metabolic syndrome, and these set cardiovascular risk and detect early kidney involvement.
  • GAD and islet antibodies — ordered when diabetes is diagnosed in a lean adult or progresses quickly, to detect autoimmune (type 1 or LADA) diabetes.
  • Liver enzymes (ALT) and ferritin — fatty liver disease is present in most people with impaired glucose tolerance; very high ferritin raises the question of hemochromatosis.
  • Postpartum OGTT at 4–12 weeks — for anyone diagnosed with gestational diabetes; up to half develop type 2 diabetes within 10 years and need testing every 1–3 years.

What usually happens next

  1. A diagnosis of diabetes from a single OGTT is confirmed with a second test (repeat OGTT, fasting glucose or A1C) within a few weeks unless glucose was unequivocally high with symptoms; a result that was borderline and taken after illness, steroids or a low-carbohydrate week is simply repeated with proper preparation.
  2. For impaired glucose tolerance, expect a conversation about a structured lifestyle program (the Diabetes Prevention Program model is covered by many US insurers and Medicare), metformin if you are under 60 with a BMI over 35 or had gestational diabetes, and repeat testing every year.
  3. For newly confirmed diabetes, baseline tests usually include A1C, lipids, kidney function with urine albumin, liver enzymes, a retinal exam and a foot check, plus antibodies if type 1 is possible.
  4. In pregnancy, a positive 3-hour test leads within days to a glucose meter, a meal plan from a dietitian, targets of fasting under 95 and 1-hour post-meal under 140 or 2-hour under 120 mg/dL, and insulin or metformin if those are not met; one abnormal value is often managed with the same diet and monitoring.
  5. Prepare properly for any repeat: at least 150 g of carbohydrate a day for 3 days, an 8–14 hour fast with water only, usual medicines unless told otherwise, no smoking or exercise that morning, and sit for the duration; bring a snack for afterward.
  6. If you had symptomatic low glucose during the test, ask whether a mixed-meal test, continuous glucose monitor or, after bariatric surgery, a dietitian review for dumping syndrome is appropriate.

Diet, lifestyle and natural levers with evidence

What can move OGTT once the cause is known. Nothing here replaces finding the cause.

Structured lifestyle change: 5–7 percent weight loss and 150 minutes a week of activity

Strong evidence

In the Diabetes Prevention Program, 3,234 US adults with impaired glucose tolerance were randomized to intensive lifestyle coaching, metformin or placebo. Over 2.8 years, lifestyle change reduced progression to diabetes by 58 percent (71 percent in those over 60) and metformin by 31 percent; the lifestyle benefit was still 27 percent at 15 years. Average weight loss was 5.6 kg. Similar trials in Finland, China and India found reductions of 40 to 60 percent. The CDC-recognized National Diabetes Prevention Program delivers the same curriculum.

Walking after meals

Moderate evidence

Muscle contraction takes up glucose without needing insulin. In a crossover trial in older adults with impaired glucose tolerance, three 15-minute walks after meals lowered 24-hour and 3-hour post-dinner glucose more than a single 45-minute walk, and a meta-analysis of post-meal activity studies found reductions in post-meal glucose of about 0.5 to 1.0 mmol/L (10–18 mg/dL) from as little as 2 to 5 minutes of light walking. Resistance training two or three times a week adds a further improvement in insulin sensitivity of 10–15 percent.

Do not exercise during the test itself; walking around between draws lowers the result and defeats the purpose.

Mediterranean or high-fiber, lower-glycemic eating pattern

Moderate evidence

In the PREDIMED trial, a Mediterranean diet with extra-virgin olive oil or nuts reduced new-onset diabetes by about 30 percent over 4 years without calorie restriction or weight loss targets. Each 10 g a day of cereal fiber is associated with about 25 percent lower diabetes incidence in cohort studies, and replacing refined grains and sugar-sweetened drinks with whole grains, legumes, vegetables and nuts lowers 2-hour glucose by 10–20 mg/dL in feeding studies. In gestational diabetes, this pattern keeps most people at target without medication.

Sleep of 7–8 hours and treating sleep apnea

Moderate evidence

Restricting healthy adults to 4–5 hours of sleep for less than a week reduces insulin sensitivity by 20–25 percent and raises 2-hour OGTT glucose into the impaired range in some, reversing with recovery sleep. Habitual short sleep (under 6 hours) is associated with about 30 percent higher diabetes risk. Obstructive sleep apnea independently worsens glucose tolerance; CPAP for 8 weeks or more lowered 2-hour glucose and A1C modestly in randomized trials of people with prediabetes.

Berberine

Moderate evidence

Berberine, a plant alkaloid, activates AMP-kinase and slows carbohydrate absorption. Meta-analyses of randomized trials in type 2 diabetes (typically 0.9 to 1.5 g a day in divided doses for 2 to 3 months) report reductions in fasting glucose of about 15 mg/dL, 2-hour post-load glucose of about 25–30 mg/dL and A1C of about 0.7 percent, comparable to metformin in small head-to-head studies. Trials in prediabetes are fewer and smaller.

Not for use in pregnancy or breastfeeding (crosses the placenta and can displace bilirubin in newborns). Causes constipation or diarrhea in up to a third of users and inhibits CYP3A4 and CYP2D6, raising levels of cyclosporine, some statins, warfarin and many other drugs; discuss with a clinician or pharmacist first.

Vinegar before a carbohydrate meal

Preliminary evidence

About 20 g (1–1.5 tablespoons) of vinegar in water taken immediately before a high-carbohydrate meal reduced the post-meal glucose rise by roughly 20 to 30 percent in small crossover trials in people with insulin resistance or type 2 diabetes, probably by slowing gastric emptying and starch digestion. The effect is on the meal that follows and does not change fasting glucose or A1C meaningfully in longer trials.

Dilute it to protect tooth enamel and the esophagus; avoid in gastroparesis, where slowing gastric emptying further worsens control.

When to see a clinician promptly

  • A 2-hour value of 200 mg/dL or more, or a fasting value of 126 or more: arrange confirmation and a plan within a week or two; sooner if you have thirst, frequent urination or weight loss.
  • Any glucose above 300 mg/dL, or above 250 with vomiting, abdominal pain, deep rapid breathing, fruity breath or drowsiness: possible diabetic ketoacidosis or hyperosmolar state, go to an emergency department.
  • Glucose below 54 mg/dL during the test, or any low value with confusion, slurred speech or fainting: the lab should treat it immediately and the ordering clinician should be told the same day.
  • In pregnancy, a positive 3-hour or one-step test: contact your prenatal clinic within a few days so that monitoring and a meal plan start without delay; reduced fetal movements at any time are a same-day emergency.
  • Recurrent episodes of sweating, shakiness and confusion 1 to 3 hours after meals after gastric bypass or sleeve surgery, which need evaluation for post-bariatric hypoglycemia.
  • New diabetes after age 50 with weight loss, abdominal or back pain, or jaundice, which can be an early sign of pancreatic cancer and warrants prompt imaging.

Frequently asked questions

What is a normal glucose tolerance test result?

For a non-pregnant adult, a fasting glucose under 100 mg/dL and a 2-hour glucose under 140 mg/dL after the 75 g drink. A 2-hour value of 140 to 199 is impaired glucose tolerance (prediabetes) and 200 or more is diabetes; a fasting value of 100 to 125 is impaired fasting glucose and 126 or more is diabetes. Some labs also report the 1-hour value, where under 155 mg/dL is considered normal. Labs may differ slightly in the fasting cut-off they print.

What are normal results for the 3-hour glucose test in pregnancy?

Using the Carpenter-Coustan criteria that most US practices follow, all four values should be below the thresholds: fasting under 95 mg/dL, 1-hour under 180, 2-hour under 155 and 3-hour under 140. Gestational diabetes is diagnosed when two or more values are at or above the thresholds. Some practices use the older National Diabetes Data Group thresholds of 105, 190, 165 and 145. If your practice used the one-step 75 g 2-hour test instead, the thresholds are 92, 180 and 153 and a single abnormal value is diagnostic.

How do I prepare for (or 'pass') a glucose tolerance test?

Eat your normal diet with at least 150 g of carbohydrate a day for the three days before; cutting carbohydrates makes the result worse, not better. Fast for 8 to 14 hours overnight with only water, take usual medicines unless told otherwise, do not smoke, skip the morning workout, arrive rested and sit quietly for the whole test. Postpone it if you have been ill, had surgery or taken steroids in the previous two weeks. These steps give a true result; a falsely reassuring one would only delay treatment that prevents complications.

What happens if I fail one value on the 3-hour glucose test?

One value at or above threshold does not meet the usual two-value definition of gestational diabetes, so technically the test is negative, but it is not entirely normal: studies show that one abnormal value carries about a 1.5 to 2 fold higher risk of a large baby and other outcomes compared with four normal values. Many clinicians therefore offer the same dietary advice, a period of home glucose monitoring, or a repeat test at 32 to 34 weeks. ACOG notes that some practices diagnose gestational diabetes on one abnormal value. Ask how your practice handles it.

Do I have to fast for a glucose tolerance test, and can I drink water?

Yes, you fast for 8 to 14 hours before the first blood draw, and plain water is allowed and encouraged. No food, coffee, tea, juice, gum or mints, and no smoking, because each can change glucose or gastric emptying. After the drink, you remain fasting and seated until the last sample, so bring a snack to eat afterward. The exception is the 1-hour 50 g screening test in pregnancy, which does not require fasting.

What if my blood sugar drops low during the glucose tolerance test?

A dip into the 60s at 2 to 4 hours with shakiness and sweating happens in up to one in ten healthy people and reflects an enthusiastic insulin response rather than disease; eating a snack afterward resolves it. The lab will treat and record a value under 70 with symptoms, and a value under 54 is reported to your clinician. Repeated post-meal lows in daily life, especially after bariatric surgery, are evaluated with a mixed-meal test or a continuous glucose monitor rather than another OGTT.

What is the difference between the 1-hour and the 3-hour glucose test?

The 1-hour test is a non-fasting screen with 50 g of glucose and a single blood draw; a result at or above 130–140 mg/dL flags you for the 3-hour test. The 3-hour test is the diagnostic test: fasting overnight, a baseline draw, 100 g of glucose and draws at 1, 2 and 3 hours, with gestational diabetes diagnosed when two or more values reach the thresholds. About three in four people who fail the screen pass the 3-hour test. Some practices replace both with a single fasting 75 g 2-hour test.

What does a 1-hour value over 155 mg/dL mean if my 2-hour value is normal?

A 1-hour glucose of 155 or more on a 75 g test, with a normal 2-hour value, identifies people whose early insulin response is already impaired. In long-term studies they progress to type 2 diabetes at two to four times the rate of people with a 1-hour value under 155 and have more cardiovascular disease and fatty liver. The International Diabetes Federation proposed in 2024 that this be treated as intermediate hyperglycemia, meaning the same lifestyle measures as prediabetes and retesting within a year. It is not a diagnosis of diabetes.

References

  1. Glucose tolerance test - non-pregnant. MedlinePlus Medical Encyclopedia, US National Library of Medicine (2024)
  2. American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2025. Diabetes Care (2025)
  3. Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia: report of a WHO/IDF consultation. World Health Organization (2006)
  4. American College of Obstetricians and Gynecologists Committee on Practice Bulletins—Obstetrics. ACOG Practice Bulletin No. 190: Gestational diabetes mellitus. Obstetrics & Gynecology (2018)
  5. Metzger BE, Gabbe SG, Persson B, et al.. International Association of Diabetes and Pregnancy Study Groups recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care (2010)
  6. Carpenter MW, Coustan DR. Criteria for screening tests for gestational diabetes. American Journal of Obstetrics and Gynecology (1982)
  7. Knowler WC, Barrett-Connor E, Fowler SE, et al.; Diabetes Prevention Program Research Group. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. New England Journal of Medicine (2002)
  8. Bergman M, Manco M, Satman I, et al.. International Diabetes Federation position statement on the 1-hour post-load plasma glucose for the diagnosis of intermediate hyperglycaemia and type 2 diabetes. Diabetes Research and Clinical Practice (2024)
  9. Ko GT, Chan JC, Woo J, et al.. The reproducibility and usefulness of the oral glucose tolerance test in patients with impaired glucose tolerance. Annals of Clinical Biochemistry (1998)
  10. Landon MB, Spong CY, Thom E, et al.. A multicenter, randomized trial of treatment for mild gestational diabetes. New England Journal of Medicine (2009)
  11. DiPietro L, Gribok A, Stevens MS, Hamm LF, Rumpler W. Three 15-min bouts of moderate postmeal walking significantly improves 24-h glycemic control in older people at risk for impaired glucose tolerance. Diabetes Care (2013)
  12. Lan J, Zhao Y, Dong F, et al.. Meta-analysis of the effect and safety of berberine in the treatment of type 2 diabetes mellitus, hyperlipemia and hypertension. Journal of Ethnopharmacology (2015)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.