Table of ContentsView AllTable of ContentsUsesRisksBefore the TestDuring the TestAfter the TestResults
Table of ContentsView All
View All
Table of Contents
Uses
Risks
Before the Test
During the Test
After the Test
Results
The oral glucose tolerance test (OGTT) measures how well the body can break down and use sugar (glucose) as well as clear it from the bloodstream. It’s also called a glucose tolerance test and is safe for adults and children.
For the test, you drink a syrupy solution after not eating for a while (fasting). A few blood samples are taken to see how your body is handling the sugar in the drink.
The oral glucose tolerance test can diagnose type 1 diabetes,type 2 diabetes, pregnancy-related diabetes (gestational diabetes), and high blood sugar levels that put someoneat risk for type 2 diabetes(prediabetes).
This article will go over why the oral glucose tolerance test is used, what to expect if you have to have it done, and what your results mean.
Is My Blood Sugar Normal?
Why the Oral Glucose Tolerance Test Is Used
The oral glucose tolerance test shows how well your body deals with the sugar (glucose) after a meal.Glucoseis a type of sugar that is made when the body breaks downcarbohydratesfrom the food that you eat. Some of the glucose will be used for energy and the rest is stored to use later.
If you have too little blood sugar, the pancreas secretes glucagon to release the stored glucose back into your bloodstream.
The body is usually able to maintain the ideal balance of blood glucose. However, if any parts of the system do not work, glucose can build up fast and lead tohigh blood sugarlevels (hyperglycemia) and diabetes.
Lack of insulin or insulin resistance causes higher than normal levels of glucose in the blood.
The oral glucose tolerance test is can spot imbalances in this process that other tests can miss. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommends that the oral glucose tolerance test be used for:
The oral glucose tolerance test can also be used to diagnose:
Oral Glucose Tolerance Test Types
The way the oral glucose tolerance test is done depends on the goals of the test.
The concentration of the oral glucose solution that you will drink, the timing of the test, and how manyblood drawsare done can be different for each patient. Some patients may need tofollow a low-carbohydrate dietfor the test.
There are two standard variations of the test that are used for screening and diagnosis:
How Do I Check My Blood Sugar?
Oral Glucose Tolerance Test During Pregnancy
The American College of Obstetricians and Gynecologists (ACOG) recommends that you be screened for gestational diabetes between 24 and 28 weeks of pregnancy.
However, rather than going right to a three-hour oral glucose tolerance test, healthcare providers usually recommend starting with a one-hour glucose challenge first, which you do not have to fast for.
The one-hour glucose challenge can be done before 24 weeks if a person has obesity, has a family history of diabetes, has been diagnosed withpolycystic ovary syndrome (PCOS), or had gestational diabetes during previous pregnancies.
If the test shows that your blood glucose values are equal to or greater than 140 milligrams per deciliter (mg/dL), the results are abnormal. You would then move on to doing the full, three-hour oral glucose tolerance test. Some providers set the threshold for requiring a second test as low as 130 mg/dL.
What Is a Normal Nonfasting Glucose?
Oral Glucose Tolerance Test Advantages and Disadvantages
The oral glucose tolerance test has some advantages:
How Accurate is the Oral Glucose Tolerance Test?The oral glucose tolerance test is very accurate. The percentage of correct positive test results (sensitivity) is between 81% and 93%. That’s far better than the FGP, which has a sensitivity of between 45%and 54%.
How Accurate is the Oral Glucose Tolerance Test?
The oral glucose tolerance test is very accurate. The percentage of correct positive test results (sensitivity) is between 81% and 93%. That’s far better than the FGP, which has a sensitivity of between 45%and 54%.
However, the oral glucose tolerance test also has some limitations:
Do I Need to Check My Blood Sugar While I’m Pregnant?
Oral Glucose Tolerance Test Risks
The oral glucose tolerance test is safe. While you will need to go to a healthcare facility or provider’s office, it’s usually an outpatient test (that means you do not need to be admitted to the hospital).
You do need to have a few blood samples taken, but they do not take long and do not usually lead to any problems (complications).
There are very few side effects or risks if you’re doing the oral glucose tolerance test. Some people have a hard time drinking the oral glucose solution. They may feel nauseated and even vomit. If this happens, the test may not be able to be completed that day.
People are watched closely during the test to make sure that they are OK. If they get low blood sugar (hypoglycemia), they will be treated right away.
What Are the Symptoms of Glucose Intolerance?
Who Should Not Do the Oral Glucose Tolerance Test?
While the oral glucose tolerance test is safe for most people, there are some reasons why a person would not be able to do it safely.
You should not have an oral glucose tolerance test if you:
If you cannot do the oral glucose tolerance test, your provider will have to use other tests to look at your blood glucose levels. The alternatives to an oral glucose tolerance test may include blood or urine tests to check your blood sugar or monitoring your levels at home with aglucometer.
Can Urine Tests Check Blood Sugar Levels?
Your provider will tell you how to prepare for your oral glucose tolerance test. If you have questions about their instructions, ask them to go over what they mean to make sure you’ve got it right.
If you need to fast before the test, you can have your dinner the night before the test, but then you will not be able to eat again until after you have finished the test the next day.
If you wake up sick on the day of your glucose tolerance test or you were sick in the days leading up to it, tell your provider. You may need to reschedule your appointment, as it can be unsafe to do the oral glucose tolerance test when you’re sick.
Timing
Oral glucose tolerance tests are usually scheduled in the morning because you will have been fasting since the night before and you might be quite hungry. The earlier you can do the test, the sooner you will be able to have something to eat.
Depending on which version of the test you’re doing, you’ll need to set aside three or four hours out of your day to do it.
Stress and anxiety can also affect your blood sugar levels. Try to arrive for your appointment at least 30 minutes early to give yourself some time to settle in and relax.
How Long Does Blood Work Take?
Location
You can’t do the oral glucose tolerance test at home. You’ll need to go to a healthcare provider’s office, a clinic, a hospital, or an independent lab facility.
What to Wear
On the day of your test, wear short sleeves or a top that allows you to roll up your sleeves easily to have your blood drawn. You can bring a sweatshirt or sweater to put on in case you get chilly.
How Can I Find Out If I Have Low Blood Sugar?
Food and Drink
You will need to stop eating and drinking eight to 12 hours before the oral glucose tolerance test, but that includes the time you spend asleep. Your provider will give you instructions. You will usually be allowed to have some water if you need to take medication.
Other than not having coffee and breakfast, you can do your normal routine like showering and brushing your teeth before the oral glucose tolerance test.
If you smoke, you will need to stop on the day of your appointment and not start again until after the test is done.Smoking increases insulin output and impairs glucose tolerance, as well as raises blood pressure.
Medications
Tell your provider about all the medications and supplements that you take, whether they are prescription, over-the-counter (OTC), nutritional, homeopathic, traditional, or recreational.
Certain medications affect your blood sugar levels. If you take any of these medications, your provider might tell you to stop taking them before your glucose tolerance test:
If you are not sure if you can take one of your medications before your oral glucose tolerance test, ask your provider. Never stop taking your medications without talking to your provider first.
What to Bring
When you arrive for your appointment, you may have to show your ID and health insurance cards when you check in.
However, activities like video games might be too overstimulating, especially for kids. Instead, watch a calming video on your tablet or phone or read a picture book to young children.
You may want to bring a water bottle and a light snack, like aprotein bar, to have after your test— especially if it will take you a while to drive home.
What Causes High Blood Sugar in People Who Don’t Have Diabetes?
Cost and Health Insurance
Prior authorizationis usually not required for an oral glucose tolerance test, but it’s still a good idea to ask your insurance carrier how much thecopay or coinsurancefor the test will be.
If youdo not have health insurance, shop around for the best price. Independent labs tend to have lower costs than healthcare providers' offices or hospitals.
You can also ask if the facility has apatient assistance programthat offers a tiered price structure or monthly payment plans.
It can be helpful to find out this information before you need it, especially if there’s a chance you’ll need to have more tests in the future.
An oral glucose tolerance test is a little different from other blood sugar tests.
For example, a fasting glucose test only looks at your blood sugar levels when you haven’t eaten for a while. An oral glucose tolerance test looks at both your fasting and non-fasting blood sugar levels.
How the oral glucose tolerance test is done will be a little different depending on whether you are an adult, a child, or pregnant.
Pre-Test
After signing in and confirming your insurance information, you will be taken to an exam room where your height and weight will be recorded. Your temperature andblood pressuremight also be taken.
Next, you will be asked to roll up your sleeve for the firstblood draw. The healthcare worker who takes blood samples (phlebotomist) will tie a piece of elastic (a tourniquet) around your upper arm to prepare to take your blood.
Can Stress Make Your Blood Sugar Go Up?
Throughout the Test
To get a sample of your blood, the phlebotomist willchoose a veinin either the crook of your arm or wrist. They will clean the spot with an antiseptic wipe.
Usually, abutterfly needleis used to take your blood. About 2 milliliters (mL) of blood will be taken and tested to get your baseline fasting blood sugar level.
Once the needle is taken out and the puncture wound is bandaged, you will be given a glucose solution to drink.
The drinks used for each type of test are:
When you finish drinking the solution, you will go back to the reception area to wait. You usually are not allowed to leave until you are done with the test.
If you are being tested for diabetes or prediabetes, you will wait for two hours after drinking the solution. After two hours, you will have blood taken again.
If you are being tested for gestational diabetes, blood samples will be taken one, two, and three hours after you finish drinking the solution.
If You Get Low Blood SugarWhile you are waiting, tell a nurse if you have symptoms of low blood sugar (hypoglycemia), including weakness, sweating, anxiety, shakiness, pale skin, hunger, or irregular heartbeat.
If You Get Low Blood Sugar
While you are waiting, tell a nurse if you have symptoms of low blood sugar (hypoglycemia), including weakness, sweating, anxiety, shakiness, pale skin, hunger, or irregular heartbeat.
Once all of the blood samples have been taken, you can go home. However, if you feel lightheaded or dizzy, you might be asked to wait a little longer before going home to make sure you’re OK.
You can go back to your normal routine and diet after the test.
What Are Normal Blood Sugar Levels for Older Adults?
You may have a little pain, swelling, orbruisingon your arms where blood was taken. If you have unusual pain, swelling, or a lot of bleeding where you had blood taken or havesigns and symptoms of infection, like a high fever, shivering chills, fast heartbeat/breathing; or shortness of breath, call your provider right away.
Continuous Glucose Monitoring: Pros and Cons
What Your Results Mean
Your provider will get the results of your oral glucose tolerance test within two to three days. Your test results will include reference ranges with high and low numeric values.
Any result that’s between the high and low values is considered normal. Results that are outside of the reference range can be abnormally high (noted with the letter “H” on the results) or abnormally low (noted with “L” on the results).
Two-hour oral glucose tolerance test results for adults or children are:
If your blood glucose value is over 200 mg/dL, your provider will have you do the test again or use another test to confirm a diabetes diagnosis. If both tests are positive, the diagnosis is definitive.
How Is Hyperglycemia Diagnosed?
The normal reference ranges for a three-hour oral glucose tolerance test are:
If any of the values are high, the test is repeated in four weeks. After the second test, if two or more values are high, then gestational diabetes is definitively diagnosed.
Symptoms of High Blood Sugar In People Who Don’t Have Diabetes
Once you have been definitively diagnosed with diabetes, your provider may need to figure out if you havetype 1ortype 2 diabetes.
Type 1 diabetes is anautoimmune disorder. With this form of diabetes, your immune system attacks the insulin-producing beta cells in your pancreas. Your provider can order blood tests to see if you haveautoantibodiesthat are associated with the disease. Another test called aC-peptide testcan also be used.
If you have type 1 diabetes, you will need to give yourself insulin oruse an insulin pump, plan your meals carefully, and check your blood sugar often orwear a continuous glucose monitorto control your blood sugar levels.
Type 2 diabetes is not an autoimmune disease. It’s a condition that you get later in life because your body does not respond to or use insulin normally. Your provider will talk to you about your symptoms and look at your blood sugar levels to find out if you have type 2 diabetes.
No matter which type of diabetes you have, your provider will want to do other tests to get a baseline that will help themmonitor how the disease progresses. One of the most important tests is theA1C, which measures your average blood sugar levels over the last three months.
Depending on your results, your provider may want you to make changes in your diet and exercise (medical nutritional therapy). You may need to have routine blood tests every three to six months.
Your provider may also recommend drug therapies, likemetforminorinsulin, to help you better control your blood sugar levels. The current treatment recommendations are:
Learning that you have diabetes can be overwhelming. If you have any questions or concerns, talk to your provider about your treatment plan.
Type 2 Diabetes Doctor Discussion GuideGet our printable guide for your next healthcare provider’s appointment to help you ask the right questions.Download PDFEmail AddressSign UpThank you, {{form.email}}, for signing up.There was an error. Please try again.
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Summary
The oral glucose tolerance test can help diagnose different types of diabetes. During the test, you’ll drink a beverage that has sugar (glucose) in it. Over the next few hours, you will be monitored and tested to see how your body handles the sugar.
The results of the test can help your provider determine if you have diabetes and make sure that you get the right treatment.
A Word From Verywell
According to the Centers for Disease Control and Prevention (CDC), over 38 million Americans are living with diabetes and over 97 million have prediabetes. However, only one in four people with diabetes know they have it, and only one in nine people with prediabetes have been diagnosed.
If you havesymptoms of diabetes, your provider may want you to do the oral glucose tolerance test because it’s very good at picking up the different types of diabetes Early diagnosis can significantly reduce your risk of diabetes-relatedcomplicationsand death.
New Technology Can Measure Glucose Levels in Sweat, No Needles Required
8 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.American Diabetes Association.2. Classification and diagnosis of diabetes: Standards of medical care in diabetes—2022.Diabetes Care. 2022;45(Supplement 1):S17-S38. doi:10.2337/dc22-S002National Institute of Diabetes and Digestive and Kidney Diseases.Diabetes & Prediabetes Tests.American College of Obstetrics and Gynecology.AGOG Practice Bulletin No. 190: Gestational Diabetes Mellitus.Gynecol Obstetrics.2018;131(2):e49-e64. doi:10.1097/AOG.0000000000002501ACOG releases guideline on gestational diabetes.Am Fam Physician.2014 Sep 15;90(6):416-417.Aekplakorn, W. Tantayotai, V.; Numsangkul, S. et al.Detecting prediabetes and diabetes: Agreement between fasting plasma glucose and oral glucose tolerance test in Thai adults.J Diabetes Res.2015;2015:396505. doi:10.1155/2015/386505Yuen L, Bontempo S, Wong VW, Russell H.Hypoglycaemia on an oral glucose tolerance test in pregnancy - Is it clinically significant?Diabetes Res Clin Pract. 2019;147:111-117.U.S. National Library of Medicine.Fasting before a blood test.Centers for Disease Control and Prevention.National diabetes statistics report.
8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.American Diabetes Association.2. Classification and diagnosis of diabetes: Standards of medical care in diabetes—2022.Diabetes Care. 2022;45(Supplement 1):S17-S38. doi:10.2337/dc22-S002National Institute of Diabetes and Digestive and Kidney Diseases.Diabetes & Prediabetes Tests.American College of Obstetrics and Gynecology.AGOG Practice Bulletin No. 190: Gestational Diabetes Mellitus.Gynecol Obstetrics.2018;131(2):e49-e64. doi:10.1097/AOG.0000000000002501ACOG releases guideline on gestational diabetes.Am Fam Physician.2014 Sep 15;90(6):416-417.Aekplakorn, W. Tantayotai, V.; Numsangkul, S. et al.Detecting prediabetes and diabetes: Agreement between fasting plasma glucose and oral glucose tolerance test in Thai adults.J Diabetes Res.2015;2015:396505. doi:10.1155/2015/386505Yuen L, Bontempo S, Wong VW, Russell H.Hypoglycaemia on an oral glucose tolerance test in pregnancy - Is it clinically significant?Diabetes Res Clin Pract. 2019;147:111-117.U.S. National Library of Medicine.Fasting before a blood test.Centers for Disease Control and Prevention.National diabetes statistics report.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
American Diabetes Association.2. Classification and diagnosis of diabetes: Standards of medical care in diabetes—2022.Diabetes Care. 2022;45(Supplement 1):S17-S38. doi:10.2337/dc22-S002National Institute of Diabetes and Digestive and Kidney Diseases.Diabetes & Prediabetes Tests.American College of Obstetrics and Gynecology.AGOG Practice Bulletin No. 190: Gestational Diabetes Mellitus.Gynecol Obstetrics.2018;131(2):e49-e64. doi:10.1097/AOG.0000000000002501ACOG releases guideline on gestational diabetes.Am Fam Physician.2014 Sep 15;90(6):416-417.Aekplakorn, W. Tantayotai, V.; Numsangkul, S. et al.Detecting prediabetes and diabetes: Agreement between fasting plasma glucose and oral glucose tolerance test in Thai adults.J Diabetes Res.2015;2015:396505. doi:10.1155/2015/386505Yuen L, Bontempo S, Wong VW, Russell H.Hypoglycaemia on an oral glucose tolerance test in pregnancy - Is it clinically significant?Diabetes Res Clin Pract. 2019;147:111-117.U.S. National Library of Medicine.Fasting before a blood test.Centers for Disease Control and Prevention.National diabetes statistics report.
American Diabetes Association.2. Classification and diagnosis of diabetes: Standards of medical care in diabetes—2022.Diabetes Care. 2022;45(Supplement 1):S17-S38. doi:10.2337/dc22-S002
National Institute of Diabetes and Digestive and Kidney Diseases.Diabetes & Prediabetes Tests.
American College of Obstetrics and Gynecology.AGOG Practice Bulletin No. 190: Gestational Diabetes Mellitus.Gynecol Obstetrics.2018;131(2):e49-e64. doi:10.1097/AOG.0000000000002501
ACOG releases guideline on gestational diabetes.Am Fam Physician.2014 Sep 15;90(6):416-417.
Aekplakorn, W. Tantayotai, V.; Numsangkul, S. et al.Detecting prediabetes and diabetes: Agreement between fasting plasma glucose and oral glucose tolerance test in Thai adults.J Diabetes Res.2015;2015:396505. doi:10.1155/2015/386505
Yuen L, Bontempo S, Wong VW, Russell H.Hypoglycaemia on an oral glucose tolerance test in pregnancy - Is it clinically significant?Diabetes Res Clin Pract. 2019;147:111-117.
U.S. National Library of Medicine.Fasting before a blood test.
Centers for Disease Control and Prevention.National diabetes statistics report.
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