Oral Glucose Tolerance Test: Uses and 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 someone at 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.

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. Glucose is a type of sugar that is made when the body breaks down carbohydrates from the food that you eat. Some of the glucose will be used for energy and the rest is stored to use later.

The amount of glucose in your blood is controlled by the hormones insulin and glucagon. If you have too much glucose in your blood, your pancreas puts out (secretes) insulin to help cells absorb and store it.

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 to high blood sugar levels (hyperglycemia) and diabetes.

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:

• Screening and diagnosis of prediabetes or impaired glucose tolerance (IGT)       
• Screening and diagnosis of type 2 diabetes
• Screening and diagnosis of gestational diabetes

The oral glucose tolerance test can also be used to diagnose:

• Reactive hypoglycemia (when blood sugar drops after eating)
• Acromegaly (an overactive pituitary gland)
• Beta cell dysfunction (when insulin is not being secreted)
• Rare disorders that affect carbohydrate metabolism (e.g. hereditary fructose intolerance)

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 many blood draws are done can be different for each patient. Some patients may need to follow a low-carbohydrate diet for the test.

There are two standard variations of the test that are used for screening and diagnosis:

• A two-hour oral glucose tolerance test: This version is done with two blood draws and is used to diagnose diabetes or prediabetes in non-pregnant adults and children.
• A three-hour oral glucose tolerance test: This version is done with four blood draws and is used to screen for gestational diabetes.

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 with polycystic 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.

Oral Glucose Tolerance Test Advantages and Disadvantages

The oral glucose tolerance test has some advantages:

• It is more sensitive than the fasting plasma glucose test (FPG). It is often ordered when a provider thinks a person has diabetes but they had a normal FPG test.
• It can help detect blood sugar problems early. That means that people with prediabetes can often treat the condition with diet and exercise and may not need to take medications.
• It is the only test that can definitively diagnose impaired glucose tolerance (IGT).

However, the oral glucose tolerance test also has some limitations:

• The test is time-consuming, requires people to fast, and takes a long time.
• The test results can be affected by stress, illness, and medications.
• Blood is less stable after it's collected, so the results can sometimes be skewed if the sample was not handled or stored correctly.

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.

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:

• Have already been diagnosed with diabetes
• Have an allergy to sugar or dextrose
• Are recovering from surgery, trauma, or infection
• Are under extreme psychological stress
• Have ever experienced hypokalemic paralysis (from low potassium levels)

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 a glucometer.

Before the Test

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.

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.

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:

• Anticonvulsants like Topamax (topiramate) or Depakote (valproate)
• Atypical antipsychotics like Clozaril (clozapine) or Seroquel (quetiapine)
• Corticosteroids like prednisone or Medrol (methylprednisolone)
• Diuretics ("water pills")
• Quinolone antibiotics like Cipro (ciprofloxacin) or levofloxacin
• Statin drugs like Crestor (rosuvastatin) and Lipitor (atorvastatin)
• Salicylates (including aspirin)
• Tricyclic antidepressants like Anafranil (clomipramine) or Tofranil (imipramine)

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.

You'll be waiting around for several hours throughout the test, so you might want to bring something relaxing to do like a book or magazine to read, music to listen to, or a hobby like knitting.

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 a protein bar, to have after your test— especially if it will take you a while to drive home.

Cost and Health Insurance

The oral glucose tolerance test might be covered in part or in full by your health insurance.

Prior authorization is usually not required for an oral glucose tolerance test, but it's still a good idea to ask your insurance carrier how much the copay or coinsurance for the test will be.

If you do 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 a patient assistance program that 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.

During the Test

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 and blood pressure might also be taken.

Next, you will be asked to roll up your sleeve for the first blood 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.

Throughout the Test

To get a sample of your blood, the phlebotomist will choose a vein in either the crook of your arm or wrist. They will clean the spot with an antiseptic wipe.

Usually, a butterfly needle is 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:

• For the two-hour oral glucose tolerance test in adults: An 8-ounce solution with 75 grams of sugar
• For the two-hour oral glucose tolerance test in children: The dose of sugar is calculated based on a child's weight: 1.75 grams of sugar per kilogram of weight (1.75 g/kg), with a maximum dose of 75 grams
• For the three-hour oral glucose tolerance test: An 8-ounce solution with 100 grams of sugar

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.

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.

After the Test

The oral glucose tolerance test usually does not have side effects, but some people have bloating, nausea, upset stomach, and diarrhea after drinking the solution. You can manage these symptoms by taking an over-the-counter anti-diarrheal medication, sipping ginger tea, or chewing peppermint gum.

You may have a little pain, swelling, or bruising on your arms where blood was taken. If you have unusual pain, swelling, or a lot of bleeding where you had blood taken or have signs and symptoms of infection, like a high fever, shivering chills, fast heartbeat/breathing; or shortness of breath, call your provider right away.

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:

• Normal: Below 140 mg/dL
• Prediabetes or IGT: 140 and 199 mg/dL
• Diabetes (presumed): 200 mg/dL and above

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.

Three-hour oral glucose tolerance test results are interpreted differently. For this, a preliminary diagnosis is made based on one or more high glucose values during one or more of the four blood draws. Abnormal values need to be confirmed by having you do the test again.

The normal reference ranges for a three-hour oral glucose tolerance test are:

• Normal in a fasted state: Less than 95 mg/dL
• Normal after one hour: Less than 180 mg/dL
• Normal after two hours: Less than 155 mg/dL
• Normal after three hours: Less than 140 mg/dL

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.

Follow-Up

Once you have been definitively diagnosed with diabetes, your provider may need to figure out if you have type 1 or type 2 diabetes.

Type 1 diabetes is an autoimmune 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 have autoantibodies that are associated with the disease. Another test called a C-peptide test can also be used.

If you have type 1 diabetes, you will need to give yourself insulin or use an insulin pump, plan your meals carefully, and check your blood sugar often or wear a continuous glucose monitor to 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.

Unlike type 1 diabetes that a person has for life, type 2 diabetes can sometimes be reversed by making lifestyle changes and taking medication.

No matter which type of diabetes you have, your provider will want to do other tests to get a baseline that will help them monitor how the disease progresses. One of the most important tests is the A1C, 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, like metformin or insulin, to help you better control your blood sugar levels. The current treatment recommendations are:

• For prediabetes: Studies have shown that making lifestyle changes is the best way to reduce your risk of developing type 2 diabetes. However, some people will need to take medication to help manage their blood sugar. Metformin is recommended if your fasting blood glucose is between 100 to 125 mg/dL and/or your blood glucose two hours after a meal is between 140 and 199 mg/dL.
• For type 2 diabetes: If you have type 2 diabetes, metformin is typically the first oral drug that is prescribed to control your blood sugar. Other classes of drugs like sulfonylureas, meglitinides, thiazolidinediones, DPP-4 inhibitors, and GLP-1 receptor agonists can be added if necessary. Insulin therapy should be started if you are on dual oral therapy and your A1C has been over 7% for two to three months.
• For gestational diabetes: If you are pregnant, ACOG recommends that insulin treatment is started if your fasting blood glucose goes over 95 mg/dL and/or your blood glucose two hours after a meal is higher than 120 mg/dL.

Learning that you have diabetes can be overwhelming. If you have any questions or concerns, talk to your provider about your treatment plan.

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 30 million Americans are living with diabetes and over 84 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 have symptoms 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-related complications and death.