What Is an Oral Glucose Tolerance Test?

What to expect during to the diabetes test

Nurse drawing woman's blood
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The oral glucose tolerance test (OGTT), also known as the glucose tolerance test, gauges the body’s ability to metabolize sugar (glucose) and clear it from the bloodstream. The test requires you to drink a syrupy solution after a period of fasting. A blood sample is then drawn to determine whether you are metabolizing glucose as you should be. The OGTT can be used to diagnose diabetes, gestational diabetes (diabetes during pregnancy), or prediabetes (elevated blood sugar predictive of type 2 diabetes), among other things.

The OGTT can be performed safely in both adults and children.

Purpose of Test

The OGTT evaluates how the body manages glucose after a meal. Glucose is a type of sugar produced when the body breaks down carbohydrates consumed in food. Some of the glucose will be used for energy; the rest will be stored for future use.

The amount of glucose in your blood is controlled by the hormones insulin and glucagon. If you have too much, the pancreas secretes insulin to help cells absorb and store glucose. If you have too little, the pancreas secretes glucagon so that stored glucose can be released back into the bloodstream.

Under normal circumstances, the body will be able to maintain the ideal balance of blood glucose. However, if any parts of the system are impaired, glucose can rapidly accumulate, leading to high blood sugar (hyperglycemia) and diabetes.

The OGTT is a highly sensitive test that can detect imbalances that other tests miss. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommends the OGTT for the following purposes:

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

The OGTT is only used for diagnosis and screening. It has no role in the monitoring of diabetes or prediabetes.

Among its other uses, the OGTT can be ordered to diagnose reactive hypoglycemia (in which blood sugar drops after eating), acromegaly (an overactive pituitary gland), beta cell dysfunction (in which insulin is not being secreted), and rare disorders affecting carbohydrate metabolism (such as hereditary fructose intolerance).

Types

The OGTT procedure can vary considerably based on the goals of the test. The concentration of the oral glucose solution can differ as can the timing and number of blood draws required. There are even variations in which a low-carbohydrate diet may be prescribed.

There are two standard variations used for screening and diagnostic purposes:

  • A two-hour OGTT, comprised of two blood draws, is used to diagnose diabetes/prediabetes in non-pregnant adults and children.
  • A three-hour OGTT, comprised of four blood draws, is used to screen gestational diabetes.

Pregnancy Recommendations

The American College of Obstetricians and Gynecologists (ACOG) recommends the routine screening for gestational diabetes in all pregnant women between 24 and 28 weeks of gestation.

With that being said, rather than proceeding directly to a three-hour OGTT, doctors will often recommend a one-hour glucose challenge first, which does not require fasting. The one-hour glucose challenge may be ordered before 24 weeks if you are obese, have a family history of diabetes, are diagnosed with polycystic ovary syndrome (PCOS), or have experienced gestational diabetes in the past. If the results of the test are abnormal—with blood glucose values equal to or greater than 140 milligrams per deciliter (mg/dL)—you would be stepped up to the full, three-hour OGTT. Some doctors set the threshold as low as 130 mg/dL.

Advantages and Disadvantages

The OGTT is far more sensitive than the fasting plasma glucose test (FPG) and is often ordered when diabetes is suspected but the FPG returns a normal result. Its ability to detect early impairment means that people with prediabetes can often treat their condition with diet and exercise rather than drugs.

The OGTT is also the only test that can definitively diagnose IGT.

Despite these advantages, the OGTT has its limitations:

  • The OGTT is a time-consuming test, requiring extensive pre-test fasting and a prolonged testing and waiting period.
  • The test results can be influenced by stress, illness, or medications.
  • Blood is less stable after collection, meaning that the results can sometimes be skewed as a result of improper handling or storage of the sample.

In terms of accuracy, the OGTT has a sensitivity (the percentage of correct positive test results) of between 81 percent and 93 percent. This is far better than the FGP, which has a sensitivity of between 45 percent and 54 percent.

Risks and Contraindications

The OGTT is a safe and minimally invasive test that requires two to four blood draws. Infection is uncommon, but may occur.

However, some may have a reaction to the oral glucose solution, most commonly nausea or vomiting. If vomiting occurs during testing, the test may not be completed.

Though you will be monitored and treated accordingly, if needed, know that the OGTT can cause hypoglycemia in some people.

An OGTT should not be performed if you:

  • Already have a confirmed diabetes diagnosis
  • Have an allergy to sugar or dextrose
  • Are recovering from surgery, trauma, or infection
  • Are under extreme psychological stress
  • Have ever experienced hypokalemic paralysis

Before the Test

If you are ill or have been recently ill, even with something as simple as a cold, you cannot take the test. If unsure, call the lab or your doctor.

Timing

Because you have to arrive at the lab in a fasted state, OGTTs are typically scheduled in the morning. You should be prepared to set aside three to four hours, depending on whether you are taking the two-hour or three-hour test.

Since stress and anxiety can affect your blood sugar levels, arrive for your appointment at least 30 minutes in advance so that you have time to settle in and relax.

Location

An OGTT can be performed at a doctor’s office, a clinic, a hospital, or an independent lab facility.

What to Wear

As blood will need to be drawn, either wear short sleeves or a top that allows you to roll up your sleeves easily

Food and Drink

You will need to stop eating and drinking eight to 12 hours before the test (time spent sleeping counts); follow your doctor's advice. You can take the occasional sip of water, if desired.

If you smoke, you will need to stop the day of your appointment until the test is complete. Smoking not only increases insulin output, but it impairs glucose tolerance and raises blood pressure.

Medications

Be sure to advise your doctor about any drugs you may be taking, whether they are prescription, over-the-counter, nutritional, homeopathic, traditional, or recreational. Certain drugs can affect blood glucose and may need to be temporarily stopped.

These may include:

You should never stop taking any chronic medication without first consulting with your doctor.

What to Bring

In addition to your ID and health insurance cards, you may want to bring something to read since you will be sitting for a couple of hours between blood draws. Some people bring headphones and calming music to listen to.

However, avoid video games or anything that may overstimulate you. This is especially true if your child is being tested. Instead, bring a storybook or toys, or download a video onto your laptop or tablet.

You may also want to bring a protein bar or snacks to eat once you have finished, especially if you have a long drive home.

Cost and Health Insurance

An OGTT costs around $50 but may be as much as three to five times more, depending on where you are tested. The test may be covered in part or in full by your health insurance. Prior authorization is usually not required, but, just to be safe, contact your insurance company beforehand to double-check and assess what your co-pay or coinsurance costs will be.

If you are uninsured, shop around for the best price. Independent labs tend to have the best prices compared to doctor's offices or hospitals. You should also ask if the lab has a patient assistance program that either offers a tiered price structure or monthly payments. This is especially useful if you expect to have ongoing lab tests.

During the Test

Unlike a fasting glucose test, which only evaluates your blood in a fasted state, an OGTT includes both fasted and non-fasted results. The test procedures can vary based on whether you are an adult, child, or pregnant.

Pre-Test

On the day of the test, after signing in and confirming your insurance information, you will be taken to an examination room where your height and weight will be recorded. Your temperature and blood pressure may also be taken.

At this point, you will be asked to roll up your sleeve for the blood draw. To do so, the phlebotomist will place an elastic tourniquet around your upper arm.

Throughout the Test

A vein in either the crook of your arm or wrist will be selected and cleansed with an antiseptic wipe. A butterfly needle would then be inserted and 2 milliliters (mL) of blood would be extracted for the baseline fasting result.

Once the needle is removed and the puncture wound is bandaged, you or your child will be given a sugary glucose solution to drink. The formulation used varies as follows:

  • For the two-hour OGTT in adults: An 8-ounce solution comprised of 75 grams of sugar
  • For the two-hour OGTT in children: The dose is calculated at 1.75 grams of sugar per kilogram of weight (1.75 g/kg), with a maximum dose of 75 grams.
  • For the three-hour OGTT: An 8-ounce solution comprised of 100 grams of sugar

Upon drinking the solution, you will return to the reception area for the prescribed waiting time; typically, you cannot leave.

If you are an adult or child being tested for diabetes or prediabetes, you will wait for two hours after drinking the solution and return to the examining room for another blood draw (for a total of two blood draws).

If you are being tested for gestational diabetes, blood samples will be taken one, two, and three hours after drinking the solution (for a total of four blood samples).

While you will be monitored throughout the test to ensure that your glucose levels don’t drop too low, advise the nurse or phlebotomist if you experience any signs of hypoglycemia, including weakness, sweating, anxiety, shakiness, pale skin, hunger, or irregular heartbeat.

Once the necessary samples are obtained, you can return home and resume your normal activities and diet. If you are feeling lightheaded or dizzy, the medical team may ask that you rest a bit before leaving.

After the Test

While side effects are uncommon, some people may experience bloating, nausea, upset stomach, and diarrhea as a result of the oral solution. These can often be relieved with an over-the-counter anti-diarrheal, by sipping ginger tea, or chewing peppermint gum. Some may also experience pain, swelling, or bruising at the site of the blood draw.

Call your doctor if you experience any unusual pain, swelling, or excessive bleeding at the puncture site or have symptoms of infection, including high fever, shivering chills, rapid heart rate, rapid breathing, or shortness of breath.

Interpreting the Results

Your doctor should receive the test results within two to three days. Along with the results will be reference ranges with high and low numeric values. Anything between the high and low values is considered normal. Anything outside of the reference range is either considered abnormally high (often denoted with the letter "H") or abnormally low (denoted with "L").

Two-hour OGTT results for adults or children are interpreted as follows:

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

If the blood glucose value is over 200 mg/dL, the doctor will repeat the test or use another test to confirm the diabetes diagnosis. If both tests are positive, the diagnosis can be considered definitive.

Three-hour OGTT 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 with a repeat OGTT.

The normal reference ranges for a three-hour OGTT are described as follows:

  • 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 one of these values is high, the test is repeated in four weeks. If, after the second test, two or more values are elevated, gestational diabetes is definitively diagnosed.

Follow-Up

Once you have been definitively diagnosed with diabetes, your doctor will sometimes need to distinguish whether you have type 1 or type 2 diabetes. Since type 1 diabetes is an autoimmune disorder in which your immune system attacks insulin-producing beta cells of the pancreas, your doctor can order tests to detect whether you have autoantibodies associated with the disease. Another test, called a C-peptide test, may be used.

Irrespective of the type of diabetes you have, your doctor will perform other tests to obtain baseline values by which to monitor disease progression. Chief among these is the A1c test, which measures your average blood sugars over the previous three months.

Depending on your baseline results, your doctor may recommend changes in your diet and exercise (referred to as medical nutritional therapy, or MNT), and schedule routine blood tests every three to six months.

At other times, your doctor may recommend drug therapies, like metformin or insulin, to better control your blood sugar. The current treatment recommendations are as follows:

  • For prediabetes, metformin is recommended when 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 diabetes, metformin is typically the first oral drug prescribed to control your blood sugar. If needed, other classes of drugs (sulfonylureas, meglitinides, thiazolidinediones, DPP-4 inhibitors, and GLP-1 receptor agonists) may be added. Insulin therapy should be started if you are on dual oral therapy and your A1c has been over 7 percent for two to three months.
  • For gestational diabetes, ACOG recommends that insulin treatment be started when your fasting blood glucose exceeds 95 mg/dL and/or your blood glucose two hours after a meal exceeds 120 mg/dL.

A Word From Verywell

The OGTT is a valuable test that can often confirm diabetes when other tests can’t. If you have symptoms of diabetes but tests fail to provide definitive proof, ask your doctor if the OGTT is an appropriate option.

According to statistics from the Centers for Disease Control and Prevention, over 100 million Americans today are living with diabetes or prediabetes. Of these, only one in four with diabetes are aware of their condition, while only one in nine with prediabetes has been diagnosed.

As such, it important to see a doctor if you experience some or all of the following symptoms:

  • Increased thirst
  • Frequent urination
  • Persistent fatigue
  • Blurred vision
  • Frequent infections or sores that are slow to heal
  • Increased hunger
  • Unexplained weight loss
  • Dark patches of velvety skin, usually around the armpits or neck

Early diagnosis can significantly reduce your risk of diabetes-related disease complications and death.

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