Oral Glucose Tolerance Test (OGTT) | Endocrinology

Oral Glucose Tolerance Test (OGTT)

What is the Oral Glucose Tolerance Test (OGTT)?

  • The oral glucose tolerance test (OGTT) is the gold standard for diagnosing type 2 diabetes.
  • During pregnancy, it is still used to diagnose gestational diabetes.
  • With an oral glucose tolerance test, the person fasts overnight (at least 8 hours, but no more than 16 hours).
  • The next morning, fasting plasma glucose is tested.
  • After this test, the person receives a dose of oral glucose (the dose depends on the duration of the test).
  • There are several methods obstetricians use to perform this test, but what is described here is standard.
  • Generally, glucose is present in the sweetened liquid that a person drinks.
  • Blood samples are taken four times at different times after taking sugar to measure the blood glucose.

Types of oral glucose tolerance test

The oral glucose tolerance test approach varies significantly depending on the objectives of the test. The ratio varies depending on the oral glucose solution and the time required and the number of blood draws. There are also variations in which a low carb diet is recommended.

There are two standard variations for screening and diagnostic purposes:

  • The two-hour OGTT with two blood draws is used to diagnose diabetes/prediabetes in non-pregnant adults and children.
  • A three-hour OGTT with four blood draws is used to evaluate gestational diabetes.

Pregnancy recommendations

The American College of Obstetricians and Gynecologists (ACOG) recommends routine testing for gestational diabetes in pregnant women at 24 and 28 weeks’ gestation.

That said, instead of going straight to a three-hour OGTT, doctors recommend a one-hour glucose test that does not require fasting. If you have these delays, have a family history of diabetes, have polycystic ovarian syndrome (PCOS), or have had gestational diabetes before, you can request the glucose challenge one hour before 24 weeks. The test is unusual: blood glucose values ​​are more than 140 milligrams per deciliter (mg / dL) or more; you go to a full three hour OGTT. Some doctors have determined that the intake is less than 130 mg / dL.

Advantages and disadvantages of oral glucose tolerance test

The oral glucose tolerance test fast is much more sensitive than the plasma glucose test (FPG) and is often ordered when diabetes is suspected but the FPG gives a normal result. 6 Its ability to detect weakness early means that people with prediabetes can often treat their condition with diet and exercise rather than medication. The oral glucose tolerance test is also the only test that can accurately diagnose IGT.

Despite these advantages, the oral glucose tolerance test has its limitations:

  • The oral glucose tolerance testis a time-consuming test that requires an extensive pre-test fast and a long period of testing and waiting.
  • Test results can be affected by stress, illness, or medications.
  • Blood is less stable after collection, which means that results can sometimes be distorted as a result of improper handling or storage of the sample.
  • In terms of precision, OGTT has a sensitivity of between 81 and 93 per cent (percentage of optimal positive test results). It is much better than FGP with a sensitivity of between 45% and 54%.

Risk factors for oral glucose tolerance test

OGTT is a safe and minimally invasive test that requires two to four blood draws. Infection is rare but can occur. However, some may have a reaction to the oral glucose solution, usually nausea or vomiting. If vomiting occurs during the test, the test may not be complete.

They will monitor and treat you if necessary, but be aware that some people experience hypoglycemia during an oral glucose tolerance test.

Do not oral glucose tolerance test if it is:

  • Diabetes has already been diagnosed
  • You have an allergy to sugar or dextrose.
  • Recovering from surgery, injury, or infection
  • They are subject to severe psychological stress.
  • Have you ever experienced hypokalemic paralysis?

Before the oral glucose tolerance test

If you are sick or have a recent illness, even if it is as common as a cold, you may not be tested. If you are not sure, call the laboratory or your doctor.


Since you have to come to the lab when you are fasting, OGTTs are usually scheduled in the morning. You should be prepared to allocate three to four hours, depending on whether you are taking a two or three-hour test.

Since stress and anxiety can affect your blood sugar levels, arrive at least 30 minutes before your appointment so that you have time to calm down and relax.


Oral glucose tolerance test can be done in the doctor’s office, clinic, hospital, or an independent laboratory.

What to wear

Because it is necessary to draw blood, wear short sleeves or a blouse that allows you to easily wrap your sleeves.

Food and drink

You should stop eating and drinking eight to 12 hours before the test (time spent sleeping); Follow your doctor’s advice. You can take an occasional sip of water if you like.

If you smoke, you must suspend the day of your appointment until the test is over. Smoking not only increases insulin production but also weakens glucose tolerance and increases blood pressure.


Inform your doctor about the medications you are taking, their prescription, over-the-counter, nutritional, homoeopathic, traditional, or recreational. Some medications affect blood glucose and may need to be temporarily stopped.

These may include:

  • Blood thinners such as Topamax (Topiramate) or Depocoat (Valproate)
  • Different antipsychotics such as clozaril (clozapine) or serocquel (cutiapine)
  • Corticosteroids such as prednisone or medrol (methylprednisolone)
  • Urination
  • Quinolone antibiotics such as cipro (ciprofloxacin) or levaquin (levofloxacin)
  • Statin medications such as Crester (rosuvastatin) and lipids (atorvastatin)
  • Salicylates including aspirin
  • Tricyclic antidepressants such as anafranil (clomipramine) or tofranil (imipramine)

You should never stop taking chronic medications without first consulting your doctor.

What Brig

In addition to your ID and health insurance cards, you may have to sit for a few hours between blood drawings, so you may want to bring something to read. Some headphones and relaxing music to listen to.

However, avoid video games or anything that exaggerates you. This is especially true if your child is being tested. Instead, bring a storybook or toys or download a video to your laptop or tablet. You may want to bring a protein bar or snack to eat after you’re done, especially if you have a long drive home.

Cost and medical insurance

The test may be covered in whole or in part by your health insurance. Prior authorization is generally not required, but to be safe, check with your insurance company ahead of time to verify and estimate what your copay or coins will cost.

If you are not insured, buy at the best price. Independent laboratories have the best prices compared to doctor’s offices or hospitals. You should also ask if there is a patient support program in the lab that provides tire structure or monthly payments. This can be very helpful if you think there should be lab tests in progress.

During the oral glucose tolerance test

Unlike a fasting glucose test, it only tests your blood in the fasted state, with fasting OGTT and non-fasting results. Testing procedures may vary depending on whether you are an adult, a child, or if you are pregnant.


On the day of the test, after registering and verifying your insurance information, you will be taken to the test room, where your height and weight will be recorded. Your temperature and blood pressure can also be taken.

At this point, you will be asked to lift your sleeve for a blood draw. To do this, the phytotomist places an elastic tourniquet around the upper arm.

Throughout the test

Select the vein on the curve of your arm or wrist and wipe it with an antiseptic wipe. The butterfly needle is then inserted and 2 millilitres (ml) of blood is drawn to obtain the initial fasting result.

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

  • For two hours of OGTT in adults: 8 ounce solution containing 75 grams of sugar
  • For the two-hour OGTT in children: the dose is 1.75 grams of sugar per kilogram of body weight (1.75 g / kg), with a maximum dose of 75 grams.
  • For three hours OGTT: 8-ounce solution with 100 grams of sugar
  • After drinking the solution, you will return to the reception area for the indicated waiting time; In general, you cannot leave.
  • If you are an adult or child tested for diabetes or prediabetes, wait two hours after drinking the solution and return to the testing room for another blood draw (for a total of two blood draws).
  • If you are being tested for gestational diabetes, take blood samples one, two, and three hours after drinking the solution (for a total of four blood samples).
  • You will be monitored throughout the test to make sure your glucose levels are not too low. Consult a nurse or phlebotomist if you experience any signs of hypoglycemia, such as weakness, sweating, anxiety, chills, pale skin, or irregular appetite.
  • After obtaining the necessary samples, you can return home and resume your normal activities and diet. If you feel light-headed or dizzy, the medical team may ask you to get some rest before leaving.

After the oral glucose tolerance test

Although the side effects are unusual, some people experience bloating, nausea, abdominal pain, and diarrhea as a result of the oral solution. These can be alleviated with an over-the-counter antidiarrheal by drinking 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 symptoms of infection such as high fever, chills, rapid heart rate, fast breathing, or shortness of breath.

Explaining the results of oral glucose tolerance test

Your doctor should get the test results in two to three days. The results are accompanied by reference ranges with higher and lower numerical values. Any value between high and low values ​​is considered normal. Anything outside the reference range is considered abnormally high (often indicated by the letter “H”) or abnormally low (indicated by “L”).

The two-hour OGTT results for adults or children are described below:

  • Normal: less than 140 mg / dL
  • Prediabetes or IgT: 140 and 199 mg / dL
  • Diabetes (reversible): 200 mg / dL and more

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

The three-hour OGTT results are interpreted differently. To do this, a preliminary diagnosis is made based on one or more high glucose values ​​in one or more of the four blood draws. Abnormal values ​​should be verified with a repeat oral glucose tolerance test.

Typical forecast ranges for three-hour OGTT are as follows:

  • Common fasting: 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 these values ​​are higher, the test is repeated in four weeks. After the second test, if two or more values ​​increase, gestational diabetes is definitely diagnosed.

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