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Oral Glucose Tolerance Test: Uses And Results


The oral glucose tolerance test measures how well the body can break down and use sugar 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 . 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 , and high blood sugar levels that put someone at risk for type 2 diabetes .

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.

What Is The 2

In some cases, the 2-hour glucose tolerance test may be used in place of the 1-hour glucose tolerance test during pregnancy, and the protocol is basically the same. You fast overnight and then go to the lab in the morning to give a blood sample. Then, you drink a 75-gram glucose solution. One hour and two hours after consumption blood draws are taken to measure the blood glucose.

If levels are higher than normal at any stage of the test, additional testing may be ordered to rule out, or definitively diagnose gestational diabetes.

The test requires at least three days of normal food consumption and activity before fasting. Smoking is not allowed during the test as results may be affected.

How Is The Glucose Screening Test Done Do I Have To Fast

No, you don’t have to fast for the screening test. You can eat and drink that day as you normally would.

Here’s what to expect during and after the test:

  • A sweet drink: When you arrive for the test, you’ll drink a sugar solution that contains 50 grams of glucose. The stuff tastes like a very sweet soda pop , and you have to get all of it down in five minutes. Some providers keep it chilled or let you pour it over ice and drink it cold.
  • A blood draw: An hour later, a blood sample is taken to check your blood sugar level. The result indicates how efficiently your body processes sugar.
  • Results: These should be available in a few days. If the reading is too high, which happens an estimated 15 to 23 percent of the time, you’ll be asked to return for a three-hour glucose tolerance test to see whether you have gestational diabetes. The good news is that most women whose screening test shows elevated blood sugar don’t turn out to have gestational diabetes.

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

Taking A Glucose Tolerance Test

An observational study comparing 2

When you receive a glucose tolerance test, a phlebotomist will draw a sample of your blood through a needle from a vein in your arm after you have fasted for at least eight hours, typically overnight. Then you will drink a sweet drink, and the technician will take more blood samples over the next two to three hours.

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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 . 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 .

Prevent Type 2 Diabetes

If your test results show you have prediabetes, ask your doctor or nurse if there is a lifestyle change program offered through the CDC-led National Diabetes Prevention Program in your community. You can also search for an online or in-person program. Having prediabetes puts you at greater risk for developing type 2 diabetes, but participating in the program can lower your risk by as much as 58% .

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What Are The Preparations Needed For Gtt

GTT is an elaborate blood test, that requires frequent testing and as the special requirements need for GTT are as follows

  • Have a normal diet like any other day.
  • Inform the doctor about the varied prescription drugs you are taking, as certain drugs like corticosteroids, diuretics and anti- depressants can cause false results.
  • Fasting is required for 8 to 10 hours prior to the test and only water is allowed during this period.
  • You might want to avoid using the washroom prior to testing as urine samples might be needed
  • On the morning of the test do not smoke or have coffee or caffeine based product.
  • The GTT is not to be done on a sick person

Beaker Location Container And Temperature

Glucose Tolerance Test (GTT) for Diabetes


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What Does The Blood Glucose Test Involve

All pregnant women are offered this screening test between 24-28 weeks gestation. When other risk factors are present, such as obesity, the test may be offered earlier and then repeated if initially normal. The screening test for GD is called a glucose challenge, which aims to see how your body is handling sugars. The measurement is taken 1 hour after you consume a glucose drink. This test can be conducted at any time of day as it is not a fasting test. If your blood sugar is normal after the challenge, you will not require any more testing. If it is high, you will have a second test. This test is done in the morning while you are fasting and will require you not to eat prior to the test. You will have your glucose tested before consuming a glucose drink and then tested again 1 and 2 hours later. If your glucose is higher than specific cut-off values, it means you have GD. See the table for more specific information.

Screening for gestational diabetes

Higher than 9.0 mmol/L = GD

What Can I Expect If Getting An Oral Glucose Tolerance Test

Before doing an OGTT, a doctor might perform a glucose challenge test first. This is a shortened version of the OGTT.

A glucose challenge test requires no fasting. During the test, youll drink a beverage containing 50 grams of glucose. After 1 hour, a healthcare professional will take a blood sample to check your blood sugar level.

If your blood glucose is higher than 140 mg/dL, it might indicate diabetes. In this case, youll need an OGTT.

An OGTT requires some preparation. Youll need to fast for about 8 hours before the test is performed. This means you cant eat breakfast or drink any liquids, except water, beforehand.

An OGTT includes the following steps:

  • A health worker will take a blood sample from your fingertip, earlobe, or a vein. Theyll test the sample for blood glucose, which will serve as a baseline measurement.
  • Youll drink a concentrated glucose beverage. Most solutions contain 75 grams of glucose.
  • Youll sit or lay down for 1 hour.
  • After 1 hour, healthcare staff will take a blood sample.
  • A healthcare professional may take another blood sample after 2 hours and again at 3 hours.
  • A medical team will measure the blood glucose levels at each test time.

Between testing times, youll need to stay still and avoid drinking a lot of water. Thats because excessive movement and hydration can alter the results.

You might receive an OGTT without receiving a glucose challenge test first.

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What Is The Glucose Challenge Screening Test

No preparation is required prior to the test. During the test, the mother is asked to drink a sweet liquid and then will have blood drawn one hour from having the drink, as blood glucose levels normally peak within one hour. No fasting is required prior to this test.

The test evaluates how your body processes sugar. A high level in your blood may indicate your body is not processing sugar effectively . If the results of this screen are positive, the woman may have the Glucose Tolerance Test performed. It is important to note that not all women who test positive for the Glucose Challenge Screening test are found to have diabetes upon further diagnosis.

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How Effective Are Oral Glucose Tolerance Tests

Specimen Collection Procedure

An OGTT can be used to effectively diagnose various types of diabetes, including prediabetes.

Its useful for confirming a diagnosis after an abnormal HbA1c test result. An A1C test is often part of routine blood tests, but its less sensitive than an OGTT. Because of this, an OGTT can help with making an official diagnosis.

This can alter the results, potentially leading to a misdiagnosis.

An OGTT is also time consuming. You may need to take off from work or school. Theres currently no alternative for the test, according to 2018 research.

An OGTT is considered to be a safe test.

Be sure to eat a meal the night before. This may help reduce discomfort caused by fasting.

For some people, drinking the concentrated glucose solution causes side effects such as:

Additional side effects, such as diarrhea and heart palpitations, are more likely to affect people whove had bariatric surgery.

Other OGTT risks are the same as getting your blood drawn for any reason:

  • skin irritation from adhesive on a bandage

If you have any concerns, talk with a doctor before taking the test.

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What Is The Glucose Tolerance Test Like Do I Have To Fast

Fast overnight: Typically you’re instructed to eat a late meal the night before the test and then to eat or drink nothing after that, except for sips of water. The fasting period is usually 8 to 14 hours before the test, so you’ll want to schedule the test for first thing in the morning.

Here’s what to expect at the lab:

  • First blood draw: When you arrive for the test, a blood sample is taken to measure your fasting blood glucose level.
  • An even sweeter drink: Next, you’ll drink either a more concentrated dose or a larger volume of glucose solution.
  • Three more blood draws: Brace yourself for three more arm pricks, as your blood is tested every hour for the next three hours. The lab tech should alternate arms each time your blood is drawn.

Some tips to make your test more comfortable:

  • Have someone drive you to and from the test in case you feel lightheaded or are low on energy due to fasting.
  • Bring something to distract you, because you have to stay nearby when your blood is not being drawn.
  • Bring something to eat right after the final blood sample is taken, because you’ll probably be starving.

Results: After the test, if one of the readings is abnormal, you may have to take another test later in your pregnancy. Or your provider may ask you to make some changes in your diet and exercise routine. See “What is the treatment for gestational diabetes?” below.

Provocative Glucose Testing Benefits And Challenges

Provocative testing: Oral glucose tolerance test is performed with 75 g of anhydrous glucose in solution, although a lowered dose for young children can be used: 1.75 g/kg, up to 75 g. Blood sugar levels should be obtained at baseline, prior to glucose load and at 2 h after glucose intake. These controlled conditions provide a more comprehensive assessment of dynamic glucose handling. The reproducibility of OGTT results however is not guaranteed, specifically in obese, insulin-resistant children. Some studies have shown only a 30% concordance with findings of impaired glucose tolerance on repeat testing. Confirmation of diabetes diagnosis is still required with a second value in the absence of symptoms.22,23,32

B. Ahrén, in, 2013

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Differential Tests For Glucose Intolerance

As an alternative to OGTT, glucose tolerance may also be determined by administering glucose intravenously. In the intravenous glucose tolerance test , glucose is injected intravenously, usually at a dose of 0.3, 0.5, or 1 g kg1, and circulating glucose is determined before and 8, 10, 15, 20, 30, 40, 50, 60, and 80 min after injection. Glucose tolerance is estimated from the elimination rate, where a glucose elimination constant is calculated. The theory behind this is that the glucose elimination after intravenous glucose displays an exponential function . kg is thus calculated as the slope for the glucose curve following logarithmic transformation of the individual glucose values and is calculated from the formula kg=/t½ where t½ is the half-time of glucose elimination . The unit for kg is percentage of glucose decay per minute. Figure 3 shows this variable. Before OGTT was routinely used, IVGTT was undertaken more frequently. Unless very specific questions are asked, it is currently not used in clinical practice because it is more cumbersome to perform and it identifies only some of the metabolic processes underlying glucose tolerance, mainly insulin secretion, insulin sensitivity, and glucose uptake. Thus, other important aspects, such as glucagon secretion, release of incretin hormones, and hepatic glucose output, which are involved in the overall glucose tolerance and included in the 2-h glucose value after OGTT, contribute only marginally to the kg after IVGTT.

Oral Glucose Tolerance Test Risks

Glucose Tolerance Test (OGTT)

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 .

You do need to have a few blood samples taken, but they do not take long and do not usually lead to any problems .

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 , they will be treated right away.

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What Happens Next

If you have prediabetes, your doctor will talk to you about ways to stop it from turning into a full-blown case. Exercise and weight loss are two ways to lower your risk for type 2 diabetes.

If the test shows you have diabetes, you might get whatâs called an âA1Câ or other tests to confirm the diagnosis. Diet, exercise, and medicine can help control your blood sugar.

Good foods and physical activity can also help control diabetes during pregnancy. Your blood sugar should go back to normal after your baby is born.

But gestational diabetes increases your risk of getting type 2 diabetes after your pregnancy. You’ll need to stay on a healthy diet and exercise plan to avoid a future diabetes diagnosis.

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What Is A 2

The 2-hour glucose tolerance test is used to determine how well a person’s body processes glucose. Glucose is the sugar the body uses for energy. This test is designed to measure how glucose is processed over a period of time by measuring fasting glucose levels as well as levels several hours after ingesting a glucose solution. This type of testing is typically done to aid in the diagnosis of type 2 diabetes.

A glucose tolerance test is typically ordered when someone is believed to be at risk for diabetes. For a more thorough test which includes measurements for 4 separate blood draws, customers may order a 3-hour glucose tolerance test.

Turnaround for the 2-hour glucose tolerance test is typically 1-2 business days.

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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 ever experienced hypokalemic paralysis

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.

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