Information

Glucose screening and glucose tolerance tests

Glucose screening and glucose tolerance tests

Why do I need a glucose screening test during pregnancy?

Most healthcare providers routinely recommend a glucose screening test (also called a one-hour glucose challenge test, or GCT) to check for gestational diabetes.

Gestational diabetes is a high blood sugar (glucose) condition that some women get during pregnancy. Between 2 and 10 percent of expectant mothers develop this condition, making it one of the most common health problems during pregnancy. And because the condition rarely causes any symptoms, testing is the only way to find out whether you have it.

The GCT is a screening test, which means it won't give you a diagnosis. Instead, it's designed to identify as many women as possible who might have gestational diabetes but need more testing to find out. So a positive result doesn't mean that you have gestational diabetes.

In fact, only about a third of women who test positive on the glucose screening test actually have the condition. If you test positive, you'll need to take the glucose tolerance test (GTT) – a longer, more definitive test that tells you for sure whether you have gestational diabetes.

Some providers skip the glucose screening test altogether and instead order a shortened version of the glucose tolerance test, which is also called a one-step test (see below).

When is the glucose screening test taken during pregnancy?

You typically get a glucose screening test between 24 and 28 weeks of pregnancy. However, your provider may want you to be screened earlier than 24 weeks if a routine urine test shows a high level of sugar in your urine or if you're considered high risk. If the results are normal, you'll be screened again at 24 to 28 weeks.

Of course, if you were diagnosed with diabetes before pregnancy, you won't need to be screened. Instead, you'll continue to work with your provider to manage your condition during pregnancy.

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 (it comes in cola, orange, or lime flavor), 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 (see below) 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.

I have the results of my glucose screening test. What's the normal range for blood sugar?

Different providers use different standards for determining whether your level is too high. Some will say that if your one-hour blood sugar level is 140 milligrams of glucose per deciliter of blood plasma (mg/dL) or more, you need to have the glucose tolerance test. Others put the cutoff at 130 mg/dL to catch more women who may have gestational diabetes, even though there are likely to be more false positives this way.

If your blood glucose level for this screening is higher than 200 mg/dL, most providers will consider you diabetic and you won't be required to take the glucose tolerance test.

But any score between 140 and 200 means that you'll have to take the three-hour glucose tolerance test for a definite diagnosis.

What is the glucose tolerance test?

The glucose tolerance test (GTT) is also called the oral glucose tolerance test (OGTT) or the three-hour glucose test. The GTT determines for sure whether you have gestational diabetes. Usually, you'll be asked to take this test if you've received a positive result on the glucose screening test.

In many ways, the GTT is similar to the screening test, but it takes longer and requires multiple blood draws.

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.

If two or more of your readings are abnormal, you'll be diagnosed with gestational diabetes. Your provider will talk to you about a treatment plan.

I have the results of my glucose tolerance test. What's the normal range, and what's considered a high blood sugar level?

This chart shows the levels that the American Diabetes Association considers abnormal at each interval of the three-hour glucose tolerance test:

IntervalAbnormal reading
Fasting95 mg/dl or higher
1 hour180 mg/dl or higher
2 hours155 mg/dl or higher
3 hours140 mg/dl or higher

What is the two-hour glucose tolerance test, or one-step test? Do I have to fast?

The two-hour glucose tolerance test is basically a shortened version of the three-hour tolerance test. If you get this test, you likely won't do the screening test first, which is why the two-hour version is sometimes referred to as the "one-step" glucose tolerance test.

Overnight fast: 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.
  • Sweet drink: Next, you'll drink a very sweet drink, which usually contains 75 grams of glucose.
  • Two more blood draws: Your blood will be drawn at one hour and two hours after the drink.
  • Results: If any of your blood sugar readings are abnormal you will be diagnosed with gestational diabetes.

This chart shows the levels that the American Diabetes Association considers abnormal at each interval of the two-hour glucose tolerance test (you'll notice these differ only slightly from the three-hour glucose tolerance test):

IntervalAbnormal reading
Fasting92 mg/dl or higher
1 hour180 mg/dl or higher
2 hours153 mg/dl or higher

You're slightly more likely to get a gestational diabetes diagnosis with the two-hour test than with the three-hour test because the diagnosis criteria are a little different. The American College of Obstetricians and Gynecologists (ACOG) favors the more traditional two-step approach: glucose screening followed by a three-hour tolerance test if the screening is positive. However, a small number of providers use the one-step test.

Will the glucose tests make me feel sick?

Some moms-to-be feel nauseated after drinking the glucose solution, and a few even throw up. It may help to eat something a few hours before the screening test. If you vomit soon after you've gotten the drink down, you'll have to come back on another day and repeat the test.

It's actually more common for women to feel sick during the three-hour glucose tolerance test, because the solution for that test is twice as sweet or contains twice as much liquid as the one for the screening test, and you have to drink it after fasting.

Does glucose testing carry any risks?

Glucose testing and screening is very low risk. As with any blood draw, there is a slight risk of:

  • Feeling lightheaded or fainting
  • Excessive bleeding
  • Infection at the blood draw site, or buildup of blood under the skin

Is there anything I can do to help me pass the glucose tests?

No. If you have gestational diabetes, the glucose tolerance test will detect it. However, there are steps you can take to reduce your chances of developing gestational diabetes, as well as to keep your blood glucose levels stable even if you do have the condition. These are:

  • Healthy eating: Opt for whole grains, fruits, and vegetables, which are high in fiber, and cut back on foods and drinks that are high in added sugars.
  • Exercise: Moderate exercise, such as walking briskly and swimming, can help keep your blood sugar under control.
  • Maintaining a healthy weight: Use our pregnancy healthy weight gain calculator to estimate how much you can expect to gain and whether you are on target.

Can I skip taking glucose tests?

Yes, you can decline a glucose screening or test, but opting out is not recommended. Since most women with gestational diabetes don't have any symptoms, getting tested may be the only way to find out if you have the condition.

Gestational diabetes puts you and your baby at risk of complications. Detecting gestational diabetes allows you to work with your provider on managing and reducing those risks so you can enjoy a healthy pregnancy.

What is the treatment for gestational diabetes?

If you fail the glucose tolerance test and are diagnosed with gestational diabetes, you'll work with your provider or a diabetes specialist, and possibly a nutritionist. Together you'll come up with a treatment plan, which will cover:

  • Strategies for managing gestational diabetes
  • Diet for gestational diabetes
  • Medication for gestational diabetes

Your high blood sugar should last only as long as your pregnancy. But some women who develop diabetes during pregnancy still have it after delivery, so you'll have another glucose test six to eight weeks after your baby is born.

Learn more

Watch the video: Glucose Tolerance Test GTT for Diabetes (November 2020).