Gestational diabetes mellitus (GDM) is any form of glucose intolerance disorder which occurs only during pregnancy. There are two types of glucose tests used for the diagnosis of gestational diabetes. This includes the glucose challenge test and the glucose tolerance test (GTT).

The GTT can be used alone for diagnosis of gestational diabetes in what is called the one-step approach or single-step glucose test. This appraoch is recommeded by the International Association of Diabetes and Pregnancy Study Groups (IADPSG). It is done over 2 hours and uses 75 g of glucose. However, it usually leads to false postitive. To overcome this limitation, the American College of Obstetricians and Gynecologists (ACOG) recommends a two-step appraoch to diagnose GDM. This involves screening using the glucose challenge test followed by a 3-hour GTT done using 100 g of glucose. 

Diagnosis of gestational diabetes is done between the 24 and 28 weeks of pregnancy. However, it can also be done during the first antenatal visit to the doctor if mother is at high risk of developing gestational diabetes. If mother previously tested negative between the 24 and 28 weeks, then the test can be repeated between the 32 and 36 weeks of pregnancy

Glucose challenge test

Glucose challenge test, also called the one-hour glucose challenge test, is a glucose screening test used to determine the probability of developing gestational diabetes by evaluating how your body responds to glucose. It does not diagnose gestational diabetes. Therefore, a positive result does not mean you have GDM, but your healthcare provided will recommend a second test (glucose tolerance test) to diagnose the condition. However, a negative result might mean no other test is necessary.

Other names for glucose challenge test are gestational diabetes screening, one-hour glucose tolerance test, one-hour glucose screening, glucose screening, glucose test, or one-hour glucose test.

How to prepare for a glucose challenge test

No special preparation is required before a glucose challenge test, and you do not need to fast.  However, avoid food high in sugar before the test as this can affect your results. Examples of food to avoid before a glucose challenge test include pancakes, donuts, biscuits, cakes, chocolate, sweets, and sugary drinks.

How a glucose challenge test is done

On the day of the days, you will be given an oral glucose solution containing 1.8 ounces (50 grams) of sugar to drink within 5 to 10 minutes. After 1 hour, a blood sample will then be drawn from your vein for analysis. You can’t eat or drink anything from the time you drink the glucose solution until after your blood is drawn. But a few sips of water is usually OK.

How to interpret results for a glucose challenge test

Blood sugar levels less than 140 mg/dL is said to be normal, and no further testing is required. However, a blood glucose level equal or greater than 140 mg/dL is said to be abnormal and hence further testing is required. The healthcare provider will then request a glucose tolerance test (GTT) to confirm diagnosis. A positive result for the glucose challenge test does not mean you have gestational diabetes mellitus. It must be confirmed using a glucose tolerance test.

The range for abnormality may vary depending on the provider. For example, some providers consider blood sugar levels greater than 135 mg/dL to be abnormal.

Glucose tolerance test (GTT)

The glucose tolerance test (GTT), also called oral glucose tolerance test (OGTT), is used to confirm diagnosis of gestational diabetes mellitus following a glucose challenge test. This test evaluates and identifies abnormalities in how your body responds to glucose after a meal. Unlike the glucose challenge test, the GTT is done over a period of 3 hours and requires that you fast for at least 8 to 12 hours (no eating or drinking before the test). The glucose tolerance test is also called one-step glucose test or three-hour glucose test.

How to prepare for a glucose tolerance test during pregnancy

There are steps to be aware of or to follow before taking a glucose tolerance test when pregnant. These instructions will be provided by your healthcare provider and usually include the following:

  • Days before the test, the patient needs to eat or drink normally as usual.
  • Eat at least 150 milligrams (mg) of carbohydrates for the next three days before the test.
  • Do not eat or drink for at least 8 to 12 hours before the test but not more than 16 hours.
  • You may drink few sips of water in the morning before the test.
  • Schedule for the test first thing in the morning.
  • You will be given a glucose solution containing 100 g of glucose.
  • The test will last for at least 3 hours, and you cannot drink or eat anything during the test. Some providers can however allow for few sips of water during the test.
  • You are not allowed to leave until the last blood is drawn.

How a glucose tolerance test is performed during pregnancy

The test begins with blood being drawn to determine your fasting blood glucose level. Afterward, you’ll be given a sweetened drink containing 100 g of glucose for you to drink within 5 minutes (do not exceed 10 minutes). Your blood will then be drawn after one, two and three hours from the time you finished taking your drink.

How to interpret results for a glucose tolerance test

A pregnant woman is said not to have gestational diabetes mellitus if:

  1. Fasting blood glucose is lower than 95 mg/dL.
  2. One-hour glucose is lower than 180 mg/dL
  3. Two-hour glucose is lower than 155 mg/dL
  4. Three-hour glucose is lower than 140 mg/dL

If any two or more of the above values are higher than normal, then you have gestational diabetes. But if only one of the above values is higher than normal, then the test will need to be repeated on a different day.

If you are diagnosed with gestational diabetes, then you will be tested for diabetes 4 to 12 weeks after delivery. Thereafter, you will be screened after every 3 years for the rest of your life.

Two-hour glucose tolerance test for diagnosis of gestational diabetes

Instead of 100 grams, 75 grams of glucose can be used in GTT for the diagnosis of gestational diabetes in a single step as recommeneded  by IADPSG. But this is done over two hours instead of three hours. In this case, a pregnant woman is said to have gestational diabetes if any of the following is true; fasting blood glucose is equal to or greater than 92 mg/dL (5.1 mmol/L), 1 h GTT is equal or greater than 180 mg/dL, or 2 h GTT is equal to or greater than 153 mg/dL (8.5 mmol/L).