Glucose Screening and Glucose Tolerance Tests

During pregnancy, females may develop a medical condition referred to as Gestational Diabetes, which is characterized by increased blood glucose levels due to pregnancy. The health of both the mother and the baby becomes at risk due to this disease. Common tests used to detect the presence of gestational diabetes in a pregnant female are glucose screening and glucose tolerance test.

Gestational diabetes during pregnancy is one of the most common illnesses that develop during pregnancy, with about 2 to 5% of pregnant females developing the condition. The condition is rarely associated with any symptoms; hence, the only way to detect its presence is by way of blood tests.

Glucose Screening vs. Glucose Tolerance Tests

Several tests exist that can detect the presence of gestational diabetes in a pregnant female. The first preliminary test used for screening is referred to as the Glucose Challenge Screening Test (GCT) and is usually done between 26 to 28 weeks of pregnancy. The second test referred to as the Glucose Tolerance Test (GTT) is performed when the first screening test is positive in a female. GTT helps to diagnose diabetes in a pregnant female by determining whether glucose is used effectively by the body of not.

Similar to most of the screening tests, a diagnosis cannot be obtained by the GCT. Instead, the purpose of GCT is to find out the maximum number of females with the problem and who may need more testing for further diagnosis. Hence, a positive GCT does not indicate the presence of gestational diabetes. In fact, only about 33% of the females who have a positive GCT actually have gestational diabetes.

When Are You Diagnosed with Gestational Diabetes ?

In case only one of your readings of the GTT are abnormal, then your physician will advise you dietary modifications and he may ask you to repeat the test at a later date during pregnancy. However, in case two or more of your readings of the GTT are abnormal then you are diagnosed with gestational diabetes and you will be started on a treatment plan by your physician. It is extremely important to treat diabetes during pregnancy so as to prevent adverse side effects to the health of both the mother and the baby.

Glucose Screening Tests

1. How Is It Done?

During the test you will be given 50 g of sugar in a solution that you have to drink in around 5 minutes. After an hour, your blood sample is taken and is checked for the sugar level. You will get the results in a couple of days. This test is to check the efficiency of your body to utilize sugar. If the test results are high (happens in around 15-23% of the cases), then you will be called for a GTT to diagnose gestational diabetes. It happens quite often that the females who have a high GCT have a normal GTT.

2. How Does It Feel?

Somepregnant females may get a feeling of nausea after drinking the sugar solution and may actually vomit. It is good to eat a couple of hours before you go for the screening test. In case you vomit after drinking the sugar solution then you will be asked to come again on another day for repeat test. It is more common for females to get a feeling of sickness during the three hour glucose tolerance test as the sugar solution for GTT is almost two times sweeter than the solution used for GCT. Moreover, GTT is done after 14 hours of fasting, thereby increasing the chances of nausea even more.

3. How to Interpret the Results

Different standards are used by different physicians to determine normal and abnormal GCT. Some have put the cut off at 140 mg/dL, while some consider any value above 130 mg/dL to be high and requiring GTT (though, with this value as the cut off there are chances of more false positives). However, a reading of higher than 200 mg/dL is considered to be high enough to confirm gestational diabetes and do not require a GTT to confirm the diagnosis. But any value between 140 and 200 mg/dL qualifies you for a GTT to confirm the diagnosis of gestational diabetes.

Glucose Tolerance Tests

1. How to Prepare

Your physician will recommend you to eat at least 150 mg of carbohydrates for three days before the time you will be required to fast for the GTT. You will be asked to fast (you can drink sips of water) for 14 hours before the test; hence, you should schedule the test in the morning hours. Moreover, since you may have low energy levels, you should ask someone to drive you to and fro from the clinic. You may experience slight lightheadedness due to fasting.

2. What Happens in the Process?

During the test, your fasting blood sample will be collected to measure your fasting blood sugar level. Then you will be asked to drink the sugar solution (usually a larger volume than you drank for the GCT or the solution will be more concentrated) for GTT. After you have completed the sugar solution, your blood will be drawn every hour for the next three hours and tested for blood sugar levels.

3. How to Interpret the Results

According to the American Diabetes Association, the following values during the Glucose tolerance test are considered abnormal:


Abnormal Reading


95 mg/dL or higher

One hour after drinking the solution

180 mg/dL or higher

Two hours after drinking the solution

155 mg/dL or higher

Three hours after drinking the solution

140 mg/dL or higher

Watch a video to learn about glucose tolerance test: