Updated: October 28, 2019
GCT or 1-hour glucose challenge test , measures the effect of a glucose solution on blood glucose levels. A glucose solution containing 50 to 100 grams of glucose or sugar is injected and blood sample is taken after one hour.
This screening test does not usually diagnose diabetes but is performed to identify those who require additional testing for gestational diabetes. Normally it is done in the 28th week of pregnancy. Gestational diabetes develops during pregnancy (gestation). Gestational diabetes causes blood sugar levels to become too high like Type 1 and Type 2 diabetes. No additional testing is required if the test results are below 140 mg/dl. If the test is above 140 mg then a 3-hour glucose tolerance test or GTT is ordered which uses a 100 gram glucose solution.
Fasting glucose levels greater than 120 mg/dL or normal daily glucose levels greater than 200 mg/dL clearly indicates diabetes. Glucose tolerance testing is not necessary in this case.
The glucose challenge test is used to screen for gestational diabetes and is generally done between 24 to 28th week of pregnancy. However, it can be done as early as your first prenatal visit if you are at high risk of gestational diabetes due to obesity, a personal history of gestational diabetes, a family history of diabetes or other risk factors.
Abnormal test results early in pregnancy might indicate that you have pre-existing type 2 diabetes that was not previously recognized, rather than gestational diabetes. Gestational diabetes needs to be recognized and treated quickly because it can cause health problems for both mother and baby. Gestational diabetes can lead to various pregnancy complications, such as preeclampsia or excess fetal growth which might increase the risk of birth injuries, birth defects or prompt a C-section delivery if not managed well.
With careful management most women who have gestational diabetes deliver healthy babies. Gestational diabetes can be controlled by eating healthy foods, exercising and, if necessary, taking medication. Controlling blood sugar can prevent a difficult birth and keep you and your baby healthy.
Fasting is not required for this test. You can eat and drink normally before the glucose challenge test.
The glucose challenge test is done in two steps. At first you will drink about 148 milliliters of a syrupy glucose solution that contains 50 grams of sugar when you arrive at your health care provider's lab. You will have to wait there for your blood sugar level to be tested.
A blood sample will be taken from a vein in your arm one hour later which will be used to measure your blood sugar level. Fasting is not required for this test. A band will be tied by a technician around your arm so that the veins of the hand are more prominent to prick the injection.The vein is punctured with a fresh, disposable syringe and the blood is withdrawn after the site is cleaned with an alcohol swab. A cotton swab will be given to you after drawing the blood to place on the place where the injection was pricked. You will be asked to apply pressure on the cotton in order to stop the bleeding.
You might notice a bruise in the place where the injection was pricked on your skin. This is normal and should fade away in a few days. But if you experience unbearable pain, or if the area around the puncture becomes red and swollen, immediately follow up with your doctor. These could be signs of an infection. You can return to your usual activities immediately after the test.
Results of the glucose challenge test are given in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L). A blood sugar level below 140 mg/dL (7.8 mmol/L) is considered normal where as blood sugar level of 140 mg/dL (7.8 mmol/L) or higher might indicate gestational diabetes.
Typically the glucose tolerance test is done if the results of your glucose challenge test indicate the possibility of gestational diabetes. This can help in proper diagnosis.