Also called: One Hour Glucose Screening Test, GCT, Gestational Diabetes Screening, 50 Gram Glucose Challenge
A glucose challenge test is a blood test used to screen women for diabetes during pregnancy. The test evaluates the body’s ability to metabolize glucose (blood sugar).
The test is usually ordered between the 24th and 28th weeks of pregnancy, sometimes earlier. Blood is drawn one hour after the patient drinks a sugary solution. Results are compared to normal values.
Abnormal results may indicate gestational diabetes. However, not all women with abnormal test results have diabetes. To diagnose gestational diabetes, a physician will order an additional glucose test.
The terms “glucose challenge test” and “oral glucose tolerance test” (OGTT) are sometimes used interchangeably. However, the OGTT is a more involved blood test that can be used in diagnosing type 2 diabetes and prediabetes in addition to gestational diabetes.
About glucose challenge tests
The glucose challenge test is a blood test used to screen pregnant women for gestational diabetes. The test evaluates the ability to metabolize glucose (blood sugar), the body’s primary source of energy. A person with undiagnosed diabetes will have abnormally high glucose in the blood.
The American Diabetes Association recommends that all women be screened for diabetes during pregnancy. However, some physicians prefer to test only high-risk women, such as those who are overweight or have a family history of diabetes.
The glucose challenge test is typically ordered between the 24th and 28th weeks of pregnancy. Pregnant women who are at a particularly high risk of developing gestational diabetes may be screened earlier, between the 12th and 14th weeks of pregnancy.
During pregnancy, hormones from the placenta begin to temporarily block the action of the mother’s insulin. Gestational diabetes usually appears around the 24th week of pregnancy, a time when the placenta begins producing large quantities of these hormones. The glucose challenge test is used to determine if the mother’s body is able to overcome the insulin resistance on its own, or if glucose is building up in the blood, causing hyperglycemia. Excess glucose can lead to health problems for the mother and the fetus.
The terms “glucose challenge test” and “oral glucose tolerance test” (OGTT) are sometimes used interchangeably. However, the OGTT is a more involved blood test that requires fasting and other preparation. It can be used in diagnosing type 2 diabetes and impaired glucose tolerance (prediabetes) in addition to gestational diabetes.
Before and during the test
A glucose challenge test requires little preparation. Physicians typically do not instruct patients to fast, but may in some cases. They may also recommend withholding certain medications. The test can be performed at a physician’s office or at a laboratory.
On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. The medical professional will ask questions about the patient’s medical history before the test, to determine if the patient is taking any medications that will interfere with the test’s accuracy. The patient will also be asked about any medical problems she may have.
The patient will drink 50 grams of a glucose solution within a few minutes. The solution is extremely sweet and may cause nausea or vomiting in some patients. If vomiting occurs, the test is usually rescheduled for another day. An hour after the patient has ingested the glucose solution, blood will be drawn, usually from a vein (venipuncture) from the inside of the elbow or the back of the hand. Sometimes patients are given the solution at a previous medical appointment and keep it at home so they can drink it before the test and avoid the one-hour wait.
First the puncture site is sterilized with antiseptic and an elastic band is wrapped around the patient’s upper arm. As a result, the pressure restricts blood flow through the vein and causes the veins below the band to fill with blood.
A needle is then inserted into a vein. Occasionally more than one puncture is necessary to locate a vein. Some people may feel some discomfort when the needle is inserted, but most feel only a prick or stinging sensation. Once the needle is inserted into a vein, a tube is attached to collect the blood as it begins to flow out. The elastic band is then removed.
The needle is withdrawn after the blood is collected and a small cotton ball is applied with light pressure over the puncture site. After several minutes a small bandage will be placed on the puncture wound. The entire process takes less than 10 minutes. The sample is sent to a laboratory for analysis and results are returned in several days.
Despite the precautions taken to avoid bruising and soreness, it does sometimes occur. Typically, this is not a cause for concern. To minimize soreness, patients may apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.
After the test
After the glucose challenge test, the sample of blood is analyzed for its glucose (blood sugar) content and compared to a range of glucose levels that indicate whether the patient’s results are normal, or indicate gestational diabetes. The patient may visit the physician to obtain the test results or receive them by telephone.
A blood glucose level below 140 milligrams per deciliter (mg/dL) is considered normal. A patient with a normal test result will usually require no further testing. However, a woman being tested early, between the 12th and 14th weeks of pregnancy, will require another test around the 24th week.
A glucose level of 140 mg/dL or above is considered abnormal and may indicate gestational diabetes. However, not all women with abnormal test results have diabetes. To diagnose gestational diabetes, a physician will order a three-hour oral glucose tolerance test (OGTT).
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about the glucose challenge test:
- Do you recommend a glucose challenge test for me?
- When during my pregnancy should the test take place?
- If I have the test early in my term, will I need to repeat it later?
- Do I need to fast, withhold medication or make any other preparations for the blood test?
- How long does the test take? May I be given the solution in advance so I can drink at home the day of the test and avoid the one-hour wait before blood is drawn?
- If I feel ill after drinking the sugary solution, will I be able to complete the test, or will I need to reschedule it for another day?
- When will my test results be ready? Who will explain them to me, and how?
- What do my test results show?
- Is further testing needed to diagnose if I have gestational diabetes?
- What are my treatment options if I have gestational diabetes?
- How does a glucose challenge test differ from an oral glucose tolerance test? When would I have an oral glucose tolerance test or another glucose blood test instead?
- Is it OK for me to drive myself to and from this test?