What is a Glucose Challenge Test?

A glucose challenge test (GCT), also called a one hour glucose screening test, 50 gram glucose challenge, or gestational glucose screening test, is a test that measures the blood sugar levels of pregnant women. It can also be used to evaluate the body's ability to metabolize blood sugar or glucose to be used by the different cells inside the body. Some pregnant women are prone to develop high sugar levels during pregnancy, the condition also known as gestational diabetes. In order to identify pregnant women with gestational diabetes, obstetricians frequently request their patients to undergo routine testing. Other obstetricians may only test pregnant women with a strong family history of diabetes or those with increased risk to develop the disease, including those who are obese.

Pregnant women are usually advised to have the GCT between the 24th to the 28th week of pregnancy, sometimes earlier. The test is often done in a lab or clinic, and usually requires no prior preparation. Most medical professionals will give instructions to their patients to stop some medications if there is the possibility of them interfering with the test results.

During the test, the woman is given 50 grams (about 1.8 ounces) of glucose solution, usually orange flavored, to drink in a few minutes. The solution is typically very sweet and may induce nausea and vomiting in some pregnant women. If the woman vomits, the glucose challenge test is often rescheduled to another date. At approximately one hour after drinking the solution, a blood sample is taken from the arm or by pricking the finger. Through that blood sample, the sugar level is then measured and compared to the normal value.

A normal glucose value generally falls below 130 to 140 mg/dl (7.2 to 7.8 mmol/L). If a woman's sugar level falls around 140 mg/dl (7.8 mmol/L) or above, it may indicate that she has developed gestational diabetes. To confirm the high result, obstetricians may request for another test to be done, such as the oral glucose tolerance test (OGTT), which is usually scheduled on another day. This is because not all women with high results may actually have gestational diabetes. The OGTT involves fasting, drinking a glucose solution, and then taking blood samples one, two, and three hours later.