Oral glucose tolerance test: A test to determine the body’s ability to handle glucose . In the test, a person fasts overnight (at least 8 but not more than 16 hours). Then first, the fasting plasma glucose is tested.After this test, the person receives 75 grams of glucose (100 grams for pregnant women). Usually, the glucose is in a sweet-tasting liquid that the person drinks. Blood samples are taken up to four times to measure the blood glucose.

Most pregnant women are screened for gestational diabetes, a temporary form of hyperglycemia, between their 24th and 28th week of pregnancy using a version of the OGTT, a 1-hour glucose challenge. If either fasting glucose or a random glucose is above the values used to diagnose diabetes in those who are not pregnant, the woman is considered to have gestational diabetes and neither the screening nor the glucose tolerance test is needed. If the 1-hour level is higher than the defined value, a longer OGTT is performed to clarify the patient’s status.

Why It Is Done?

The oral glucose tolerance test (OGTT) is done to: Check pregnant women for gestational diabetes. When done for this purpose, the test is called a glucose challenge screening test, and it is usually done during the 24th to the 28th week of pregnancy. You have an increased chance of developing gestational diabetes if you: Have had gestational diabetes during a previous pregnancy. Have previously given birth to a baby who weighed more than 8.8lb. Are younger than age 25 and were overweight before getting pregnant. Confirm the presence of gestational diabetes if other blood glucose measurements are high. Screen women who have polycystic ovary syndrome (PCOS) for diabetes.Diagnose prediabetes and diabetes.

How to Prepare for the Test? 

Make sure you eat normally for several days before the test. Do not eat or drink anything for 12 hours prior to the test. You can not eat during the test. Ask your health care provider if you are using medications that can interfere with the test results.The oral glucose tolerance test is conducted by measuring blood glucose levels five times over a period of 3 hours.  In a person without diabetes, the glucose levels in the blood rise following drinking the glucose drink, but then then fall quickly back to normal (because insulin is produced in response to the glucose, and the insulin has a normal effect of lowing blood glucose.)  In a diabetic, glucose levels rise higher than normal after drinking the glucose drink and come down to normal levels much slower (insulin is either not produced, or it is produced but the cells of the body do not respond to it).

Pregnancy and the Oral Glucose Tolerance Test

Pregnancy affects a woman’s ability to metabolize blood sugar. This is why the American Diabetes Association recommends an oral glucose tolerance test, which checks for gestational diabetes, for all expectant mothers. This test is common during the 24th to the 28th week of pregnancy. Typically, the dose of glucose that is given is 50 or 100 grams. Normal values for pregnancy are described below. Values above this range indicate gestational diabetes: For the 50-gram oral glucose tolerance test that is used to screen for gestational diabetes: 1 hour: less than 140 mg/dL

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