Glucose tolerance tests are used to diagnose diabetes. A random blood glucose test should show levels below 200 mg/dL, while fasting plasma tests should have results of 60 to 99 mg/dL. The oral glucose tolerance test (OGTT) results vary based on the type of diabetes being screened for. Pregnant women undergo an OGTT to screen for gestational diabetes, with normal results being less than 140 mg/dL. The three-hour OGTT is used if glucose levels are higher, with optimal levels being below 180 mg/dL in the first hour, below 155 mg/dL in the second, and no higher than 140 mg/dL in the third hour.
If a patient is suspected of having diabetes, his or her doctor may order a glucose tolerance test. During this procedure, blood is drawn one or more times, often after a period of fasting or consuming a sugary drink. Normal results for a glucose tolerance test vary by test type. A random blood glucose test should typically show glucose levels below 200 mg/dL, and a fasting plasma test typically has results of 60 to 99 mg/dL. Oral glucose tolerance test (OGTT) results, on the other hand, vary based on the strength of the beverage used and whether the screening is for type 2 or gestational diabetes.
No fasting or special diet is required before a random blood glucose test. The patient can usually come to the laboratory for blood sampling at any convenient time of day. If your blood glucose level reads as 200 mg/dL or more, it is outside the normal range. A diagnosis of diabetes typically isn’t made until the patient undergoes further testing, however, which may include a repeat plasma test, fasting plasma test, or an OGTT.
Prior to a fasting plasma test, the patient should refrain from eating or drinking anything other than a small amount of water for eight hours prior to blood sampling. Fasting blood tests are typically scheduled in the morning to allow for fasting while you sleep. Normal results for a glucose tolerance test after fasting can be between 60 and 99 mg/dL; diabetes is usually not confirmed until two fasting plasma tests give results above 126 mg/dL.
The OGTT also requires the patient to fast for eight hours before a blood draw. After the initial blood draw, the patient should drink a liquid that usually contains 75 mg of glucose. Blood is drawn again one and two hours after the drink has been consumed. The fasting glucose level should be between 60 and 99 mg/dL, the one-hour level no more than 200 mg/dL, and the two-hour level no more than 140 mg/dL.
Pregnant women typically undergo an OGTT to screen for gestational diabetes, commonly known as a “one-hour” test, when they’re 24 to 28 weeks pregnant. The patient must fast again for eight hours before the test and have a fasting blood drawn. The drink given to pregnant women generally contains only 50 mg of glucose, and blood is drawn just one hour after ingesting the liquid. Normal results for a glucose test for gestational diabetes are less than 140 mg/dL. If glucose levels are higher, the patient will have a more extensive OGTT, called the “three hour” test.
The three-hour OGTT requires a special diet high in carbohydrates for three days before blood work. Blood samples are taken before taking the glucose drink and every hour for three hours after. Optimal glucose levels are below 180 mg/dL in the first hour, below 155 mg/dL in the second, and no higher than 140 mg/dL in the third hour. Anything above those numbers at this point indicates the patient has gestational diabetes.
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