If a patient is suspected of having diabetes, his healthcare provider may order a glucose tolerance test. During this procedure, blood is drawn one or more times, often after a period of fasting or consumption of a sugary drink. Normal results for a glucose tolerance test vary by test type. A random plasma glucose test typically should show glucose levels of less than 200 mg/dL, and a fasting plasma test normally has results from 60 to 99 mg/dL. Oral glucose tolerance test (OGTT) results, on the other hand, vary by concentration of the drink used and whether the screening is for type 2 or gestational diabetes.
No fasting or special diet is required before a random plasma glucose test. The patient usually can report to the laboratory for the blood draw at any time of the day that is convenient. If his blood glucose level is found to be at 200 mg/dL or greater, this is outside of the normal range. A diagnosis of diabetes is typically not made until the patient undergoes additional testing, however, which may include a repeat plasma test, a fasting plasma test or an OGTT.
Before a fasting plasma test, the patient must abstain from eating or drinking anything other than a small amount of water for eight hours before the blood draw. Fasting blood tests are typically scheduled in the morning to allow the fasting to occur during sleep. Normal results for a glucose tolerance test after fasting can be anywhere from 60 to 99 mg/dL; diabetes isn't usually confirmed until two fasting plasma tests give results greater than 126 mg/dL.
OGTT also requires the patient to fast for eight hours prior to a blood draw. After the initial blood draw, the patient must drink a liquid that usually contains 75 mg of glucose. Blood is drawn again at one and two hours after the drink is consumed. The fasting glucose level should be 60 to 99 mg/dL, the one-hour level at no more than 200 mg/dL, and the two-hour level must not exceed 140 mg/dL.
Pregnant women typically undergo an OGTT to screen for gestational diabetes, commonly known as the "one-hour" test, when they are 24 to 28 weeks pregnant. The patient must again fast for eight hours before the test and undergo a fasting blood draw. The drink given to pregnant women generally contains only 50 mg of glucose, and blood is only drawn one hour after ingesting the liquid. Normal results for a glucose test for gestational diabetes are under 140 mg/dL. If the glucose levels are higher, the patient will take a more extensive OGTT, called the "three-hour" test.
The three-hour OGTT requires a special diet high in carbohydrates for three days before the blood test. Blood draws take place before taking the glucose drink and hourly for three hours afterward. Optimal glucose levels are less than 180 mg/dL in the first hour, under 155 mg/dL in the second hour, and no more than 140 mg/dL in the third hour. Anything above those numbers at this point indicates the patient has gestational diabetes.
