Blood Sugar Tests Health Article

Fasting plasma glucose test

The fasting plasma glucose test requires an eight-hour fast. The person must have nothing to eat or drink except water. The person's blood is usually collected by a nurse or phlebotomist by sticking a needle into a vein. Either serum, the liquid portion of the blood after it clots, or plasma may be used. Plasma is the liquid portion of unclotted blood that is collected. The ADA recommends a normal range for fasting plasma glucose of 55–109 mg/dL. A glucose level equal to greater than 126 mg/dL is indicative of diabetes. A fasting plasma glucose level of 110–125 gm/dL is referred to as "impaired fasting glucose."

Oral glucose tolerance test (OGTT)

The oral glucose tolerance test is done to see how well the body handles a standard amount of glucose. There are many variations of this test. A two-hour OGTT as recommended by the ADA is described below. The person must have at least 150 grams of carbohydrate each day, for at least three days before this test. The person must take nothing but water and abstain from exercise for 12 hours before the glucose is given. At 12 hours after the start of the fast, the person is given 75 grams of glucose to ingest in the form of a drink or standardized jelly beans. A healthcare provider draws a sample of venous blood two hours following the dose of glucose. The serum or plasma glucose is measured. A glucose concentration equal to or greater than 200 mg/dL is indicative of diabetes. A level below 140 mg/dL is considered normal. A level of 140–199 mg/dL is termed "impaired glucose tolerance."

The glycated (glycosylated) hemoglobin test is used to monitor the effectiveness of diabetes treatment. Glycated hemoglobin is a test that indicates how much glucose was in a person's blood during a two- to three-month window beginning about four weeks prior to sampling. The test is a measure of the time-averaged blood glucose over the 120-day life span of the red blood cells. The normal range for glycated hemoglobin measured as HbA _1c is 3 to 6 percent. Values above 8 percent indicate that a hyperglycemic episode occurred sometime during the window monitored by the test (two to three months beginning four weeks prior to the time of blood collection).

The ADA recommends that glycated hemoglobin testing be performed during a person's first diabetes evaluation, again after treatment is begun and glucose levels are stabilized, then repeated semiannually. If the person does not meet treatment goals, the test should be repeated quarterly.

A related blood test, fructosamine assay, measures the amount of albumin in the plasma that is bound to glucose. Albumin has a shorter half-life than red blood cells, and this test reflects the time-averaged blood glucose over a period of two to three weeks prior to sample collection.

Precautions

Diabetes must be diagnosed as early as possible. If left untreated, it results in progressive vascular disease that may damage the blood vessels, nerves, kidneys, heart, and other organs. Brain damage can occur from glucose levels below 40 mg/dL and coma from levels above 450 mg/dL. For this reason, plasma glucose levels below 40 mg/dL or above 450 mg/dL are commonly used as alert values. Point-of-care and home glucose monitors measure glucose in whole blood rather than plasma and are accurate generally within a range of glucose concentration between 40 and 450 mg/dL. In addition, whole blood glucose measurements are generally 10 percent lower than serum or plasma glucose.

Other endocrine disorders and several medications can cause both hyperglycemia and hypoglycemia. For this reason, abnormal glucose test results must be interpreted by a physician.

Glucose is a labile (affected by heat) substance; therefore, plasma or serum must be separated from the blood cells and refrigerated as soon as possible. Splenectomy can result in an increase and hemolytic anemia can result in a decrease in glycated hemoglobin.

Exercise, diet, anorexia, and smoking affect the results of the oral glucose tolerance test. Drugs that decrease tolerance to glucose and affect the test include steroids, oral contraceptives, estrogens, and thiazide diuretics.

Preparation

Blood glucose tests require either whole blood, serum, or plasma collected by vein puncture or finger puncture. No special preparation is required for a casual blood glucose test. An eight-hour fast is required for the fasting plasma or whole-blood glucose test. A 12-hour fast is required for the two-hour OGTT and three-hour OGTT tests. In addition, the person must abstain from exercise in the 12-hour fasting period. Medications known to affect carbohydrate metabolism should be discontinued three days prior to an OGTT test if possible, and the person must maintain a diet of at least 150 grams of carbohydrate per day for at least three days prior to the fast.