Glucose tests are used to determine the concentration of glucose in blood, urine, cerebrospinal fluid (CSF), and other body fluids. These tests are used to detect increased blood glucose (hyperglycemia), decreased blood glucose (hypoglycemia), increased glucose in the urine (glycosuria), and decreased glucose in CSF, serous, and synovial fluid glucose.
The results of glucose tests are used in a variety of situations, including:
Diabetes must be diagnosed as early as possible so that treatment can begin. If left untreated, it will result 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. They are accurate, for the most part, within a range of glucose concentration between 40 mg/dL and 450 mg/dL. In addition, whole blood glucose measurements are generally 10% lower than those of serum or plasma glucose.
Other endocrine disorders and a number of medications can cause both hyperglycemia and hypoglycemia. For this reason, abnormal glucose test results must be interpreted by a doctor.
Glucose is affected by heat; therefore, plasma or serum must be separated from the blood cells and refrigerated as soon as possible. Splenectomy , for example, can result in an increase in glycated hemoglobin, but hemolytic anemia can produce a decrease in it.
There are other factors that can also affect the OGTT, such as exercise , diet, anorexia, and smoking. Drugs that decrease tolerance to glucose and affect the test include steroids, oral contraceptives, estrogens, and thiazide diuretics .
The body uses glucose to produce most of the energy it needs to function. Glucose is absorbed from the gastrointestinal tract directly and is also derived from digestion of other dietary carbohydrates. It is also produced inside cells by the processes of glycogen breakdown (glycogenolysis) and reverse glycolysis (gluconeogenesis). Insulin is made by the pancreas and facilitates the movement of glucose from the blood and extracellular fluids into the cells. Insulin also increases the formation of glucose by cells.
Diabetes may result from a lack of insulin or a subnormal (below normal) response to insulin. There are three forms of diabetes: Type I or insulin-dependent (IDDM); type II or noninsulin dependent (NIDDM); and gestational diabetes (GDM). Type I diabetes usually occurs in childhood and is associated with low or absent blood insulin and production of ketones. It is caused by autoantibodies to the islet cells in the pancreas that produce insulin, and persons must be given insulin to control blood glucose and prevent ketosis. Type II accounts for 85% or more of persons with diabetes. It usually occurs after age 40, and is usually associated with obesity. Persons who have a deficiency of insulin may require insulin to maintain glucose, but those who have a poor response to insulin may not. Gestational diabetes is a form of glucose intolerance that first appears during pregnancy. It usually ends after delivery, but over a 10-year span approximately 30–40% of females with gestational diabetes go on to develop NIDDM.
There are a variety of ways to measure a person's blood glucose level.
Whole blood glucose testing can be performed by a person at home or by a member of the health care team outside the laboratory. The test is usually performed using a drop of whole blood obtained by finger puncture. Care must be taken to wipe away the first drop of blood because it is diluted with tissue fluid. The second drop is applied to the dry reagent test strip or device.
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 (person trained to draw blood) by insertion of a needle into a vein in the patient's arm. 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."
The OGTT 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 health care provider draws a sample of venous blood two hours following the dose of glucose. 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 screening test for gestational diabetes is performed between 24 and 28 weeks of pregnancy. No special preparation or fasting is required. The patient is given an oral dose of 50 grams of glucose and blood is drawn one hour later. A plasma or serum glucose level less than 140 mg/dL is normal and requires no follow-up. If the glucose level is 140 mg/dL or higher, a three-hour OGTT is performed. The same pretest preparation is followed for the two-hour OGTT described previously, except that 100 grams of glucose are given orally. Blood is drawn at the end of the fast and at one-, two-, and three-hour intervals after the glucose is ingested. Gestational diabetes is diagnosed if two or more of the following results are obtained:
The glycated (glycosylated) hemoglobin test is used to diagnose diabetes and monitor the effectiveness of treatment. Glycated hemoglobin is a test that indicates how much glucose was in a person's blood during a twoto 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 lifespan of the red blood cells (RBCs). The normal range for glycated hemoglobin measured as HbA 1c is 3–6%. Values above 8% 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 begins 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 halflife than RBCs, and this test reflects the time-averaged blood glucose level over a period of two to three weeks prior to sample collection.
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 (the doctor should provide guidance on this), and the patient must maintain a diet of at least 150 grams of carbohydrate per day for at least three days prior to the fast.
After the test or series of tests is completed (and with the approval of the doctor), the person should eat and drink as normal, and take any medications that were stopped for the test.
The patient may feel discomfort when blood is drawn from a vein. Pressure should be applied to the puncture site until the bleeding stops; this will help to reduce bruising. Warm packs can also be placed over the puncture site to relieve discomfort.
The patient may experience weakness, fainting, sweating, or other reactions while fasting or during the test. If any of these reactions occur, the patient should immediately inform the doctor or nurse.
Normal values listed below are for children and adults. Results may vary slightly from one laboratory to another depending on the method of analysis used.
For the person with diabetes, the ADA recommends an ongoing blood glucose level of less than or equal to 120 mg/dL.
The following results are suggestive of diabetes mellitus, and must be confirmed with repeat testing:
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National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). National Institutes of Health, Building 31, Room 9A04, 31 Center Drive, MSC 2560, Bethesda, MD 208792-2560. (301) 496-3583. http://www.niddk.nih.gov .
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Victoria E. DeMoranville
Mark A. Best