Blood Sugar Tests
Blood sugar or plasma glucose tests are used to determine the concentration of glucose in blood. These tests are used to detect an increased blood glucose (hyperglycemia) or a decreased blood glucose (hypoglycemia).
Blood glucose tests are used in a variety of situations, including the following:
- Screening persons for diabetes mellitus: The American Diabetes Association (ADA) recommends that a fasting plasma glucose (fasting blood sugar) be used to diagnose diabetes. If the person already has symptoms of diabetes, a blood glucose test without fasting, called a casual plasma glucose test, may be performed. In difficult diagnostic cases, a glucose challenge test called a two-hour oral glucose tolerance test is recommended. If the result of any of these three tests is abnormal, it must be confirmed with a second test performed on another day. The same test or a different test can be used, but the result of the second test must be abnormal as well in order to establish a diagnosis of diabetes.
- Blood glucose monitoring: Daily measurement of whole blood glucose identifies diabetics who require intervention to maintain their blood glucose within an acceptable range as determined by their physician. The Diabetes Control and Complications Trial (DCCT) demonstrated that persons with diabetes who maintained blood glucose and glycated hemoglobin (hemoglobin with glucose bound to it) at or near normal decreased their risk of complications by 50 to 75 percent. Based on results of this study, the American Diabetes Association (ADA) recommends routine glycated hemoglobin testing to measure long-term control of blood sugar.
- Diagnosis and differentiation of hypoglycemia: Low blood glucose may be associated with symptoms such as confusion, memory loss, and seizure. Demonstration that such symptoms are the result of hypoglycemia requires evidence of low blood glucose at the time of symptoms and reversal of the symptoms by glucose. In documented hypoglycemia, blood glucose tests are used along with measurements of insulin and C-peptide (a fragment of proinsulin) to differentiate between fasting and postprandial (after a meal) causes.
The body uses glucose to produce the majority 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 promotes cellular production of lipids and glycogen and opposes the action of glucagons, which increases the formation of glucose by cells.
Diabetes may result from a lack of insulin or a subnormal 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 even in the absence of stressed metabolic conditions. 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 percent 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. Ketosis does not develop under normal metabolic conditions but may occur with stress. 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 to 40 percent of females with gestational diabetes go on to develop noninsulin dependent diabetes.
There are a variety of ways to measure a person's blood glucose.
Whole blood glucose tests
Whole blood glucose testing can be performed by a person in his or her home, or by a member of the healthcare 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 this is diluted with tissue fluid. The second drop is applied to the dry reagent test strip or device.
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
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.
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.
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.
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.
After the test or series of tests is completed (and with the approval of his or her doctor), the person should eat, drink, and take any medications that were stopped for the test.
The patient may feel discomfort when blood is drawn from a vein. Bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure should be applied to the puncture site until the bleeding stops 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 this occurs, he or she should immediately inform the physician or nurse.
Normal values listed below are for children. Results may vary slightly from one laboratory to another depending upon the method of analysis used.
- fasting plasma glucose test: 55–109 mg/dL
- oral glucose tolerance test at two hours: less than 140 mg/dL
- glycated hemoglobin: 3–6 percent
- fructosamine: 1.6–2.7 mmol/L for adults (5% lower for children)
- gestational diabetes screening test: less than 140 mg/dL
- cerebrospinal glucose: 40–80 mg/dL
- serous fluid glucose: equal to plasma glucose
- synovial fluid glucose: within 10 mg/dL of the plasma glucose
- urine glucose (random semiquantitative): negative
For the diabetic person, the ADA recommends an ongoing blood glucose goal of less than or equal to 120 mg/dL.
The following results are suggestive of diabetes mellitus and must be confirmed with repeat testing:
- fasting plasma glucose test: greater than or equal to 126 mg/dL
- oral glucose tolerance test at two hours: equal to or greater than 200 mg/dL
- casual plasma glucose test (nonfasting, with symptoms): equal to or greater than 200 mg/dL
The needle used to withdraw the blood only causes pain for a moment. If a child needs to take glucose tests
When to call a doctor
If the needle puncture site continues to bleed, or if hours or days later the site looks infected (red and swollen), then a doctor should be contacted.
Diabetes mellitus—The clinical name for common diabetes. It is a chronic disease characterized by the inability of the body to produce or respond properly to insulin, a hormone required by the body to convert glucose to energy.
Glucose—A simple sugar that serves as the body's main source of energy.
Glycated hemoglobin—A test that measures the amount of hemoglobin bound to glucose. It is a measure of how much glucose has been in the blood during a two to three month period beginning approximately one month prior to sample collection.
See also Diabetes.
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American Diabetes Association (ADA). National Service Center, 1660 Duke St., Alexandria, VA 22314. Web site: <www.diabetes.org/>.
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Mark A. Best