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Definition

Glucose tests are used to determine the concentration of glucose in blood, urine, cerebrospinal fluid, and other body fluids. These tests are used to detect an increased blood glucose (hyperglycemia), a decreased blood glucose (hypoglycemia), increased glucose in the urine (glycosuria), and a decrease in cerebrospinal, serous, and synovial fluid glucose.

Purpose

Glucose tests are used in a variety of situations including:

• Screening persons for diabetes mellitus. The American Diabetes Association (ADA) recommends that a fasting plasma glucose (fasting blood sugar) be used to diagnose diabetes. People without symptoms of diabetes should be tested when they are 45 years old and again every three years. People in high-risk groups should be tested before the age of 45 and tested more frequently. 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.
• Screening for gestational diabetes. Diabetes that occurs during pregnancy is called gestational diabetes. This condition is associated with hypertension, increased birth weight, and a higher risk for preeclampsia. Women who are at risk are screened when they are 24-28 weeks pregnant. A woman is considered at risk if she is older than 25 years, is not at her normal body weight, has a parent or sibling with diabetes, or is in an ethnic group that has a high rate of diabetes (Hispanic, Native American, Asian, African-American).
• 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 at or near normal decreased their risk of complications by 50-75%. 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 associated with neuroglycopenia produces symptoms such as confusion, memory loss, and seizure. Demonstration that such symptoms are the result of hypoglycemia requires evidence of a 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 post-prandial causes.
• Analysis of glucose in body fluids. High levels of glucose in body fluids reflect a hyperglycemic state and is otherwise not clinically significant. However, low body fluid glucose levels indicate increased glucose utilization which is often caused by infection (e.g., meningitis causes a low CSF glucose); inflammatory disease(e.g., rheumatoid arthritis causes a low pleural fluid glucose); or malignancy (e.g., a leukemia or lymphoma such as Hodgkin's disease infiltrating the central nervous system or serous cavity).

Precautions

Diabetes must be diagnosed as early as possible. 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 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% lower than serum or plasma glucose owing to the greater water content of the red blood cells. Results are not definitive beyond the manufacturer's stated measuring range, and should be repeated as soon as possible to avoid hypoglycemic shock, cardiac arrest, coma, and other complications of an extremely abnormal glucose result.

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.

A nurse or phlebotomist who collects the sample for a plasma glucose test should follow standard precautions for the prevention of transmission of bloodborne pathogens. Glucose is a labile substance; therefore, plasma or serum must be separated from the blood cells and refrigerated as soon as possible. Samples that must be transported unrefrigerated to a distant site should be collected in a tube with an additive such as sodium fluoride to inhibit glycolysis. Blood glucose methods are largely free of interferences. However, hemolysis may increase the glucose result when measured by the hexokinase method, and high levels of ascorbic acid may reduce the glucose result when measured by the glucose oxidase method. Glycated hemoglobin measurements may be affected by abnormal hemoglobin pigments, such as methemoglobin and structural hemoglobin abnormalities such as hemoglobin S. Splenectomy can result in an increase and hemolytic anemia 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.

Description

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 (gluconeo-genesis). 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 glucagon 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% 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 abates after delivery, but over a 10-year span approximately 30-40% 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 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 this is diluted with tissue fluid. The second drop is applied to the dry reagent test strip or device. All whole blood glucose analyzers use the glucose oxidase reaction to measure glucose concentration. In the home test kits, the enzymes glucose oxidase and peroxidase, a buffer, and dye are immobilized on the testing devise. When the blood contacts the reaction zone, it hydrates the reagents. The glucose oxidase utilizes oxygen to oxidize the glucose forming gluconic acid and hydrogen peroxide. The peroxidase enzyme catalyzes the oxidation of the dye by the hydrogen peroxide producing a colored product. The test strip or device is inserted into a portable analyzer that measures the amount of color produced. Concentration of gluocse is determined by comparing the color intensity, called the reflectance density, to that for a standard measured the same way. Point-of-care devices often utilize the same method. However, some devices employ the polarographic glucose oxidase method. In this procedure, the glucose oxidase is impregnated into a glucose permeable membrane that covers an electrode. Peroxidase and dye are not required. Glucose from the sample diffuses through the membrane and the glucose oxidase catalyzes the formation of hydrogen peroxide inside the electrode. The peroxide is unstable and reforms oxygen and water. The oxygen is reduced at the cathode of the electrode producing a current that is proportional to glucose concentration.

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 via venipuncture. 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 in an anticoagulant. The glucose is measured by an enzymatic glucose method. The glucose oxidase-peroxidase or glucose oxidase-polarographic methods may be used and are similar to those described above. Two additional methods used are the hexokinase and glucose dehydrogenase methods. These methods both result in the production of NADH (NADPH) in proportion to the glucose concentration in the sample. The reaction is measured in an automated chemistry analyzer which measures light absorption. The amount of light absorbed by the NADH at 340 nm is directly proportional to the glucose in the sample. Enzymatic methods measure no sugar other than glucose, and the same normal range can be used. 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

Normal findings for glucose tolerance test (GTT, oral glucose tolerance test [OGTT])
SOURCE: Pagana, K.D. and T.J. Pagana. Mosby's Diagnostic and Laboratory Test Reference. 3rd ed. St. Louis: Mosby, 1997.
Blood test
Fasting	70–115 mg/dl (<6.4 mmol/L)
30 minutes	<200 mg/dl (<11.1 mmol/L)
1 hour	<200 mg/dl (<11.1 mmol/L)
2 hours	<140 mg/dl (<7.8 mmol/L)
3 hours	70–115 mg/dl (<6.4 mmol/L)
4 hours	70–115 mg/dl (<6.4 mmol/L)
Urine test	Negative

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 health care provider draws a sample of venous blood two hours following the dose of glucose. The serum or plasma glucose is measured by an enzymatic method. 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."

Testing for gestational diabetes

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 less than 140 mg/dL is normal and requires no follow-up. If the glucose is 140 mg/dL or higher, a three-hour oral glucose tolerance test is performed. The same pretest preparation is followed as for the two-hour OGTT described earlier except that 100 grams of glucose is given orally. Blood is drawn at the end of the fast and at one, two, and three hours after the glucose is ingested. Gestational diabetes is diagnosed if two or more of the following results are obtained:• fasting plasma glucose greater than 105 mg/dL

• one-hour plasma glucose greater than 190 mg/dL
• two-hour plasma glucose greater than 165 mg/dL
• three-hour plasma glucose greater than 145 mg/dL

Glycated hemoglobin blood glucose test (G-Hgb)

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 N-terminal valine of the beta globin chain of hemoglobin forms are irreversible amide bond with glucose and other carbohydrates. The additional carbohydrate increases the negative charge of the hemoglobin molecule. When the various hemoglobins are separated by chromatography the hemoglobin bound to glucose is located in the fastest fraction called HbA_1c. Since the glucose inside the red cells is in equilibrium with the plasma glucose, a spurious increase in the plasma level will increase the percentage of glycated hemoglobin. 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 3-6%. 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 following formula estimates the average blood glucose during this window: (% G-Hgb x 33.3 mg/dL) - 86 = average blood glucose. Methods available to measure glycated hemoglobin include column and high performance liquid chromatography, electrophoresis, and ion capture. The first three are based upon the fact that glycated hemoglobin has a greater negative charge than nonglycated hemoglobin. Ion capture is a novel method based upon the ability of glycated hemoglobin to suppress the fluorescence of a dye.

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.

Preparation

Blood glucose tests require either whole blood, serum or plasma collected by venipuncture 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.

Aftercare

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.

Complications

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 their physician or nurse.

Results

Normal values listed below are for children and adults. 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%
• 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

Blood glucose test (blood sugar, fasting blood sugar [FBS])
SOURCE: Pagana, K.D. and T.J. Pagana. Mosby's Diagnostic and Laboratory Test Reference. 3rd ed. St. Louis: Mosby, 1997.
	Normal findings	Possible critical values
Cord	45–96 mg/dl (2.5–5.3 mmol/L)
Premature infant	20–60 mg/dl (1.1–3.3 mmol/L)
Newborn	30–60 mg/dl (1.7–3.3 mmol/L)	<30and>300 mg/dl
Infant	40–90 mg/dl (2.2–5.0 mmol/L)	<40 mg/dl
Child <2 years	60–100 mg/dl (3.3–5.5 mmol/L)
Child >2 years to adult	70–105 mg/dl (3.9–5.8 mmol/L)	Male: <50and>400 mg/dl Female: <40and>400 mg/dl
Elderly	increase in normal range after age 50 years

• 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
• gestational diabetes three-hour oral glucose tolerance test: two or more of the limits below are exceeded
• fasting plasma glucose: greater than 105 mg/dL
• one-hour plasma glucose greater than 190 mg/dL
• two-hour plasma glucose greater than 165 mg/dL
• three-hour plasma glucose greater than 145 mg/dL

Health care team roles

Prior to the test the health care professional administering an OGGT should describe the symptoms of hypoglycemia to the patient and tell the patient to alert a health care worker should he or she experience any of those symptoms. A phlebotomist, or sometimes a nurse, collects the blood, and a clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) or clinical laboratory technician CLT(NCA)/medical laboratory

technician MLT(ASCP) performs the testing. Results are interpreted by a physician. Critically high or low glucose levels should be immediately called to the attention of the patient's nurse or doctor. Physicians and nurses are responsible for educating patients about how to best manage their diabetes.

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KEY TERMS

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Diabetes mellitus—A disease in which a person cannot effectively use glucose to meet the needs of the body. It is caused by a lack of the hormone insulin.

Glucose—The main form of sugar (chemical formula C₆H₁₂O₆) used by the body for 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 2-3 month period beginning approximately one month prior to sample collection.

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Victoria E. DeMoranville