Blood glucose test are ordered to measure the amount of glucose in the blood right at the time the sample is collected. They are used to detect both hyperglycemia (high blood sugar) and hypoglycemia (low blood sugar) and to help diagnose diabetes.
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. Risk groups include people with high blood pressure, a family history of diabetes, women who had diabetes during a pregnancy, and non-Caucasian ethnic groups. 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).
- Home blood glucose monitoring is done by diabetics to keep their glucose in an acceptable range. Two landmark studies, the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS) demonstrated that persons with diabetes who maintained blood glucose and hemoglobin A1c (an average of the person's blood glucose for the last three months) as close to normal as possible decreased their risk of complications by 50-75%. Based on results of this study, the American Diabetes Association (ADA) recommends routine hemoglobin A1c testing to measure long-term control of blood sugar.
- Diagnosis and differentiation of hypoglycemia. Low blood glucose associated with 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. Low glucose can occur before eating (fasting) or after eating (post prandial).
- Analysis of glucose in body fluids. High levels of glucose in body fluids reflect a hyperglycemic state. However, low body fluid glucose levels indicate increased glucose utilization which is often caused by infection (e.g., meningitis causes a low CSF glucose in the fluid around the brain); inflammatory disease (e.g., rheumatoid arthritis causes low glucose in the fluid in a joint); or cancer.
Diabetes must be diagnosed as early as possible. If left untreated, it will result in damage to blood vessels, nerves, kidneys, heart, and other organs. 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 red blood cells diluting the glucose in the liquid part of the blood. If a diabetic's glucose is outside the normal range or the range recommended by the health care team, the patient should contact their diabetes health care provider.
|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.|
|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)|
A nurse or phlebotomist who collects the sample for a plasma glucose test should follow standard precautions for the prevention of transmission of blood borne pathogens. Glucose is a changeable substance; therefore, plasma or serum must be separated from the blood cells and refrigerated as soon as possible. Samples that must be transported unrefridgerated to a distant site should be collected in a tube with an additive such as sodium fluoride to prevent the blood cells from using the glucose in the blood serum. 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.
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.
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. Newer blood glucose testing devises use the forearm as the testing site, which uses less blood and causes less pain than the fingertip.
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 (the liquid portion of unclotted blood) can be used. The ADA recommends a normal range for fasting plasma glucose of 55-94 mg/dL. A glucose level equal to greater than 126 mg/dL is indicative of diabetes. A fasting plasma glucose level of 100-125 mg/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. 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. Hemoglobin A1c is a test that indicates how much glucose was in a person's blood during the two- to three-months before sampling. Glucose will stick to hemoglobin (the substance that makes red blood cells red) and the amount of hemoglobin with glucose stuck to it is measured and reported as a percentage. 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 hemoglobin A1c is generally 3-6%. The following formula estimates the average blood glucose during this window: (% G-Hgb × 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 hemoglobin A1c to suppress the fluorescence of a dye.
The ADA recommends that hemoglobin A1c testing be performed during a person's first diabetes evaluation, again after treatment is begun and glucose levels are stabilized, then repeated at least 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.
Studies done in 2001 and 2004 of a new continuous monitoring device in children with type 1 diabetes found frequent and profound episodes of both hypoglycemia and postprandial hyperglycemia, even when hemoglobin A1c levels were satisfactory and beforemeal glucose levels were near the target range. Researchers at Yale found that almost 90% of the peak glucose levels after every meal were above target (more than 180 mg/dl), and half of them were higher than 300 mg/dl. The continuous monitoring also revealed asymptomatic hypoglycemia in almost 70% of the children. Noting that this new device has enabled examination of the limitations of conventional monitoring, the study authors suggest that repeated use of the continuous monitor may help to optimize insulin dosing. Test results are stored in the device's memory and downloaded in a doctor's office. A similar devise which can be read at home has been approved by the Food and Drug Administration (FDA) and was undergoing marketing tests in selected U.S. cities as of late 2005. Also available is a blood glucose monitor wristwatch, called the GlucoWatch, which measures glucose levels through the skin and displays the results on the wristband face
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.
|Blood glucose test (blood sugar, fasting blood sugar [FBS])|
|Normal findings||Possible critical values|
|Source: Pagana, K.D. and T.J. Pagana. Mosby's Diagnostic and Laboratory Test Reference. 3rd ed. St. Louis: Mosby, 1997.|
|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)||<30 and >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: <50 and >400 mg/dl|
Female: <40 and >400 mg/dl
|Elderly||increase in normal range after age 50 years|
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 their physician or nurse.
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: 55-99 mg/dL
- oral glucose tolerance test at two hours: less than 140 mg/dL.
- hemoglobin A1c: 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
- 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
Diabetes mellitus— A disease in which a person can't 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.
Hemoglobin A1c— 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 before the blood was drawn.
Health care team roles
Prior to the test the health care professional administering an OGTT 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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