At a Glance
Why Get Tested?
To determine if your blood glucose level is within a healthy range; to screen for, diagnose, and monitor high blood glucose (hyperglycemia) or low blood glucose (hypoglycemia), diabetes, and pre-diabetes; to check for glucose in your urine
When to Get Tested?
Blood glucose: as part of a regular physical, when you have symptoms suggesting hyperglycemia or hypoglycemia, and during pregnancy; if you are diabetic, self-checks up to several times a day to monitor blood glucose levels
Urine glucose: usually as part of a urinalysis
A blood sample drawn from a vein in your arm or a drop of blood from a skin prick; sometimes a random urine sample is used. Some diabetic patients may use a continuous glucose monitor, which is a small sensor wire inserted beneath the skin of the abdomen that measures blood glucose every five minutes.
Test Preparation Needed?
In general, it is recommended that you fast - nothing to eat or drink except water - 8 hours before having a blood glucose test. In persons with diabetes, glucose levels are often checked both while fasting and after meals to provide the best control of diabetes. For random, timed, and post-meal glucose tests, follow your doctor's instructions.
The Test Sample
What is being tested?
This test measures the amount of glucose in the blood or urine. Glucose is the primary energy source for the body’s cells and the only energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood.
During digestion, fruits, vegetables, breads and other carbohydrates are broken down into glucose (and other nutrients); they are absorbed by the small intestine and circulated throughout the body. Using glucose for energy production depends on insulin, a hormone produced by the pancreas. Insulin facilitates transport of glucose into the body's cells and directs the liver to store excess energy as glycogen for short-term storage and/or as triglycerides in adipose (fat) cells.
Normally, blood glucose rises slightly after a meal and insulin is released by the pancreas into the blood in response, with the amount corresponding to the size and content of the meal. As glucose moves into the cells and is metabolized, the level in the blood drops and the pancreas responds by slowing, then stopping the release of insulin.
If the blood glucose level drops too low, such as might occur in between meals or after a strenuous workout, glucagon (another pancreatic hormone) is secreted to induce the liver to turn some glycogen back into glucose, raising the blood glucose level. If the glucose/insulin feedback mechanism is working properly, the amount of glucose in the blood remains fairly stable. If the balance is disrupted and the glucose level in the blood rises, then the body tries to restore the balance, both by increasing insulin production and by eliminating excess glucose in the urine.
There are a few different conditions that may disrupt the balance between glucose and the pancreatic hormones, resulting in high or low blood glucose. The most common cause is diabetes. Diabetes is a group of disorders associated with insufficient insulin production and/or a resistance to insulin. People with untreated diabetes are not able to process and use glucose normally. Those who are not able to produce enough insulin to process glucose are diagnosed as having type1 diabetes while people who are resistant to insulin have type 2. Either type of diabetic may have acute and/or chronically increased blood glucose levels.
Severe, acute high blood glucose (hyperglycemia) or low blood glucose (hypoglycemia) can be life-threatening, causing organ failure, brain damage, coma, and, in extreme cases, death. Chronically high blood glucose levels can cause progressive damage to body organs such as the kidneys, eyes, heart and blood vessels, and nerves. Chronic hypoglycemia can lead to brain and nerve damage.
Some women may develop hyperglycemia during pregnancy, which is termed gestational diabetes. If untreated, this can cause these mothers to give birth to large babies who may have low glucose levels. Women who have had gestational diabetes may or may not go on to develop diabetes.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm or a drop of blood is taken by pricking the skin, typically on a finger, with a small, pointed lancet (fingerstick). Sometimes, a random urine sample is collected. Some diabetics may use a continuous glucose monitor, which is a small sensor wire inserted beneath the skin of the abdomen and held in place with an adhesive patch. The sensor measures blood glucose levels every five minutes and sends the results to a device that is attached to the person's clothing. A digital readout on the device lets the person know the blood glucose level in real time.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
For screening purposes, fasting is generally recommended - nothing to eat or drink except water - at least 8 hours before a blood glucose test. Those who have been diagnosed with diabetes and are monitoring glucose levels are often tested both while fasting and after meals. For random and timed tests, follow the doctor's instructions. A glucose tolerance test requires that you fast for the first blood sample and then drink a liquid containing a specified amount of glucose. Subsequent blood samples are drawn at specified times.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
(Revised 2011 February). Know your Blood Sugar Numbers. National Diabetes Education Program [On-line information]. PDF available for download at http://ndep.nih.gov/media/knownumbers_eng.pdf through http://ndep.nih.gov. Accessed May 2011.
Olatunbosun, S. (Updated 2011 April 19). Glucose Intolerance. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/119020-overview through http://emedicine.medscape.com. Accessed May 2011.
Dugdale, D. (Updated 2010 May 23). Glucose test – blood. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003482.htm. Accessed May 2011.
Eckman, A. (Updated 2010 April 19). Blood glucose monitoring. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003438.htm. Accessed May 2011.
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Khardori, R. (Updated 2011 May 19). Type 2 Diabetes Mellitus. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/117853-overview through http://emedicine.medscape.com. Accessed May 2011.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby’s Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 497–499.
(Octobert 2008) National Institute of Diabetes and Digestive and Kidney Diseases. Glucose Monitoring. Available online at http://diabetes.niddk.nih.gov/dm/pubs/glucosemonitor/index.htm through http://diabetes.niddk.nih.gov. Accessed September 2011.
American Diabetes Association. Standards of Medical Care in Diabetes—2011. Diabetes Care January 2011 34:S11-S61. Available online at http://care.diabetesjournals.org/content/34/Supplement_1/S11.full through http://care.diabetesjournals.org. Accessed September 2011.
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Sources Used in Previous Reviews
Juvenile Diabetes Research Foundation. Continuous Glucose Monitors (Online information) Available online through http://www.jdrf.org. Accessed March 2008.
Johns Hopkins Health Alert. Using a Continuous Glucose Monitor (Online information). Available online through http://www.johnshopkinshealthalerts.com. Accessed March 2008.
National Institute of Diabetes and Digestive and Kidney Diseases. National Diabetes Information Clearinghouse, Diagnosis of Diabetes (Online information). PDF available for download at http://diabetes.niddk.nih.gov. Accessed March 2008.
Recommendations of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Diabetes Care, Vol 20(7), Pp 1183-97 (1997).
David E. Goldstein, Hsiao-Mei Wiedmeyer, Randie R. Little, Victor Vargas, Satish S Nair, John Reid. Relationship Between Glycohemoglobin (GHB) and Mean Blood Glucose (MBG) in the Diabetes Control and Complications Tiral (DCCT). Diabetes 46 (Sup 1), 8A (1997).
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