At a Glance
Why Get Tested?
To determine if your blood glucose level is within a healthy range; to screen for and diagnose diabetes and prediabetes and to monitor for high blood glucose (hyperglycemia) or low blood glucose (hypoglycemia); to check for glucose in your urine
When to Get Tested?
Blood glucose: when you are older than 45 years or have risk factors for diabetes; when you have symptoms suggesting high or low blood glucose; during pregnancy; when 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 diabetics may use a continuous glucose monitor, which uses a small sensor wire inserted beneath the skin of the abdomen to measure blood glucose at frequent intervals and provides a result.
Test Preparation Needed?
In general, it is recommended that you fast (nothing to eat or drink except water) for at least 8 hours before having a blood glucose test. For people 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 health practitioner's instructions.
The Test Sample
What is being tested?
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. A few different protocols may be used to evaluate the glucose level in the blood. See "How is it used?" for more information on these. Sometimes, glucose may be tested in urine.
During digestion, fruits, vegetables, breads and other dietary sources of 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 the effects of insulin. People with untreated diabetes are not able to process and use glucose normally. Those who are not able to produce any or enough insulin (and typically have diabetes autoantibodies) are diagnosed as having type 1 diabetes. Those who are resistant to insulin and may or may not be able to produce sufficient quantities of it may have prediabetes or type 2 diabetes.
Severe, acute changes in blood glucose, either high (hyperglycemia) or low (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 gestational diabetes, which is hyperglycemia that occurs only during pregnancy. 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 a finger with a small, pointed lancet (fingerstick). Sometimes, a random urine sample is collected. Some diabetics may use a continuous glucose monitor, which uses 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 at frequent intervals 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) for at least 8 hours before a blood glucose test. Those who have been diagnosed with diabetes and are monitoring their glucose levels are often tested both while fasting and after meals. For random and timed tests, follow the health practitioner's instructions. A glucose tolerance test requires that the person 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
(2014 September 22). Diagnosing Diabetes and Learning About Prediabetes. American Diabetes Association [On-line information]. Available online at http://www.diabetes.org/diabetes-basics/diagnosis/?loc=db-slabnav through http://www.diabetes.org. Accessed October 2014.
(2014 January). Standards of Medical Care in Diabetes 2014. Diabetes Care Volume 37, Supplement 1 [On-line information]. Available online at http://care.diabetesjournals.org/content/37/Supplement_1/S14.full.pdf+html through http://care.diabetesjournals.org. Accessed September 2014.
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Kishore, P. (Reviewed 2014 June). Diabetes Mellitus (DM). The Merck Manual Professional Edition [On-line information]. Available online through http://www.merckmanuals.com. Accessed October 2014.
Screening for Gestational Diabetes Mellitus: U.S. Preventive Services Task Force Recommendation Statement. Annals of Internal Medicine,18 March 2014, Vol 160, No. 6. Available online at http://annals.org/article.aspx?articleid=1813285&resultClick=24#SummaryofRecommendationsandEvidence through http://annals.org. Accessed November 2014.
(October 2014) U.S. Preventive Services Task Force Draft Recommendation Statement. Abnormal Glucose and Type 2 Diabetes Mellitus in Adults: Screening. Available online at http://www.uspreventiveservicestaskforce.org/Page/Document/RecommendationStatementDraft/screening-for-abnormal-glucose-and-type-2-diabetes-mellitus through http://www.uspreventiveservicestaskforce.org. Accessed November 2014.
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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(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.
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