Glucose Tests

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Also known as: Blood sugar; Fasting blood sugar; blood glucose; Oral Glucose Tolerance Test (OGTT or GTT); urine glucose
Related tests: Insulin, C-Peptide, Urine glucose, HbA1c, Microalbumin, Home Testing

At a Glance

Why Get Tested?

To determine whether or not your blood glucose level is within normal ranges; to screen for, diagnose, and monitor diabetes, and hypoglycaemia (low blood glucose)

When to Get Tested?

If you have symptoms suggesting hyperglycaemia (high blood glucose) or hypoglycemia, or if you are pregnant; if you are diabetic, up to several times a day to monitor glucose levels

Sample Required?

A blood sample taken from a vein in your arm, or for a self check, a drop of blood from your finger. A few 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.

Note: In general, it is recommended that you fast 8 hours before having a blood glucose test , however, persons with diabetes glucose levels are often checked both while fasting and after meals to provide the best control of diabetes.

The Test Sample

What is being tested?

Glucose is a simple sugar that serves as the main source of energy for the body. The carbohydrates we eat are broken down into glucose (and a few other simple sugars), absorbed by the small intestine and circulated throughout the body. Most of the body's cells require glucose for energy production; the brain and nervous system cells rely on glucose for energy, and can only function when glucose levels in the blood remain within a certain range.

The body's use of glucose depends on the availability of insulin, a hormone produced by the pancreas. Insulin acts to control the transport of glucose into the body's cells to be used for energy. Insulin also directs the liver to store excess glucose as glycogen for short term energy storage and promotes the synthesis of fats, which form the basis of a longer term store of energy. We cannot live without glucose or insulin, and they must be in balance.

Normally blood glucose levels rise slightly after a meal, and insulin is released to lower them, with the amount of insulin released dependent upon the size and content of the meal. If blood glucose levels drop too low, such as might occur between meals or after a strenuous exercise, glucagon (another hormone from the pancreas) is produced to tell the liver to release some of its glucose stores, raising the blood glucose levels. If the glucose/insulin system is working properly the amount of glucose in the blood remains fairly stable.

Hyperglycaemia and hypoglycaemia, caused by a variety of conditions, are both hard on the body. Severe, sudden high or low blood glucose levels can be life threatening, causing organ failure, brain damage, coma, and, in extreme cases, death. Long-term high blood glucose levels can cause progressive damage to body organs such as the kidneys, eyes, blood vessels, heart and nerves. Untreated hyperglycaemia that arises during pregnancy (known as 'gestational diabetes') can cause mothers to give birth to large babies who may have low glucose levels. Long-term hypoglycaemia can lead to brain and nerve damage.

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 from your finger by pricking it with a small pointed lancet.  A small number of diabetic patients 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 patient’s clothing.  A digital readout on the device lets the patient know the blood glucose level.

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.

The Test

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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.