Glucose

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 following birth. 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.

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 (generally 8-10 hours fasting) 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 instructions given to you by your healthcare professional.

There is another test called an oral glucose tolerance test (OGTT). It requires that the person fasts (as described above) for the first blood sample and then drink a liquid containing a specified amount of glucose; a further blood sample is then taken after 2 hours. This test cannot be performed at home, and must be done under supervision of a healthcare professional.