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; brain and nervous system cells not only rely on glucose for energy, they can only function when glucose levels in the blood remain above a certain level.

The body’s use of glucose hinges on the availability of insulin, a hormone produced by the pancreas. Insulin acts as a traffic director, transporting glucose into the body’s cells, directing the body to store excess energy as glycogen for short-term storage and/or as triglycerides in adipose (fat) cells. 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 secreted to lower them, with the amount of insulin released matched up with the size and content of the meal. If blood glucose levels drop too low, such as might occur in between meals or after a strenuous workout, glucagon (another pancreatic hormone) is secreted to tell the liver to turn some glycogen back into glucose, raising the blood glucose levels. 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 glucose levels in the blood rise, then the body tries to restore the balance, both by increasing insulin production and by excreting glucose in the urine.

Severe, acute hyperglycemia or 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 your skin with a small, pointed lancet. Sometimes, a random urine sample is collected. Some 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 in real time.

Is any test preparation needed to ensure the quality of the sample?
For screening purposes, it is generally recommended that you fast 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. 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.