What It Is

A blood glucose test measures the amount of glucose (the main type of sugar in the body) in a blood sample.

Glucose is the body's major source of energy. Our bodies break down food into glucose and other nutrients, which are then absorbed into the bloodstream from the gastrointestinal tract. Glucose levels in the blood rise after a meal and trigger the pancreas to make the hormone insulin and release it into the blood.

Insulin works like a key that opens the doors to cells and allows glucose in. Without insulin, glucose can't get into cells and it stays in the bloodstream. As a result, levels of sugar in the blood remain higher than normal.

High blood sugar (hyperglycemia) is a concern because, if left untreated, it can cause health problems, both short-term (such as extreme thirst, frequent urination, and fatigue) and long-term (such as organ failure and nerve damage). Blood sugar that's too low (hypoglycemia) can also be a problem, causing symptoms such as sweating, trembling, and lightheadedness.

Diabetes is the most common cause of abnormal rises in blood sugar. People with diabetes either can't make or can't respond to insulin properly. This means they must carefully monitor their glucose levels and follow a doctor-prescribed management plan that uses diet, medications (such as insulin shots), and exercise to keep those levels within a healthy range.

Why It's Done

The blood glucose test is ordered to measure the amount of sugar in the blood. It may be performed as part of a routine physical, to help diagnose type 1 or type 2 diabetes, or during pregnancy to check for gestational diabetes (high glucose levels that can affect the health of both mother and baby).

In a person with diabetes, frequent glucose testing (both self-testing with a home monitor and testing at the doctor's office) is an important part of any good management plan.

Preparation

Your doctor will let you know if any special preparations are needed for this test. Sometimes it's necessary for kids to fast (not having anything to eat or drink for 8 hours prior to the test). This is known as a fasting blood sugar test. Other times, doctors may want to check levels at specific times, such as right after a meal.

The Procedure

A health professional will usually draw the blood from a vein or a finger prick. For an infant, the blood may be obtained by puncturing the heel with a small needle (lancet). If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic, and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.

What to Expect

Either method (heel or finger prick or vein withdrawal) of collecting a blood sample is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.

Getting the Results

Many doctors, especially those who specialize in treating diabetes, have blood-analysis equipment in their office and will be able to analyze the results right away. Sometimes, though, the doctor also may send a blood sample to the lab.

Risks

The blood glucose test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or bruise)
• pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many children are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help for your child to look away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the blood glucose test, speak with your doctor.

Reviewed by: Steven Dowshen, MD
Date reviewed: February 2011