Blood Glucose Test

Written by Janelle Martel | Published on July 2, 2012
Medically Reviewed by George Krucik, MD

What Is a Blood Glucose Test?

A glucose test measures the amount of glucose in your blood. Glucose, also known as sugar, is your body’s main source of energy. Your body converts the carbohydrates you eat into glucose.

Glucose testing is done primarily to check for type 1 diabetes, type 2 diabetes, and gestational diabetes. Diabetes is a condition that causes your blood glucose to rise. In healthy individuals, the glucose level is managed in the body by a hormone called insulin.

However, if you have diabetes, your body does not make enough insulin or the insulin does not work properly. This causes your blood glucose to rise. If left untreated, high levels of blood glucose can cause organ damage.

More rarely, glucose testing may also be used to test for hypoglycemia, which occurs when the levels of glucose in the blood are too low.

Diabetes and the Blood Glucose Test

Type 1 diabetes is usually diagnosed in children and teenagers whose bodies are not able to produce enough insulin. It is a chronic condition and requires treatment throughout an individual’s life. Late onset Type 1 diabetes has been shown to affect individuals in the 30 to 40 age group as well.

Type 2 diabetes is usually diagnosed in overweight and obese adults. This condition occurs when the body does not make enough insulin, or when the insulin produced is not working properly. The impact of Type 2 diabetes may be reduced through weight loss.

Gestational diabetes occurs when a pregnant woman develops diabetes. Gestational diabetes usually goes away after the woman gives birth.

After receiving a diagnosis, people with diabetes may have to get blood glucose tests to determine if their condition is being managed well. A high glucose level in a person with diabetes may mean that the diabetes is not being managed correctly.

Other possible causes of high blood glucose levels include:

  • pre-diabetes (an individual is at an increased risk of developing type 2 diabetes)
  • hyperthyroidism (overactive thyroid)
  • kidney problems
  • pancreatitis (inflammation of the pancreas)
  • pancreatic cancer

In rare cases, high blood glucose levels could signal acromegaly, Cushing syndrome, kidney failure, or adrenal tumor.

It is also possible to have levels of blood glucose that are too low. However, this result is not as common. Low blood glucose or hypoglycemia may be caused by:

  • insulin overuse
  • starvation
  • hypopituitarism
  • hypothyroidism (underactive thyroid)
  • Addison’s disease, alcohol abuse, liver disease, or insulinoma (in rare cases)

How to Prepare for the Test

There are two different types of glucose blood testing—a fasting glucose test and a random glucose test.

If you are having a fasting glucose test, you should not eat or drink for eight hours before your test. You can only drink plain water. You may want to schedule a fasting glucose test first thing in the morning, so you do not have to fast during the day.

You may eat and drink before a random glucose test.

Severe stress can cause a temporary increase in blood glucose. This stress is usually due to surgery, trauma, stroke, or heart attack. Certain medications can also affect blood glucose levels.

Always tell your doctor about the medications you are on—including prescriptions, over-the-counter drugs, and herbal supplements. He or she may ask you to stop taking some medications or decide to change the dosage before your test.

Medications that can affect blood glucose levels include:

  • acetaminophen
  • corticosteroids
  • steroids
  • diuretics
  • oral contraceptives (birth control pills)
  • hormone therapy
  • aspirin
  • atypical antipsychotics
  • lithium
  • epinephrine
  • tricyclic antidepressants
  • monoamine oxidase inhibitors (MAOIs)
  • phenytoin
  • sulfonylurea medications

What to Expect During the Test

This simple test involves giving only a small sample of blood.

A technician will draw blood from your vein, usually from inside the elbow or on the back of your hand. Before drawing blood, the technician will clean the area with antiseptic to kill any germs. He or she will tie an elastic band around your upper arm, causing your blood to pool in the vein.

The technician will then insert a sterile needle into your vein. The blood will be drawn into a tube and the elastic band removed from around your arm. You may feel slight to moderate pain, similar to a needle prick or a burning sensation. You can reduce your pain by trying to relax your arm.

When the technician is finished drawing blood, he or she will remove the needle, apply pressure to the insertion spot, and apply a bandage. Continue applying pressure for a few minutes to prevent bruising. The sample of blood you gave will then be sent to the lab for testing. Your doctor will follow-up with you to discuss the results.

Test Risks

The chance of experiencing a problem during or after a blood test is low. Possible risks include:

  • multiple puncture wounds if it is difficult to find a vein
  • excessive bleeding
  • lightheadedness or fainting
  • hematoma (blood collecting under the skin)
  • infection
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