blood glucose tests Alternative Names
Blood glucose, or blood sugar, is the amount of glucose circulating in the blood. Glucose is another name for sugar and is an important energy source for the body. The body forms glucose when it breaks down the foods that are eaten. For example, bread or cereal is not in a form of energy that the body can use. The body converts these foods into a useable form, glucose.
Measuring the amount of blood glucose in the blood helps evaluate:
Blood glucose can be measured in a number of ways. The various tests that can measure blood glucose include: Who is a candidate for the test?
- how the body is converting and breaking down foods that are eaten
- how the organs in the body that help regulate blood glucose are working
- how the liver is working
A doctor may order a blood glucose test to evaluate the amount of glucose in the blood when a person is suspected of having diabetes. These tests may also be done to rule out other causes of high or low blood glucose.
How is the test performed?
Here's how some of the tests may be done to check blood glucose levels:
For most blood glucose tests, a blood sample is taken from a vein in the arm. First, the skin over the vein is cleaned with an antiseptic. A tourniquet, or a thin strip of rubber, is wrapped around the upper arm to enlarge the veins. A small needle is gently inserted into a vein, and blood is collected for testing in the laboratory. After the tourniquet is removed, a cotton ball will be held over the needle site until bleeding stops. The laboratory then tests the blood sample.
- random blood glucose test. This test is done shortly after a person has eaten or had something to drink. A level of 11 mmols/L (millimoles per litre) or higher may indicate diabetes. Usually if there is a level above 11 mmols/L, a fasting glucose test or oral glucose tolerance test is done to confirm the diagnosis of diabetes.
- fasting blood glucose test. This is the preferred method to diagnose diabetes and rule out other conditions. This test is done after a person has had nothing to eat or drink (except water) for at least 8 hours. This is often called "fasting" and is generally started overnight so the test can be done in the morning. Normal fasting plasma glucose levels are less than 6.0 mmols/L. Fasting plasma glucose levels of more than 7.0 mmols/L on 2 or more tests performed on different days usually indicate diabetes.
- oral glucose tolerance test. A person drinks a premeasured amount of a glucose drink. Two hours later a blood glucose measurement is done. Normal glucose levels with this test are less than 8.0 mmols/L. If the blood glucose is greater than 11 mmols/L, then another test is done on a different day to confirm if the person has diabetes or not. Usually the fasting blood glucose test or the random glucose test is done.
- HbA1c, also known as glycosylated haemoglobin. This test measures the average blood glucose over the past 3 months. It is a good measure of long-term blood glucose control. This test is generally only done in people with diabetes to assess how well their therapy is working.
- Self-monitoring of blood glucose (SMBG) or home blood glucose monitoring. These techniques allow a person to monitor blood glucose at home. This is done only by people who have diabetes. A record of daily blood glucose readings can be kept to follow changes in glucose levels throughout the day. The information collected is useful in deciding if changes need to be made in a person's medications for diabetes.
The home blood glucose monitoring system, on the other hand, uses a small drop of blood from a person's finger. The drop is put onto a special strip of paper, which is then inserted into the blood glucose monitor machine. The monitor measures and displays the blood glucose reading. This test can monitor blood glucose fairly accurately if directions are followed carefully.
What do the test results mean?
Results of each individual test will vary. The normal blood glucose levels also vary depending on which test was performed, whether a person was fasting before the test, and whether any special dietary or glucose substances were given during testing.
Increased levels of blood glucose, a condition known as hyperglycaemia, may be caused by:
Decreased levels of blood glucose, a condition known as hypoglycaemia, may indicate:
- diabetes mellitus
- gestational diabetes, or diabetes that develops during pregnancy
- stress response, including infection, severe burns, or surgery
- Cushing syndrome, a condition in which the level of the hormone cortisol is too high and causes fatigue, weakness, protein loss, swelling, and DM
- pheochromocytoma, which is a noncancerous tumour that causes an increase in certain chemicals that can cause high blood pressure
- kidney failure, such as chronic renal failure
- infection in the pancreas, known as pancreatitis
- diuretics, or water pills
- steroid medications, such as prednisone
- acromegaly, a condition that causes elongation of the bones of the limbs and head
- liver disease, such as cirrhosis
- inadequate therapy for diabetes mellitus
Author: Eileen McLaughlin, RN, BSN
- hypothyroidism, a condition in which too little thyroid hormone is present in the blood
- insulinoma, which is a tumour in the pancreas that causes too much insulin to be produced
- hypopituitarism, which is a condition in which the pituitary gland does not release enough hormone
- Addison's disease, a condition in which there is a decreased amount of the adrenocorticol hormone
- extensive liver disease
- insulin overdose
- malabsorption, or inadequate absorption of nutrients from the stomach or intestines
- blood loss
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request