The blood glucose test may be used to:
Depending on the purpose of testing, glucose may be measured on a fasting basis (collected after an 8- to 10-hour fast), randomly (anytime), post prandial (after a meal), and/or as part of an oral glucose challenge or tolerance test (OGTT / GTT).
Blood glucose is often measured as part of a group of tests, such as a CMP (Comprehensive Metabolic Panel), during routine physicals. This is done to screen for diabetes, which often causes no symptoms early in its course, and for pre-diabetes – moderately increased blood glucose levels that indicate an increased risk of developing type 2 diabetes. For screening purposes, a CMP or blood glucose test is performed on a fasting basis (fasting blood glucose, FBG).
Many pregnant women are screened for gestational diabetes, a temporary form of hyperglycemia, between their 24th and 28th week of pregnancy using a version of the OGTT, a 1-hour glucose challenge (GCT). For this test, a woman is given a standard amount of a glucose solution to drink. After one hour, her glucose level is measured and if the level is higher than a defined value, then a longer OGTT (usually 2 or 3 hours) is performed to clarify the woman's status. This is the recommendation of the American Congress of Obstetricians and Gynecologists.
In 2011, the American Diabetes Association adopted guidelines that recommend changes in the way women are tested for gestational diabetes. Instead of the GCT described above, a 2-hour OGTT, using a 75-gram glucose drink, is performed. If one or more of her glucose levels at fasting, 1 hour, or 2 hours are above a certain level, then she is diagnosed as having gestational diabetes.
The ADA’s new recommendations follow those proposed in 2010 by the International Association of Diabetes and Pregnancy Study Group (IADPSG). This group based their recommendations on results of the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) trial. This large study found that risk to babies increases with the gradual increase of maternal glucose levels; it determined that there is an increased risk of adverse outcomes for a baby even when only one of the mother’s glucose levels is below the cutpoints used to diagnose gestational diabetes, and slightly lowered some of the cutpoints from those used in older guidelines. (For more on this, read the article Panel Suggests New Criteria for Gestational Diabetes.)
The American Diabetes Association recommends a fasting glucose or a different test, the hemoglobin A1c (A1c), to diagnose diabetes but says that testing should be done twice, at different times, preferably with the same test in order to confirm a diagnosis of diabetes. Another way to diagnose diabetes, especially if the screening test is not diagnostic, is the OGTT test. This test is a series of blood glucose tests. A fasting glucose is collected; then the person being tested drinks a standard amount of a glucose solution to "challenge" their system. This is followed by one or more additional glucose tests performed at specific intervals to track glucose levels over time.
In those with suspected hypoglycemia in which an initial FBG result is low, the glucose test is used as part of the "Whipple triad" to confirm a diagnosis. (See Common Questions #5).
Diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and to determine what oral medications or insulin(s) they may need. This is usually done by placing a drop of blood from a skin prick onto a glucose strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose level.
Urine glucose is one of the substances tested when a urinalysis is performed. A urinalysis may be done routinely as part of a physical or prenatal checkup, when a doctor suspects that a person may have a urinary tract infection or for a variety of other reasons. The doctor may follow up an elevated urine glucose test with blood glucose testing.
When is it ordered?
Blood glucose testing can be used to screen healthy, asymptomatic individuals for diabetes and pre-diabetes because diabetes is a common disease that begins with few symptoms. Screening with a glucose test may occur during public health fairs or as part of workplace health programs. It may also be ordered when someone has a routine physical exam. Screening is especially important for people at high risk of developing diabetes, such as those with a family history of diabetes, those who are overweight, and those who are more than 40 to 45 years old.
The glucose test may also be ordered to help diagnose diabetes when someone has symptoms of high blood glucose (hyperglycemia), such as:
- Increased thirst, usually with frequent urination
- Blurred vision
- Slow-healing infections
or symptoms of low blood glucose (hypoglycemia), such as:
- Blurred Vision
Blood glucose testing is also done in emergency settings to determine if low or high glucose is contributing to symptoms such as fainting and unconsciousness.
Pre-diabetes is characterized by fasting or OGTT levels that are higher than normal but lower than those defined as diabetic. The doctor may order a glucose test at regular intervals to monitor the person's status.
With known diabetics, doctors will order glucose levels periodically in conjunction with other tests such as A1c to monitor glucose control over time. Occasionally, a blood glucose level may be ordered along with insulin and C-peptide to evaluate insulin production.
Diabetics are often required to self-check their glucose, up to several times a day, to monitor glucose levels and to determine treatment options as prescribed by their doctor.
Pregnant women are usually screened for gestational diabetes late in their pregnancies, unless they have early symptoms or have had gestational diabetes with a previous pregnancy. When a woman has gestational diabetes, her doctor will usually order glucose levels throughout the rest of her pregnancy and after delivery to monitor her condition.
What does the test result mean?
High levels of glucose most frequently indicate diabetes, but many other diseases and conditions can also cause an elevated blood glucose. The following information summarizes the meaning of the test results.
Fasting Blood Glucose
|From 70 to 99 mg/dL (3.9 to 5.5 mmol/L)
||Normal fasting glucose
|From 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
||Impaired fasting glucose (pre-diabetes)
|126 mg/dL (7.0 mmol/L) and above on more than one testing occasion
Oral Glucose Tolerance Test (OGTT)
Levels applicable except during pregnancy. Sample drawn 2 hours after a 75-gram glucose drink.
|Less than 140 mg/dL (7.8 mmol/L)
||Normal glucose tolerance
|From 140 to 200 mg/dL (7.8 to 11.1 mmol/L)
||Impaired glucose tolerance (pre-diabetes)
|Over 200 mg/dL (11.1 mmol/L) on more than one testing occasion
Gestational Diabetes Screening: Glucose Challenge Test
Sample drawn 1 hour after a 50-gram glucose drink.
|Less than 140* mg/dL (7.8 mmol/L)
|140* mg/dL (7.8 mmol/L) and over
||Abnormal, needs OGTT (see below)
* Some use a cutoff of 130 mg/dL (7.2 mmol/L) because that identifies 90% of women with gestational diabetes, compared to 80% identified using the threshold of 140 mg/dL (7.8 mmol/L).
Gestational Diabetes Diagnostic: OGTT
|TIME OF SAMPLE COLLECTION
||CURRENT ACOG TARGET LEVEL
||ADA TARGET LEVEL
||Glucose load: Samples drawn after 100-gram glucose drink
||Glucose load: Samples drawn after 75-gram glucose drink
|Fasting (prior to glucose load)
||95 mg/dL (5.3 mmol/L)
||92 mg/dL (5.1 mmol/L)
|1 hour after glucose load
||180 mg/dL (10.0 mmol/L)
||180 mg/dL (10.0 mmol/L)
|2 hours after glucose load
||155 mg/dL (8.6 mmol/L)
||153 mg/dL (8.5 mmol/L)
|3 hours after glucose load
||140 mg/dL (7.8 mmol/L)
||If TWO or more values meet or exceed the target level, gestational diabetes is diagnosed.
||If ONE or more values meet or exceed the target level, gestational diabetes is diagnosed.
Moderately increased blood glucose levels may be seen in those with pre-diabetes. Left un-addressed, pre-diabetes increases the risk of developing type 2 diabetes.
Some other diseases and conditions that can result in an elevated blood glucose level include:
A low level of glucose may indicate hypoglycemia, a condition characterized by a drop in blood glucose to a level where first it causes nervous system symptoms (sweating, palpitations, hunger, trembling, and anxiety), then begins to affect the brain (causing confusion, hallucinations, blurred vision, and sometimes even coma and death). A diagnosis of hypoglycemia uses three criteria known as the Whipple triad. (See the common questions section.).
A low blood glucose level (hypoglycemia) may be seen with:
Low to undetectable urine glucose results are considered normal. Any condition that raises blood glucose such as diabetes or the other conditions listed above also has the potential to elevate the concentration of glucose in the urine.
Increased urine glucose may be seen with medications, such as estrogens and chloral hydrate, and with some forms of kidney disease.
Is there anything else I should know?
Extreme stress can cause a temporary rise in blood glucose. This can be a result of trauma, surgery, heart attack or stroke, for example.
Drugs, including corticosteroids, tricyclic antidepressants, diuretics, epinephrine, estrogens (birth control pills and hormone replacement), lithium, phenytoin, and salicylates, can increase glucose levels, while drugs such as acetaminophen and anabolic steroids can decrease levels.