The blood glucose test is ordered to measure the amount of glucose in the blood right at the time of sample collection. It is used to detect both hyperglycemia and hypoglycemia, to help diagnose diabetes, and to monitor glucose levels in persons with diabetes. Blood 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 tolerance test (OGTT / GTT). An OGTT is a series of blood glucose tests. A fasting glucose is collected; then the patient 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. The OGTT may be ordered to help diagnose diabetes and as a follow-up test to an elevated blood glucose.

The American Diabetes Association recommends either the fasting glucose or the OGTT to diagnose diabetes but says that testing should be done twice, at different times, in order to confirm a diagnosis of diabetes.

Most 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. If either fasting glucose or a random glucose is above the values used to diagnose diabetes in those who are not pregnant, the woman is considered to have gestational diabetes and neither the screening nor the glucose tolerance test is needed. If the 1-hour level is higher than the defined value, a longer OGTT is performed to clarify the patient’s status.

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.

In those with suspected hypoglycemia, glucose levels are used as part of the "Whipple triad" to confirm a diagnosis. (See “Is there anything else I should know?” section).

The urine glucose is seldom ordered by itself. At one time, it was used to monitor diabetics, but it has been largely replaced by the more sensitive and “real time” blood glucose. The urine glucose is, however, one of the substances measured 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 patient may have a urinary tract infection, or for a variety of other reasons. The doctor may follow an elevated urine glucose test with blood glucose testing.

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 for glucose may occur during public health fairs or as part of workplace health programs. It may also be ordered when a patient 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 hyperglycemia, such as:

• Increased thirst
• Increased urination
• Fatigue
• Blurred vision
• Slow-healing infections

or symptoms of hypoglycemia, such as:

• Sweating
• Hunger
• Trembling
• Anxiety
• Confusion
• 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. If a patient has pre-diabetes (characterized by fasting or OGTT levels that are higher than normal but lower than those defined as diabetic), the doctor will order a glucose test at regular intervals to monitor the patient’s status. With known diabetics, doctors will order glucose levels in conjunction with other tests such as hemoglobin A1c to monitor glucose control over a period of time. Occasionally, a blood glucose level may be ordered along with insulin and C-peptide to monitor insulin production.

Diabetics may be required to self-check their glucose, once or 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 previously have had gestational diabetes. 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.

High levels of glucose most frequently indicate diabetes, but many other diseases and conditions can also cause elevated glucose. The following information summarizes the meaning of the test results. These are based on the clinical practice recommendations of the American Diabetes Association.

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	Diabetes

Oral Glucose Tolerance Test (OGTT) [except pregnancy] (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	Diabetes

Gestational Diabetes Screening: Glucose Challenge Test (1 hour after a 50-gram glucose drink)
Less than 140* mg/dL (7.8 mmol/L)	Normal screen
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 (100-gram glucose drink)
Fasting*	95 mg/dL (5.3 mmol/L)
1 hour after glucose load*	180 mg/dL (10.0 mmol/L)
2 hours after glucose load*	155 mg/dL (8.6 mmol/L)
3 hours after glucose load* **	140 mg/dL (7.8 mmol/L)
* If two or more values are above the criteria, gestational diabetes is diagnosed. ** A 75-gram glucose load may be used, although this method is not as well validated as the 100-gram OGTT; the 3-hour sample is not drawn if 75 grams is used.

Some of the other diseases and conditions that can result in elevated glucose levels include:

• Acromegaly
• Acute stress (response to trauma, heart attack, and stroke for instance)
• Chronic renal failure
• Cushing syndrome
• Drugs, including: corticosteroids, tricyclic antidepressants, diuretics, epinephrine, estrogens (birth control pills and hormone replacement), lithium, phenytoin (Dilantin), salicylates,
• Excessive food intake
• Hyperthyroidism
• Pancreatic cancer
• Pancreatitis

Low to non-detectible urine glucose results are considered normal. Anything that raises blood glucose levels also has the potential to elevate urine glucose levels. Increased urine glucose levels may be seen with medications, such as estrogens and chloral hydrate, and with some forms of renal disease.

Moderately increased blood levels may be seen with pre-diabetes. This condition, if left un-addressed, often leads to type 2 diabetes.

Low blood glucose levels (hypoglycemia) are also seen with:

• Adrenal insufficiency
• Drinking alcohol
• Drugs, such as acetaminophen and anabolic steroids
• Extensive liver disease
• Hypopituitarism
• Hypothyroidism
• Insulin overdose
• Insulinomas
• Starvation

Hypoglycemia is 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). An actual diagnosis of hypoglycemia requires satisfying the "Whipple triad." These three criteria include:

• Documented low glucose levels (less than 40 mg/dL (2.2 mmol/L) often tested along with insulin levels and sometimes with C-Peptide levels)
• Symptoms of hypoglycemia
• Reversal of the symptoms when blood glucose levels are returned to normal.

Primary hypoglycemia is rare and often diagnosed in infancy. People may have symptoms of hypoglycemia without really having low blood sugar. In such cases, dietary changes such as eating frequent small meals and several snacks a day and choosing complex carbohydrates over simple sugars may be enough to ease symptoms. Those with fasting hypoglycemia may require IV glucose if dietary measures are insufficient.