Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Glucose


At a Glance

Why Get Tested?

To determine if your blood glucose level is within a healthy range; to screen for and diagnose diabetes and prediabetes and to monitor for high blood glucose (hyperglycemia) or low blood glucose (hypoglycemia); to check for glucose in your urine

When to Get Tested?

Blood glucose: when you are older than 45 years or have risk factors for diabetes; when you have symptoms suggesting high or low blood glucose; during pregnancy; when you are diabetic, self-checks up to several times a day to monitor blood glucose levels

Urine glucose: usually as part of a urinalysis

Sample Required?

A blood sample drawn from a vein in your arm or a drop of blood from a skin prick; sometimes a random urine sample is used. Some diabetics may use a continuous glucose monitor, which uses a small sensor wire inserted beneath the skin of the abdomen to measure blood glucose at frequent intervals and provides a result.

Test Preparation Needed?

In general, it is recommended that you fast (nothing to eat or drink except water) for at least 8 hours before having a blood glucose test. For people with diabetes, glucose levels are often checked both while fasting and after meals to provide the best control of diabetes. For random, timed, and post-meal glucose tests, follow your health practitioner's instructions. 

The Test Sample

What is being tested?

Glucose is the primary energy source for the body's cells and the only energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood. A few different protocols may be used to evaluate the glucose level in the blood. See "How is it used?" for more information on these. Sometimes, glucose may be tested in urine.

During digestion, fruits, vegetables, breads and other dietary sources of carbohydrates are broken down into glucose (and other nutrients); they are absorbed by the small intestine and circulated throughout the body. Using glucose for energy production depends on insulin, a hormone produced by the pancreas. Insulin facilitates transport of glucose into the body's cells and directs the liver to store excess energy as glycogen for short-term storage and/or as triglycerides in adipose (fat) cells.

Normally, blood glucose rises slightly after a meal and insulin is released by the pancreas into the blood in response, with the amount corresponding to the size and content of the meal. As glucose moves into the cells and is metabolized, the level in the blood drops and the pancreas responds by slowing, then stopping the release of insulin.

If the blood glucose level drops too low, such as might occur in between meals or after a strenuous workout, glucagon (another pancreatic hormone) is secreted to induce the liver to turn some glycogen back into glucose, raising the blood glucose level. If the glucose/insulin feedback mechanism is working properly, the amount of glucose in the blood remains fairly stable. If the balance is disrupted and the glucose level in the blood rises, then the body tries to restore the balance, both by increasing insulin production and by eliminating excess glucose in the urine.

There are a few different conditions that may disrupt the balance between glucose and the pancreatic hormones, resulting in high or low blood glucose. The most common cause is diabetes. Diabetes is a group of disorders associated with insufficient insulin production and/or a resistance to the effects of insulin. People with untreated diabetes are not able to process and use glucose normally. Those who are not able to produce any or enough insulin (and typically have diabetes autoantibodies) are diagnosed as having type 1 diabetes. Those who are resistant to insulin and may or may not be able to produce sufficient quantities of it may have prediabetes or type 2 diabetes.

Severe, acute changes in blood glucose, either high (hyperglycemia) or low (hypoglycemia), can be life-threatening, causing organ failure, brain damage, coma, and, in extreme cases, death. Chronically high blood glucose levels can cause progressive damage to body organs such as the kidneys, eyes, heart and blood vessels, and nerves. Chronic hypoglycemia can lead to brain and nerve damage.

Some women may develop gestational diabetes, which is hyperglycemia that occurs only during pregnancy. If untreated, this can cause these mothers to give birth to large babies who may have low glucose levels. Women who have had gestational diabetes may or may not go on to develop diabetes.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or a drop of blood is taken by pricking a finger with a small, pointed lancet (fingerstick). Sometimes, a random urine sample is collected. Some diabetics may use a continuous glucose monitor, which uses a small sensor wire inserted beneath the skin of the abdomen and held in place with an adhesive patch. The sensor measures blood glucose levels at frequent intervals and sends the results to a device that is attached to the person's clothing. A digital readout on the device lets the person know the blood glucose level in real time.

Is any test preparation needed to ensure the quality of the sample?

For screening purposes, fasting is generally recommended (nothing to eat or drink except water) for at least 8 hours before a blood glucose test. Those who have been diagnosed with diabetes and are monitoring their glucose levels are often tested both while fasting and after meals. For random and timed tests, follow the health practitioner's instructions. A glucose tolerance test requires that the person fast for the first blood sample and then drink a liquid containing a specified amount of glucose; subsequent blood samples are drawn at specified times.

The Test

How is it used?

The blood glucose test may be used to:

  • Detect high blood glucose (hyperglycemia) and low blood glucose (hypoglycemia)
  • Screen for diabetes in people who are at risk before signs and symptoms are apparent; in some cases, there may be no early signs or symptoms of diabetes. Screening can therefore be useful in helping to identify it and allowing for treatment before the condition worsens or complications arise.
  • Help diagnose diabetes, prediabetes and gestational diabetes
  • Monitor glucose levels in people diagnosed with diabetes

A few different testing protocols may be used to evaluate blood glucose levels, depending on the purpose.

Screening and Diagnosis
The following tests may be used for screening and diagnosis of type 1, type 2 or prediabetes. (Gestational diabetes testing is different—see below.) If the initial screening result from one of the tests is abnormal, the test is repeated on another day. The repeat result must also be abnormal to confirm a diagnosis of diabetes.

  • Fasting glucose (fasting blood glucose, FBG) – this test measures the level of glucose in the blood after fasting for at least 8 hours.
  • 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done (see above), then drinks a 75-gram glucose drink. Another blood sample is drawn 2 hours after the glucose drink. This protocol "challenges" the person's body to process the glucose. Normally, the blood glucose level rises after the drink and stimulates the pancreas to release insulin into the bloodstream. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose level is expected to decrease again. When a person is unable to produce enough insulin, or if the body's cells are resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells and the blood glucose level remains high.
  • A different test called hemoglobin A1c may be used as an alternative to glucose testing for screening and diagnosis. (For more, see the article on A1c.).

Sometimes a blood sample may be drawn and glucose measured when a person has not been fasting, for example, when a comprehensive metabolic panel (CMP) is performed. If the result is abnormal, it is typically followed up with a fasting blood glucose test or a GTT.

Glucose blood tests are also used to screen pregnant women for gestational diabetes between their 24th and 28th week of pregnancy. The American Diabetes Association and the U.S. Preventive Services Task Force recommend that pregnant women not previously known to have diabetes be screened and diagnosed, using either a one-step or two-step approach. The American College of Obstetricians and Gynecologists (ACOG) recommends the two-step approach.

  • One-step 2-hour oral glucose tolerance test (OGTT). After a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values needs to be above a cutoff value for diagnosis.
  • Two-step
    • Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
    • If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman's fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made.

Glucose testing is also used to test women who were diagnosed with gestational diabetes 6-12 weeks after they have delivered their baby to detect persistent diabetes.

Monitoring
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
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. The health practitioner may follow up an elevated urine glucose test with blood glucose testing. Urine glucose testing is a screening tool, but it is not sensitive enough for diagnosis or monitoring.

Other tests, such as diabetes autoantibodies, insulin, and C-peptide, may sometimes be performed along with these tests to help determine the cause of abnormal glucose levels, to distinguish between type 1 and type 2 diabetes, and to evaluate insulin production.

When is it ordered?

Several health organizations, including the American Diabetes Association (ADA) and the U. S. Preventive Services Task Force (USPSTF), recommend diabetes screening when a person is age 45 or older or when a person of any age has risk factors. Examples of risk factors include:

  • Overweight, obese, or physically inactive
  • A close (first degree) relative with diabetes
  • A woman who delivered a baby weighing more than 9 pounds or with a history of gestational diabetes
  • A woman with polycystic ovarian syndrome
  • High-risk race or ethnicity such as African American, Latino, Native American, Asian American, Pacific Islander
  • High blood pressure (hypertension) or taking medication for high blood pressure
  • Low HDL cholesterol level (less than 35 mg/dL or 0.90 mmol/L) and/or a high triglyceride level (more than 250 mg/dL or 2.82 mmol/L)
  • A1c equal to or above 5.7%
  • Prediabetes identified by previous testing
  • History of cardiovascular disease (CVD)

If the screening test result is within normal limits, the ADA recommends retesting within 3 years, while the USPSTF recommends yearly testing. People with prediabetes may be monitored with annual testing.

See the screening articles for Children, Teens, Young Adults, Adults and Adults 50 and Up for additional details.

A blood glucose test may also be ordered when someone has signs and symptoms of high blood glucose (hyperglycemia), such as:

  • Increased thirst, usually with frequent urination
  • Fatigue
  • Blurred vision
  • Slow-healing wounds or infections

or symptoms of low blood glucose (hypoglycemia), such as:

  • Sweating
  • Hunger
  • Trembling
  • Anxiety
  • Confusion
  • Blurred vision

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 health practitioner. The healthcare provider may order blood glucose levels periodically in conjunction with other tests such as A1c to monitor glucose control over time.

Pregnant women are usually screened for gestational diabetes between their 24th and 28th week of pregnancy, unless they have early symptoms or have had gestational diabetes with a previous pregnancy. A woman may be tested earlier in her pregnancy if she is at risk of type 2 diabetes (overt diabetes), says the ADA. When a woman has type 1, type 2 or gestational diabetes, her health practitioner will usually order glucose levels throughout the rest of her pregnancy and after delivery to monitor her condition.

What does the test result mean?

Blood Glucose

High levels of glucose most frequently indicate diabetes, but many other diseases and conditions can also cause elevated blood glucose.

In a person with signs and symptoms of diabetes or hyperglycemia, a non-fasting glucose level (random blood sample) that is equal to or greater than 200 mg/dL (11.1 mmol/L) indicates diabetes.

The following information summarizes the meaning of other test results.

Fasting Blood Glucose

Glucose LevelIndication
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) Prediabetes (impaired fasting glucose)
126 mg/dL (7.0 mmol/L) and above on more than one testing occasion Diabetes

2-Hour Oral Glucose Tolerance Test (OGTT)

Levels applicable except during pregnancy. Sample drawn 2 hours after a 75-gram glucose drink.
Glucose LevelIndication
Less than 140 mg/dL (7.8 mmol/L) Normal glucose tolerance
From 140 to 199 mg/dL (7.8 to 11.1 mmol/L) Prediabetes (impaired glucose tolerance)
Equal to or greater than 200 mg/dL (11.1 mmol/L) on more than one testing occasion Diabetes

Gestational Diabetes One-Step Approach (as one option recommended by the ADA)

Samples drawn fasting and then 1 hour and 2 hours after a 75-gram glucose drink. Diagnosis of GDM is made when any of the values exceed the limit.
Time of sample collectionglucose level
Fasting Equal to or greater than 92 mg/dL (5.1 mmol/L)
1 hour Equal to or greater than 180 mg/dL (10.0 mmol/L)
2 hour Equal to or greater than 153 mg/dL (8.5 mmol/L)

Gestational Diabetes Two-Step Approach (as currently recommended by ACOG and as one option from the ADA): Step One

Step One: Glucose Challenge Screen. Sample drawn 1 hour after a 50-gram glucose drink.
Glucose LevelIndication
Less than 140* mg/dL (7.8 mmol/L) Normal screen
140* mg/dL (7.8 mmol/L) and over Abnormal, needs OGTT (see Step two below)

*Some experts recommend 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). ACOG recommends a lower threshold of 135 mg/dL (7.5 mmol/L) in high-risk ethnic groups with higher prevalence of gestational diabetes.

Gestational Diabetes Two-Step Approach (as currently recommended by ACOG and as one option from the ADA): Step Two

Step Two: Diagnostic OGTT. Samples drawn at fasting and then 1, 2 and 3 hours after a 100-gram glucose drink. If two or more values meet or exceed the target level, gestational diabetes is diagnosed. One of two sets of criteria may be used to establish a diagnosis.
Time of sample collectiontarget levels**
Fasting (prior to glucose load) 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)

**Some labs may use different numbers.

Some other diseases and conditions that can result in an elevated blood glucose level include:

  • Acromegaly
  • Acute stress (response to trauma, heart attack, and stroke for instance)
  • Chronic kidney disease
  • Cushing syndrome
  • Excessive consumption of food 
  • Hyperthyroidism
  • Pancreatic cancer
  • Pancreatitis

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:

  • Adrenal insufficiency
  • Drinking excessive alcohol
  • Severe liver disease
  • Hypopituitarism
  • Hypothyroidism
  • Severe infections
  • Severe heart failure
  • Chronic kidney (renal) failure
  • Insulin overdose
  • Tumors that produce insulin (insulinomas)
  • Starvation
  • Deliberate use of glucose-lowering products

Urine Glucose

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. Some people naturally leak glucose in their urine when blood levels are normal. Some medications used to treat diabetes work by increasing the elimination of glucose in the urine.

Is there anything else I should know?

Extreme stress can cause a temporary rise in blood glucose. This can be a result of, for example, trauma, surgery, heart attack or stroke.

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.