- To screen for and diagnose prediabetes and diabetes
- To detect high blood glucose (hyperglycemia) or low blood glucose (hypoglycemia)
- To monitor blood glucose levels over time to determine whether treatment has been effective in controlling your diabetes
- When you have risk factors for diabetes or when you are 45 years of age or older, as recommended by the American Diabetes Association
- When you have symptoms suggesting high or low blood glucose
- When you have diabetes, you may be instructed by your healthcare practitioner to check your glucose level up to several times a day.
- Screening and diagnosis: a blood sample drawn from a vein; note that blood samples from fingersticks are not appropriate for diagnosing diabetes.
- Monitoring: a drop of blood from a fingerstick is tested using a home glucose monitor (glucometer). Some people with diabetes may use a continuous glucose monitoring device, which uses a small sensor wire inserted beneath the skin to measure blood glucose at frequent intervals.
- Screening and diagnosis: in general, it is recommended that you fast (nothing to eat or drink except water) for at least 8 hours (usually overnight) before having a blood glucose test. Sometimes testing may be done at a random time when you have not fasted. For example, glucose testing may be done as part of routine screening during a general health exam.
- 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.
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Glucose (commonly called "blood sugar") is the primary energy source for the body's cells and the only short-term 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. Glucose tests measure the level of glucose in your blood or detect glucose in your urine.
A few different protocols may be used to evaluate glucose levels. This article focuses on:
- Fasting blood glucose (commonly called fasting blood sugar)—this test measures the level after a fast of at least 8 hours.
- Random blood glucose—sometimes your blood glucose will be measured when you have not fasted (randomly).
Other types of glucose tests include:
- A glucose tolerance test measures glucose levels after fasting and after you drink liquid containing a specific amount of glucose (see Glucose Tolerance Test).
- A specific protocol is used to help diagnose gestational diabetes, which is diabetes that first develops during pregnancy (see Glucose Tests for Gestational Diabetes).
- Urine is routinely tested for glucose as part of a urinalysis.
During digestion, the carbohydrates that you eat are broken down into glucose (and other nutrients). They are absorbed by the digestive tract, move into the blood, and circulate throughout the body. Normally, blood glucose rises slightly after a meal and the hormone insulin is released by the pancreas into the blood in response. The amount of insulin released corresponds to the size and content of the meal. Insulin helps transport glucose into the body's cells, where it is used for energy. As glucose moves into the cells and is broken down (metabolized), the blood glucose level drops and the pancreas responds by decreasing the release of insulin.
If this glucose/insulin blood feedback system is working properly, the amount of glucose in the blood remains fairly stable. If the feedback system is disrupted and the glucose level in the blood rises, then the body tries to restore the balance by increasing insulin production.
Diabetes is the most common disease resulting from an imbalance between glucose and insulin.
- Type 1 diabetes results when the body is not able to produce sufficient insulin to control blood glucose levels. Usually in type 1 diabetes, the cells that produce insulin (beta cells) have been destroyed by the person's own immune system.
- Type 2 diabetes results from a combination of insulin resistance (the body does not react normally to insulin) and a relative decline in insulin production.
- Some women may develop gestational diabetes, which is high blood glucose that develops during pregnancy. (For more information, see the article on Glucose Tests for Gestational 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 some cases, death. Chronically high blood glucose levels that can occur with untreated or poorly controlled diabetes 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.
How is the test used?
Screening and Diagnosis
- Fasting blood glucose (FBG)—this test may be used to screen for and diagnose prediabetes and diabetes in people with signs and symptoms. In some cases, there may be no early signs or symptoms of diabetes, so an FBG may be used to screen people at risk of diabetes. Screening can be useful in helping to identify it and allowing for treatment before the condition worsens or complications arise. If the initial screening result is abnormal, the test should be repeated. The repeat result must also be abnormal to confirm a diagnosis of diabetes. Certain other tests (e.g., hemoglobin A1c) can also be used to confirm diagnosis of diabetes.
- Random blood glucose—sometimes a blood sample may be drawn and glucose measured when you have not fasted, for example, when a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP) is performed. A random blood glucose may also be used to screen for diabetes. However, if a random glucose result is abnormal, it is typically followed by a fasting blood glucose test or a glucose tolerance test (GTT) to establish the diagnosis.
Some people with diabetes 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 medications or insulin(s) they may need. Glucose monitoring may be done by inserting a glucose test strip into a small instrument called a glucose meter. A drop of blood from a skin prick is placed onto the test strip and the glucose meter provides a digital readout of the blood glucose level. Alternatively, some people may use a continuous glucose monitoring device.
When is it ordered?
Screening and Diagnosis
The American Diabetes Association (ADA) recommends diabetes screening when you are age 45 or older. Several health organizations, including the ADA, recommend screening when you have risk factors, regardless of age. You may be at risk if you:
- Are overweight, obese, or physically inactive
- Have a close relative with diabetes, such as parent, siblings, aunt or uncle
- Are a woman who delivered a baby weighing more than 9 pounds or who has a history of gestational diabetes
- A woman with polycystic ovary syndrome
- Are of a high-risk race or ethnicity, such as African American, Latino, Native American, Asian American, Pacific Islander
- Have high blood pressure (hypertension) (≥140/90 mmHg) or are taking medication for high blood pressure
- Have a low HDL cholesterol level (less than 35 mg/dL or 0.90 mmol/L) and/or a high triglyceride level
- Have prediabetes identified by previous testing
- Have a history of cardiovascular disease (CVD)
The U.S. Preventive Services Task Force (USPSTF) recommends screening for abnormal blood glucose as part of a risk assessment for cardiovascular disease in adults ages 40 to 70 who are overweight or obese.
If the screening test result is within normal limits, the ADA and USPSTF recommend rescreening every 3 years. People with prediabetes may be monitored with annual testing.
A blood glucose test may also be ordered when you have signs and symptoms of abnormal blood glucose levels.
High blood glucose (hyperglycemia):
- Increased thirst, usually with frequent urination
- Blurred vision
- Slow-healing wounds or infections
Low blood glucose (hypoglycemia):
- Blurred vision
If you have diabetes, you may be instructed to self-check your glucose levels up to several times a day. Your healthcare practitioner may order blood glucose levels periodically in conjunction with other tests such as hemoglobin A1c to monitor glucose control over time.
What does the test result mean?
Screening and Diagnosis
High levels of glucose most frequently indicate diabetes, but many other diseases and conditions can also cause elevated blood glucose.
A random glucose level (non-fasting) in a person with signs and symptoms of diabetes and hyperglycemia that is equal to or greater than 200 mg/dL (11.1 mmol/L) indicates diabetes.
The following table summarizes the meaning of fasting glucose results.
Fasting Blood Glucose
Glucose Level Indication 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
Some other diseases and conditions that can result in an elevated blood glucose level include:
- Acute stress (response to trauma, heart attack, and stroke for instance)
- Chronic kidney disease
- Cushing syndrome
- Overactive thyroid (hyperthyroidism)
- Pancreatic cancer
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 "How is hypoglycemia diagnosed?" below.)
A low blood glucose level (hypoglycemia) may be seen with:
- Adrenal insufficiency
- Drinking excessive alcohol
- Severe liver disease
- Underactive thyroid (hypothyroidism)
- Severe infections
- Severe heart failure
- Chronic kidney (renal) failure
- Insulin overdose
- Tumors that produce insulin (insulinomas)
- Deliberate use of glucose-lowering products
If you have diabetes and are monitoring your glucose levels, your healthcare practitioner will give you guidelines for how high or low your blood sugar should be at different times of the day. This may depend on several factors, such as:
- How long you have had diabetes
- Your age and life expectancy
- Other underlying conditions you may have, including heart disease
- Whether you do not experience distinct symptoms of low blood sugar (hypoglycemia unawareness)
Can a blood sample from a fingerstick be used to diagnose diabetes?
No, fingerstick blood samples are not appropriate for diagnosing diabetes. Rather, a blood sample drawn from a vein should be used for diagnostic testing. However, fingerstick samples may be used to monitor your glucose levels if you have been diagnosed with diabetes.
Can glucose testing be done when I have not fasted?
You may have non-fasting glucose testing in certain situations:
- A blood sample collected after a fast of at least 8 hours is preferred when using a glucose test for screening and diagnosing diabetes. However, you may sometimes have your blood drawn for a routine metabolic panel that includes a glucose test (e.g., CMP, BMP) when you have not fasted for 8 hours. In this case, if the glucose result is above the cutoff level, then additional testing, such as repeating the glucose test after fasting, may be required.
- If you have been diagnosed with diabetes, you may be instructed to check your glucose levels after you have eaten as part of monitoring and controlling your glucose levels over time.
I’ve recently been diagnosed with type 2 diabetes. Will I need to monitor my glucose levels every day?
Your healthcare practitioner will discuss with you whether you need to monitor your glucose levels. Not everyone with type 2 diabetes needs to monitor their glucose levels every day, especially if they are able to manage their diabetes and glucose levels with diet and exercise.
However, some people with type 2 diabetes must check their blood glucose levels, sometimes several times a day. This may be done using a glucose meter. You would place a drop of blood from a skin prick onto a glucose strip and then insert the strip into the glucose meter, a small machine that provides a digital readout of the blood glucose level. Alternatively, some people may use a continuous glucose monitoring device.
Your healthcare practitioner will give you guidelines for how high or low your blood sugar should be at different times of the day. By checking your glucose regularly, you can see if the diet and medication schedule you are following is working properly for you.
Can I test my blood glucose levels at home?
If you have not been diagnosed with diabetes or prediabetes, there is usually no reason to test glucose levels at home. Screening done as part of your regular physical should be sufficient.
If you have been diagnosed with diabetes, however, your healthcare practitioner or diabetes educator may recommend a home glucose monitor (glucometer or continuous glucose monitoring device) so that you can test your blood glucose levels at home. By checking your glucose regularly, you can see if the diet and medication schedule you are following is working properly for you.
Can urine samples be tested for glucose?
Urine samples are routinely tested for glucose as part of a urinalysis. Additional testing is usually done to identify the cause of an abnormal urine glucose result.
Glucose usually only shows up in the urine if glucose is at sufficiently high levels in the blood that some of the excess is lost in the urine, or if there is some degree of kidney damage and the glucose is leaking out into the urine.
Besides blood and urine, can other body fluids be tested for glucose?
Besides glucose, what other tests might be done?
Hemoglobin A1c is an indicator of long-term glucose control that is commonly ordered in patients with prediabetes and diabetes and can also be used as a screening test. Other less common tests, such as islet 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.
What are the usual treatments for diabetes?
For type 2 diabetes, which is the most common type of diabetes, losing excess weight, eating a healthy diet that is high in fiber and restricted in carbohydrates, and getting regular amounts of exercise may be enough to lower your blood glucose levels. In many cases, however, medications may be necessary to achieve the desired glucose level. With type 1 diabetes (and with type 2 diabetes that does not respond well enough to oral medications), insulin injections several times a day are necessary. See the article on Diabetes for more on treatment.
How can a diabetes educator help me?
A diabetes educator (often a nurse with specialized training) can make sure that you know how to:
- Recognize and know how to treat both high and low blood sugar
- Test and record your self-check glucose values
- Adjust your medications
- Administer insulin (which types in which combinations to meet your needs)
- Handle medications when you get ill
- Monitor your feet, skin, and eyes to catch diabetes-related health problems early
- Buy diabetic supplies and store them properly
- Plan meals; diet is extremely important in minimizing swings in blood glucose levels. A registered dietician can help you learn how to plan meals and a diabetes educator can help with this as well.
How is hypoglycemia diagnosed?
A medical diagnosis of hypoglycemia typically 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 when the blood glucose level is abnormally low
- 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.