What is a blood glucose (sugar) test?
A blood glucose test is a blood test that mainly screens for diabetes by measuring the level of glucose (sugar) in your blood.
There are two main types of blood glucose tests:
- Capillary blood glucose test: A healthcare professional collects a drop of blood — usually from a fingertip prick. These tests involve a test strip and glucose meter (glucometer), which show your blood sugar level within seconds.
- Venous (plasma) blood glucose test: A phlebotomist collects a sample of blood from a vein (venipuncture). These glucose tests are usually part of a blood panel, such as a basic metabolic panel. The provider will send the samples to a lab. There, a medical laboratory scientist will prepare your samples and perform the test on machines known as analyzers.
Venous blood glucose tests are generally more accurate than capillary blood glucose tests.
Healthcare providers often order fasting blood glucose tests to screen for diabetes. Since eating food affects blood sugar, fasting blood glucose tests show a more accurate picture of your baseline blood sugar.
There’s also at-home blood sugar testing (using a glucometer) for people who have diabetes. People with Type 1 diabetes especially need to monitor their blood sugar multiple times a day to effectively manage the condition. Continuous glucose monitoring devices (CGMs) are another option for this.
What is blood glucose (sugar)?
Glucose (sugar) mainly comes from carbohydrates in the food and drinks you consume. It’s your body’s main source of energy. Your blood carries glucose to all of your body’s cells to use for energy.
Several bodily processes help keep your blood glucose in a healthy range. Insulin, a hormone your pancreas makes, is the most significant contributor to maintaining healthy blood sugar.
If you have elevated blood glucose levels (hyperglycemia), it usually indicates diabetes. Diabetes develops when your pancreas doesn’t make any insulin or enough insulin or your body isn’t responding to the effects of insulin properly.
When would I need a blood glucose test?
There are three main reasons why you may need a blood glucose (sugar) test:
- Your healthcare provider may have ordered routine bloodwork called a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP), which both include a glucose blood test.
- You may be having symptoms of high blood sugar or low blood sugar, which could indicate diabetes or another condition.
- If you take a long-term medication that affects your blood sugar levels, such as corticosteroids, you may need routine glucose blood tests to monitor your levels.
The most common use of a blood glucose test is to screen for Type 2 diabetes (T2D), which is a common condition. Certain people are at risk for developing Type 2 diabetes. If you have risk factors, your provider will likely recommend regular screening no matter your age. The American Diabetes Association recommends regular screening for anyone age 35 or older.
Your provider will also order a blood glucose test if you have symptoms of high blood sugar (hyperglycemia) or low blood sugar (hypoglycemia).
Symptoms of diabetes and high blood sugar include:
- Feeling very thirsty (polydipsia).
- Frequent urination (polyuria).
- Feeling very hungry (polyphagia).
- Unexplained weight loss.
- Blurred vision.
- Slow healing of cuts or sores.
If you or your child have these symptoms in addition to vomiting, deep labored breathing and/or confusion, go to the nearest emergency room as soon as possible. You may have diabetes-related ketoacidosis, which is a life-threatening condition.
Symptoms of low blood sugar include:
- Shaking or trembling.
- Sweating and chills.
- Dizziness or lightheadedness.
- Faster heart rate.
- Intense hunger.
- Anxiousness or irritability.
You need to consume carbohydrates (sugar) to treat hypoglycemia, such as a banana or apple juice. Severe hypoglycemia can be life-threatening.
Who performs a blood glucose test?
Many healthcare providers, such as nurses, can perform a capillary (finger prick) blood glucose test. These tests involve a glucose meter and a test strip, which show your blood sugar result within seconds.
Phlebotomists typically perform venous blood glucose tests. They send the samples to a lab for testing.
How do I prepare for a blood glucose test?
If your healthcare provider has ordered a fasting blood glucose test, you’ll need to not eat or drink anything except water (fast) for eight to 10 hours before the test.
If your blood glucose test is part of a basic or comprehensive metabolic panel, you may also need to fast for several hours before your blood draw. In any case, your healthcare provider will let you know if you need to follow any special instructions.
What can I expect during a blood glucose test?
You can expect the following during a venous glucose test, or blood draw:
- You’ll sit in a chair, and a phlebotomist will check your arms for an easily accessible vein. This is usually in the inner part of your arm on the other side of your elbow.
- Once they’ve located a vein, they’ll clean and disinfect the area.
- They’ll then insert a small needle into your vein to take a blood sample. This may feel like a small pinch.
- After they insert the needle, a small amount of blood will collect in a test tube.
- Once they have enough blood to test, they’ll remove the needle and hold a cotton ball or gauze on the site to stop the bleeding.
- Finally, they’ll place a bandage over the site, and you’ll be finished.
You can expect the following during a capillary blood glucose test (finger prick):
- A healthcare provider will ask you which finger you’d like them to use.
- They’ll disinfect your fingertip with an alcohol swab and prick it with a small needle called a lancet, which is usually contained within a small plastic device.
- They’ll squeeze your fingertip to form a drop of blood.
- They’ll place your finger/the drop of blood against a test strip that’s inserted into a glucometer.
- After they have enough blood for the test, they’ll give you a cotton ball or gauze to hold against your fingertip to stop the bleeding.
- The glucometer will show your blood glucose level within seconds.
What are the risks of a blood glucose test?
Blood tests are a very common and essential part of medical testing and screening. There’s very little risk to having either type of glucose blood test. You may have slight tenderness or a bruise at the site of the blood draw or finger prick, but this usually resolves quickly.
Results and Follow-Up
What do the results of a blood glucose test mean?
Blood test reports, including blood glucose test reports, usually provide the following information:
- The name of the blood test or what was measured in your blood.
- The number or measurement of your blood test result.
- The normal measurement range for that test.
- Information that indicates if your result is normal or abnormal or high or low.
What is a normal glucose level in a blood test?
A healthy (normal) fasting blood glucose level for someone without diabetes is 70 to 99 mg/dL (3.9 to 5.5 mmol/L). Values between 50 and 70 mg/dL (2.8 to 3.9 mmol/L) for people without diabetes can be “normal” too.
What does a high blood glucose level mean?
If your fasting blood glucose level is 100 to 125 mg/dL (5.6 to 6.9 mmol/L), it usually means you have prediabetes. People with prediabetes have up to a 50% chance of developing Type 2 diabetes over the next five to 10 years. But you can take steps to prevent Type 2 diabetes from developing.
If your fasting blood glucose level is 126 mg/dl (7.0 mmol/L) or higher on more than one testing occasion, it usually means you have diabetes.
In either of these cases, your provider will likely order a glycated hemoglobin test (A1c) before diagnosing you with prediabetes or diabetes. An A1c shows your average blood sugar over a few months.
There are a few different types of diabetes. The most common forms are:
- Type 2 diabetes (T2D): T2D happens when your pancreas doesn’t make enough insulin or your body doesn’t use insulin well (insulin resistance), resulting in high blood glucose levels. This is the most common type of diabetes.
- Type 1 diabetes (T1D): T1D is an autoimmune disease in which your immune system attacks the insulin-producing cells in your pancreas for unknown reasons. Your pancreas can no longer produce insulin. At diagnosis, people with Type 1 diabetes usually have very high blood glucose (200 mg/dL, or 11.1 mmol/L, or higher).
- Gestational diabetes: This condition can develop in pregnant people — usually appearing during the middle of pregnancy, between 24 and 28 weeks. The high blood sugar (diabetes) goes away once the pregnancy is over. Pregnant people have screenings for gestational diabetes with a glucose challenge test and/or glucose tolerance test.
Other causes of high glucose levels
Other causes of high glucose levels include:
- Issues with your adrenal glands, such as Cushing syndrome.
- Issues with your pancreas, such as pancreatitis.
- Experiencing significant stress, such as from surgery or trauma.
- Certain medications, especially corticosteroids.
What does a low blood glucose level mean?
A blood sugar result of 70 mg/dL or lower is usually considered low.
Low blood sugar (hypoglycemia) episodes are common in people with Type 1 diabetes and people with Type 2 diabetes who take certain medications. They’re much less common in people who don’t have diabetes.
If you don’t have diabetes, low blood glucose levels may be a sign of:
- Liver disease.
- Kidney disease.
- Addison disease (adrenal insufficiency).
- Alcohol use disorder (AUD).
- Insulinoma (a rare tumor).
These conditions typically cause frequent low blood sugar episodes. A single low blood sugar test result usually isn’t a cause for concern in people who don’t have diabetes.
Should I be concerned if I have high or low blood glucose results?
If your glucose blood test result reveals that you have high or low levels of glucose, it doesn’t necessarily mean that you have a medical condition. Other factors, such as certain medications and not fasting, can affect your levels. There could’ve also been an error in the collection, transport or processing of the test.
Healthcare providers rely on more than a single blood glucose test to diagnose diabetes or another condition. In any case, your provider will carefully interpret your results and discuss them with you.
When should I know the results of a blood glucose test?
In most cases, you should have your venous blood glucose test results within 1 or 2 days.
Capillary blood glucose tests show the results within seconds via the glucometer.
When should I call my doctor?
If you’re experiencing concerning symptoms of low blood sugar or high blood sugar, contact your healthcare provider as soon as possible.
A note from Cleveland Clinic
Seeing an abnormal test result can be stressful. Know that having a high or low glucose level doesn’t necessarily mean you have a medical condition and need treatment. Approximately 1 in 20 healthy people will have results outside of the normal range. Your healthcare provider will let you know if you need to undergo further tests to determine the cause of the abnormal level. Don’t be afraid to ask your provider questions. They’re available to help you.