A blood glucose test measures the level of glucose (sugar) in your blood. The test can involve a finger prick or a blood draw from your vein. Healthcare providers most commonly use blood glucose tests to screen for Type 2 diabetes, which is a common condition.
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:
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.
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
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.
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.
There are three main reasons why you may need a blood glucose (sugar) test:
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:
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:
You need to consume carbohydrates (sugar) to treat hypoglycemia, such as a banana or apple juice. Severe hypoglycemia can be life-threatening.
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.
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.
You can expect the following during a venous glucose test, or blood draw:
You can expect the following during a capillary blood glucose test (finger prick):
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.
Blood test reports, including blood glucose test reports, usually provide the following information:
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.
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:
Other causes of high glucose levels include:
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:
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.
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.
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.
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.
Last reviewed by a Cleveland Clinic medical professional on 11/16/2022.
Learn more about our editorial process.