A blood glucose test measures how much sugar you have in your blood. You may have a blood glucose test in a doctor’s office, laboratory, or with a fingerstick meter at home.
When you eat carbohydrates, your body converts them into glucose for energy. Having too much or too little glucose in your blood could mean you have a serious medical condition.
Doctors often order a blood glucose test in their clinics or at a lab to help diagnose diabetes, and people already diagnosed with diabetes might check their blood sugar levels at home to manage their condition.
Glucose testing is primarily done to diagnose or manage type 1 diabetes, type 2 diabetes, prediabetes, and gestational diabetes. Diabetes is a condition that causes your blood glucose levels to rise.
The amount of sugar in your blood is usually controlled by a hormone called insulin. But if you have diabetes, your body either doesn’t make enough insulin or the insulin produced doesn’t work properly. This causes sugar to build up in your blood.
If left untreated, chronically elevated levels of blood sugar can lead to other serious conditions, including kidney disease, blindness, and heart disease.
About low blood sugar
Blood glucose checks may also be used to test for low blood sugar or hypoglycemia. This occurs when the levels of glucose in your blood are too low, usually lower than 70 milligrams per deciliter (mg/dL).
Hypoglycemia can occur in people with diabetes if they take too much of their medication or insulin, exercise more than usual, or skip a meal. But hypoglycemia can also be unrelated to diabetes, from another underlying condition or medications.
Doctors consider very low blood sugar to be a medical emergency, as it can lead to seizures, coma, and even death.
Blood glucose tests fall into several categories, including:
- fasting
- non-fasting
- hemoglobin A1C test
- oral glucose tolerance test (OGTT), a 2-hour test
- post-prandial, which means testing after consuming food or drink
Doctors use fasting, random, and OGTT tests to diagnose diabetes.
Post-prandial tests can help show how well a person with diabetes manages their blood sugar levels.
Before your test, tell your healthcare professional about your medications, including prescriptions, over-the-counter drugs, and herbal supplements. Certain medications can affect blood glucose levels. Your healthcare professional may ask you to stop taking a particular medication or to change the dosage before your test temporarily.
Medications that can affect your blood glucose levels include:
- corticosteroids
- diuretics
- birth control pills
- hormone therapy
- aspirin (Bufferin)
- antipsychotics
- lithium
- epinephrine (Adrenalin)
- tricyclic antidepressants
- monoamine oxidase inhibitors (MAOIs)
- phenytoin
- sulfonylurea medications
Severe stress can also cause a temporary increase in your blood glucose. You should tell your healthcare professional if you’ve recently had any of these:
- surgery
- trauma
- stroke
- heart attack
Fasting test preparation
For a fasting blood glucose test, you can’t eat or drink anything except water for 8 hours before your test. You may want to schedule a fasting glucose test first thing in the morning so that you don’t have to fast during the day.
Fasting before a blood glucose test is important because it will provide more accurate results that are easier for your healthcare professional to interpret.
Random testing preparation
Random (non-fasting) blood glucose tests don’t require you to not eat or drink before the test.
Your healthcare professional may have you take several random measurements throughout the day to see how your glucose levels change.
Post-prandial testing preparation
A test given 2 hours after starting a meal measures postprandial plasma glucose. When someone has diabetes, this test is most often done at home.
It can help you understand if you’re taking the right amount of insulin with meals. You must do this test 2 hours after you start eating a meal.
If you already have diabetes, this information may also help your healthcare professional understand if you need to adjust your medications.
If you haven’t been diagnosed with diabetes, your healthcare professional will likely recommend a blood sample test in their office or at a separate lab.
But if you’re managing your diabetes at home, you’ll likely administer a finger-prick test yourself.
Blood-drawn tests
A healthcare professional will most likely draw blood from a vein inside your elbow to perform a blood glucose test. The procedure is fairly simple:
- They clean the area with an antiseptic, like alcohol, to kill any germs.
- They tie an elastic band around your upper arm, causing your veins to swell with blood.
- They insert a sterile needle into a vein. You may feel slight to moderate pain when the needle goes in, but you can reduce the pain by relaxing your arm.
- Your blood is then drawn into a tube attached to the needle.
- When they’re finished drawing blood, the healthcare professional removes the needle and places a bandage over the puncture site.
- Pressure will be applied to the puncture site for a few minutes to prevent bruising.
The blood sample is then sent to a lab for testing. Your healthcare professional will then follow up with you to discuss the results.
Finger-prick blood sugar checks
If you have diabetes, your healthcare professional may have instructed you to monitor your glucose levels at home using a blood glucose meter or a continuous glucose monitor (CGM).
Your test kit should include directions for how to take blood from your finger. In general, you’ll follow these steps:
- Wash your hands thoroughly with soap and water. Dry well.
- Place the strip in the meter. This may automatically start the meter, but some devices require you to turn them on first before inserting the strip.
- Use the lancing device that came with your kit to prick the cushiest part of your fingertip.
- Touch the edge of a test strip to the blood drop.
- Wait for the meter to countdown and display a blood sugar number.
- Track and record your results on paper, a mobile app, or a different tracking method.
- Dispose of the used test strip, and make sure to put the used lancet in a sharps container for disposal.
Blood glucose targets aren’t the same for everyone.
While the American Diabetes Association (ADA) recommends targets for most non-pregnant adults, target ranges might vary depending on many factors. These factors include:
- your age
- any underlying conditions
- whether you have diabetes and for how long
- medications you’re taking
Your blood sugar result will depend on the type of blood glucose test used and when you last ate food or drank anything other than water.
The ADA’s standards of care list the following targets for blood sugars in diagnosing diabetes:
| Type of test | Target blood glucose range for people without diabetes | Target blood glucose range for people with diabetes |
| Fasting test | less than 99 mg/dL | between 80 and 130 mg/dL |
| 2 hours after beginning a meal (postprandial plasma glucose) | less than 140 mg/dL | less than 180 mg/dL |
However, diabetes is not diagnosed based on a single random test. If you had a fasting blood glucose test and the results are above typical, it will likely indicate that you have either prediabetes or diabetes:
| Fasting test | Prediabetes | Diabetes |
| Blood glucose range | 100–125 mg/dL | 126 mg/dL or above |
If you had a random blood glucose test, a level of 200 mg/dL or higher often means you have diabetes. Your healthcare professional may also order a fasting blood glucose test to confirm the diagnosis or another test like an A1C or a glucose tolerance test.
For those with type 2 diabetes, a post-prandial blood glucose reading of over 180 mg/dL could mean that your diabetes management and care plan may need adjusting.
Other conditions
High glucose levels are a sign of diabetes, but they could signal other health conditions that are causing higher blood sugars. These may include:
- hyperthyroidism, or overactive thyroid
- pancreatitis, or inflammation of your pancreas
- pancreatic cancer
- prediabetes, which happens when you’re at increased risk of developing type 2 diabetes
- stress to the body from illness, trauma, or surgery
- medications like steroids
- Cushing syndrome, in which the body produces too much cortisol
- acromegaly, when there is too much growth hormone
High blood glucose levels (hyperglycemia) can cause damage to your organs and nerves and increase your risk of heart attack and stroke, even for those without any type of diabetes.
In some cases, people who are pregnant will develop high blood sugar during their pregnancy. This is called gestational diabetes.
Most pregnant people are given a blood glucose test
To diagnose gestational diabetes, a doctor may have you participate in a test called an oral glucose challenge test. During this test, you’ll be asked to drink a sugary drink. You’ll then wait an hour before your blood is drawn. You don’t have to fast before this test.
Typical results for a gestational diabetes oral glucose challenge test are a level of 140 mg/dL or lower.
If the oral glucose challenge test results are abnormal, you’ll need to have a follow-up test to confirm a diagnosis. This test is called an oral glucose tolerance test (OGTT).
First, you’ll have a fasting glucose test. Remember not to eat or drink anything for at least 8 hours before this test.
After this test, you’ll drink a sugary drink and have your blood drawn to obtain your glucose levels three times — after 1, 2, and 3 hours. You’ll be diagnosed with gestational diabetes if two or more glucose values fall at or above the typical glucose threshold.
A doctor may recommend another test or diagnose diabetes if the person’s fasting blood sugar is 126 mg/dL or higher if non-fasting glucose is 200 mg/dL or higher, or if their A1C result is 6.5% or above.
A doctor may do a fasting blood glucose test, a random (non-fasting) blood test, an oral glucose tolerance test, or an A1C test.
The A1C shows glucose levels over time, while the others provide a snapshot of glucose levels. Post-prandial tests show how eating or drinking affects a person’s blood sugar levels and can be used to monitor diabetes and blood sugar management.
Doctors usually perform a fasting blood sugar test in the morning after 8 hours of fasting. People can also perform a random test or an OGTT at any time without fasting, but the OGTT takes 2 to 3 hours to complete.
A post-prandial test is specifically for use after eating or drinking. An A1C test shows how glucose levels have fluctuated over the last 3 months.
A blood glucose test is an important test often used to diagnose diabetes or to help people with diabetes manage their condition. Your healthcare professional may order a blood glucose test for an annual wellness checkup.
However, you may consult your healthcare team about a blood sugar check if you’re experiencing any new symptoms, including increased thirst, frequent urination, or blurred vision. These can become medical emergencies quickly, especially in the event of newly-onset and undiagnosed type 1 diabetes.