A blood glucose test measures the glucose levels in your blood. Glucose is a type of sugar. It is your body's main source of energy. A hormone called insulin helps move glucose from your bloodstream into your cells.
Too much or too little glucose in the blood can be a sign of a serious medical condition. High blood glucose levels (hyperglycemia) may be a sign of diabetes, a disorder that can cause serious, long-term health conditions.
High blood sugar may also be caused by other conditions that can affect insulin or glucose levels in your blood, such as problems with your pancreas or adrenal glands. Low blood glucose levels (hypoglycemia) are common among people with type 1 diabetes and people with type 2 diabetes who take certain diabetes medicines. Certain conditions, such as liver disease, may cause low levels of blood glucose in people without diabetes, but this is uncommon. Without treatment, severe low blood sugar can lead to major health problems, including seizures and brain damage.
Other names: blood sugar, self-monitoring of blood glucose (SMBG), fasting plasma glucose (FPG), fasting blood sugar (FBS), fasting blood glucose (FBG), random blood sugar, glucose challenge test, oral glucose tolerance test (OGTT)

What is it used for?
A blood glucose test is used to find out if your blood sugar levels are in a healthy range. It is often used to help diagnose and monitor diabetes.

Why do I need a blood glucose test?
Your health care provider may order a blood glucose test if you have symptoms of high glucose levels or low glucose levels.

Symptoms of high blood glucose levels include:

• Increased thirst and urination (peeing)
• Blurred vision
• Fatigue
• Sores that don't heal
• Weight loss when you're not trying to lose weight
• Numbness or tingling in your feet or hands

Symptoms of low blood glucose levels include:

• Feeling shaky or jittery
• Hunger
• Fatigue
• Feeling dizzy, confused, or irritable
• Headache
• A fast heartbeat or arrhythmia (a problem with the rate or rhythm of your heartbeat)
• Having trouble seeing or speaking clearly
• Fainting or seizures

You may also need a blood glucose test if you have a high risk for developing type 2 diabetes.
You're more likely to develop diabetes if you:

• Are overweight or have obesity
• Are age 45 or older
• Have a family history of diabetes
• Have high blood pressure
• Don't exercise enough
• Have a history of heart disease or stroke
• Have had gestational diabetes (diabetes that happens only during pregnancy) If you are pregnant, you will likely get a blood glucose test between the 24th and 28th week of your pregnancy to check for gestational diabetes.

What happens during a blood glucose test?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out.

For some types of glucose blood tests, you will drink a sugary liquid and wait for an hour before your blood sample is taken:

A glucose challenge test is used to test for gestational diabetes in pregnancy. If your blood glucose level is higher than normal, you may have gestational diabetes. You'll need an oral glucose tolerance test (OGTT) to get a diagnosis.

An oral glucose tolerance test (OGTT) is used to diagnose gestational diabetes, and type 2 diabetes and prediabetes in people who aren't pregnant. A blood sample will be taken before you have a sugary drink and then again, every hour for the next 2 or 3 hours.

Will I need to do anything to prepare for the test?
If your provider orders a fasting blood glucose test or an oral glucose tolerance test, you will need to fast (not eat or drink) for at least eight hours before the test. Other blood glucose tests don't require any special preparations. Ask your provider whether you need to fast before your glucose test.

Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly. After an oral glucose tolerance test, you may feel light-headed. Your provider may suggest that you plan to have someone take you home.
If your results show higher than normal glucose levels, it may mean you have or are at risk for getting diabetes. High glucose levels may also be a sign of:

Hyperthyroidism

Pancreas disorders

Stress from surgery, very serious illness, or trauma

If you have diabetes, lower than normal glucose levels may be caused by:

Side effects from certain diabetes medicines

Not eating enough, especially after taking diabetes medicine

Being more physically active than usual

If you don't have diabetes, low blood glucose levels may be a sign of:

Liver disease

Kidney disease

Underactive adrenal, pituitary, or thyroid gland (hypothyroidism)

Alcohol use disorder (AUD)
If your glucose results are not normal, it doesn't always mean you have a medical condition that needs treatment. Certain medicines and stress can affect glucose levels. To learn what your test results mean, talk with your health care provider.