Blood Glucose (Sugar) Test

What is a blood glucose test?

A blood glucose test 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 provider collects a drop of blood — usually from a fingertip needle prick. For an infant, a provider pricks their heel. This test involves a test strip and glucose meter. You get the result within seconds.
• Venous (plasma) blood glucose test. A phlebotomist collects a sample of blood from a vein. This glucose test is usually part of a blood panel, like a basic metabolic panel. The provider sends the sample to a lab for testing.

Venous blood glucose tests are generally more accurate than capillary blood glucose tests.

Other types of blood glucose tests include:

• Fasting blood sugar test. This test can screen for diabetes. Eating food affects your blood sugar. So, fasting blood glucose tests show a more accurate picture of your baseline blood sugar.
• At-home glucose monitoring. People with diabetes use a glucose meter with a finger prick or a continuous glucose monitor (CGM). Monitoring your glucose level is essential to effectively managing diabetes.

What is blood glucose (sugar)?

Some glucose (sugar) comes from carbohydrates in the food and drinks you consume. Your liver releases some, as well. Glucose is your body’s main source of energy. Your blood carries glucose to all your body’s cells.

Several bodily processes help keep your blood glucose in a healthy range. Insulin, a hormone, is key to maintaining healthy blood sugar.

If you have high glucose levels (hyperglycemia), it usually means you have diabetes. Diabetes develops when your pancreas doesn’t make any or enough insulin, or your body isn’t using insulin properly.

When would I need a blood glucose test?

There are four 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). Both of these include a glucose measurement.
• You have symptoms of high blood sugar or low blood sugar.
• You’re doing a glucose tolerance test during pregnancy to check for gestational diabetes.
• You’re taking a long-term medication that affects your blood sugar levels, like corticosteroids. Your provider may recommend routine blood glucose checks to monitor your levels.

The most common use of a blood glucose test is to screen for Type 2 diabetes (T2D). 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.

How do I prepare for a blood glucose test?

If you’re getting a fasting blood glucose test, don’t eat or drink anything except water for eight to 12 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 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:

1. You’ll sit in a chair. A phlebotomist will check your arms for an easily accessible vein — usually in your arm.
2. Once they’ve located a vein, they’ll clean and disinfect the area.
3. They’ll then insert a small needle into your vein to take a blood sample.
4. After they insert the needle, a small amount of blood will collect in a test tube.
5. Once they have enough blood to test, they’ll remove the needle. They’ll hold a cotton ball or gauze on the site to stop the bleeding.
6. 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):

1. A healthcare provider will ask you which finger you’d like them to use.
2. They’ll disinfect your fingertip with an alcohol swab. They’ll prick it with a small needle called a lancet. It’s usually contained within a small, plastic device.
3. They’ll squeeze your fingertip to form a drop of blood.
4. They’ll place your finger and the drop of blood against a test strip that’s inserted into a glucose meter.
5. 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.
6. The glucose meter will show your blood glucose level within seconds.

When should I know the results?

In most cases, you should have your venous blood glucose test results within one or two days.

Capillary blood glucose tests show the results within seconds via the glucose meter.

What is a normal glucose level in a blood test?

A healthy 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 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 5 to 10 years. But you can take steps to try to reverse prediabetes.

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. People with Type 1 diabetes usually have very high blood glucose levels (200 mg/dL, or 11.1 mmol/L, or higher).

In either of these cases, your provider will likely order an A1c test before making a diagnosis. An A1c shows your average blood sugar over a few months.

Other causes of high glucose levels

Other causes of high glucose levels may include:

• Issues with your adrenal glands, like Cushing syndrome
• Issues with your pancreas, like pancreatitis
• Hyperthyroidism
• Experiencing significant stress, like 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 considered too low for most adults who have diabetes.

Low blood sugar (hypoglycemia) episodes are common in people with Type 1 diabetes. They can also affect 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
• Hypothyroidism
• 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.