Blood Glucose Test: Preparation, Risks, and Results

A blood glucose test measures the sugar (glucose) circulating in your bloodstream. 

Blood glucose tests are common, and many healthcare professionals can order or administer them. However, an endocrinologist (doctors who specialize in hormone-related conditions) may be the provider to help manage your glucose levels based on the test findings. 

Blood glucose levels show how well your body processes glucose from food into energy. Providers may order them as a screening tool during checkups or prenatal visits (around weeks 24 and 28 of pregnancy) or to diagnose and monitor people with prediabetes or diabetes. 

With diabetes, your body doesn’t produce enough insulin or use it effectively, causing blood glucose to run high. Insulin is a hormone the pancreas releases to help glucose enter your cells. Monitoring glucose when you have diabetes helps prevent complications like eye problems, heart disease, kidney failure, and nerve damage. 

Risk factors for diabetes include a family history, being older than 45, and having had gestational diabetes (diabetes during pregnancy). Other potential risk factors include having heart disease or a stroke, high blood pressure, and obesity.

Your provider may also order blood glucose tests if you are experiencing symptoms that suggest hyperglycemia (high blood sugar). These include blurred vision, excessive thirst, fatigue, frequent urination, headaches, and unexplained weight loss.

Blood glucose tests also detect hypoglycemia (low blood sugar). Symptoms include blurred vision, confusion, dizziness or fainting, irritability, rapid heartbeat, shakiness, and sweating. 

In addition to diabetes, health conditions that may require blood glucose testing include:

• Autoimmune disorders 
• Critical illnesses 
• Gestational diabetes
• Heart, kidney, or liver disease
• Hormone imbalances
• Insulin resistance
• Medication-induced hyperglycemia (from some antibiotics, steroids, or heart medicines) 
• Thyroid, pancreas, pituitary, or adrenal gland problems (glands that secrete or regulate hormones)

There are several types of glucose tests. They serve specific purposes in monitoring blood sugar levels. Here’s an overview of the tests:

• Capillary: Blood from a finger stick (or earlobe, forearm, heel, or palm)
• Continuous: A sensor under the skin continuously monitors glucose
• Fasting: Testing after not eating for eight hours 
• Glucose tolerance: Testing before and after consuming glucose solution
• Hemoglobin A1c (HbA1c): A long-term (three months) average 
• Non-fasting (random): Testing anytime, regardless of food intake
• Point-of-care: Rapidly getting results from handheld devices
• Postprandial: One to two hours after eating 
• Venous: A healthcare provider draws blood from a vein 

For a fasting test, do not eat or drink anything but water for eight hours before the test. A random, non-fasting blood glucose test does not require fasting. 

If you are getting a blood draw, wear loose-fitting clothing with easy access to your upper arm. Blood draws are generally safe but can be stressful. Consider bringing someone for support. You can feel light-headed afterward if you also did an oral glucose tolerance test. Consider having someone drive you.

You may need to fill out your medical history or insurance information before the test. Bring a form of identification and an insurance card (if you have one). Most insurance plans cover a blood glucose test. If you need financial help, ask your healthcare team about assistance programs.

Capillary blood glucose (CBG) takes a minute or two and can occur in any setting. Venous blood glucose tests take a few minutes and are done in a clinical setting. 

During the Test

A CBG involves a lancet (a small, sharp needle), a test strip, and a glucose meter. Your healthcare team may perform these, but they may also teach you how to do a fingerstick at home.

Wash your hands, gather supplies, and put a test strip in the glucose meter. Use the lancet to prick the side of your third or fourth fingertip or an alternate site like the forearm or earlobe. Place a drop of blood on the test strip and clean the puncture site. The meter will display your blood glucose level within seconds.

During a blood glucose lab draw, you’ll sit in a chair while a healthcare professional uses a tiny, sterile needle to draw blood from your arm. You might feel a slight sting when the needle goes in or out. The person drawing your blood will:

• Place a wide rubber band on your upper arm 
• Clean your skin with an antiseptic like alcohol
• Insert the needle into your vein
• Draw blood into a tube
• Remove the needle
• Apply pressure and a bandage
• Send the blood sample to a laboratory

After the Test/The Post-Test

After a CBG, you or your provider should record the glucose value and dispose of the lancet and test strip in a sharps container. After a venous blood glucose test, you can usually leave right away.

If needles make you queasy, take a moment to rest before driving. Depending on the results, your provider may suggest further testing, medication changes, or specialist referrals. 

It’s essential to clean the area before using a lancet or needle to avoid infection. Otherwise, both tests are generally safe.

With a fingerstick, the lancet may cause your fingertip to sting, and it may be a little tender or bruise afterward. Avoiding the first finger and thumb reduces the risk of pain. During a blood draw, you may feel a little pinch or notice a bruise where the needle was, but it typically goes away quickly.

A CBG using a glucometer gives you results in seconds. Venous lab draw results may take a day or two. Your healthcare team will share the results through a patient portal, phone call, text, letter, or follow-up appointment. 

Depending on your results, your provider might recommend further testing, lifestyle changes, medication, monitoring, or a specialist referral. The following are the typical ranges for blood glucose levels.