Blood Glucose Testing

The term “blood glucose” generally refers to the amount of glucose in your blood. Blood glucose testing is done when a healthcare provider needs to see if a person has diabetes, or how well they are managing their diabetes. People who are diagnosed with diabetes need to check their blood glucose often to make sure their diabetes is in control.

To diagnose and monitor diabetes, healthcare providers test plasma glucose levels or the percentage of glycosylated hemoglobin (HbA1c) in your bloodstream with a laboratory test. People who have diabetes test their plasma glucose with a portable meter at home.

Both the fasting plasma glucose (FPG) and HbA1c tests are used to diagnose diabetes in non‑pregnant adults. For an accurate diagnosis, all results must be confirmed by a second test on a different day.

Fasting Plasma Glucose (FPG) Test

A plasma glucose (PG) test measures the amount of glucose in the liquid part of your blood — called the blood plasma — rather than the amount of glucose in your whole blood. In the United States, plasma or whole blood glucose are measured in milligrams per deciliter, or mg/dL. For the FPG test, you will need to fast (not eat or drink anything except water) for at least 8 hours before your test. A sample of your blood is taken at your healthcare provider’s office and analyzed in a lab.

The FPG test measures the amount of glucose (sugar) that stays in the blood even after going without food for at least 8 hours. A high number is a signal that your body isn’t making enough insulin to remove glucose from your blood stream.

HbA1c Test

This test can be done any time during the day and you do not need to fast. Like the FPG, a sample of blood is taken at your healthcare provider’s office an analyzed in a lab. An HbA1c test measures the percentage of glycosylated hemoglobin in your bloodstream. Here’s what it means:

• Hemoglobin (Hb) is a protein in your red blood cells. It carries oxygen and is what makes your blood red.
• Hemoglobin picks up glucose from your bloodstream, becoming “glycosylated.” As blood glucose rises, so does the level of glycosylated hemoglobin, or HbA1c. And once hemoglobin becomes glycosylated, it stays that way for the life of the red blood cell, or about three months.

The HbA1c test is used to monitor blood glucose control over a period of time and also to diagnose diabetes.

Testing your own blood glucose can tell you if your blood glucose is too high, too low, or just right. Most people need to test their own blood glucose regularly, often more than once a day.

How do I self-test?

Self-testing requires you to prick your finger to get a small sample of blood. Then you use a small machine called a glucose meter (glucometer) to read the sample and display your blood glucose level.

Since different meters work in slightly different ways, be sure to follow the instructions that come with your meter. If you have a question about your meter and want to talk directly with someone, call the toll-free number on the back of most meters — or call your diabetes educator.

When should I self-test?

Your healthcare provider will tell you when and how often to self-test. Here are some common times to self-test and what your results tell you:

• Fasting. Fasting is when you haven’t had anything to eat or drink except water for at least 8 hours. Usually, this is when you first wake up in the morning. A reading taken when you are fasting tells you how your body handled your blood glucose during the night.
• Before meals. This reading tells you how your body has handled the glucose from your most recent meal. This helps you plan for the next meal.
• Two hours after a meal (sometimes called “2 hours postprandial”). Your blood glucose peaks about one or two hours after you eat, so testing at this time will tell you how high your blood glucose goes after eating.
• Bedtime. This reading gives you a baseline for your blood glucose before you begin your overnight fast, and helps you know whether your body is ready for the fast.

On days when you’re sick or battling an episode of high or low blood glucose, you’ll need to self-test more often. Also, if you take insulin or you often get low blood glucose, you need to test before you drive a car.

If you are not sure which blood glucose meter to buy, call your diabetes educator and health insurance company. They can tell you what you need to think about when shopping for a meter and other diabetes supplies. They may even be able to help you get a meter for a reduced price (or even free).

Most of today’s home glucose meters also measure plasma glucose. If yours doesn’t — and instead measures the amount of glucose in your whole blood — then it may be an old meter. You may want to check into getting a new one. Plasma and whole blood values aren't that different, but if your home meter measures plasma glucose, it makes it easier for you and your healthcare team to compare home test results with your lab test results.

It’s important to keep a record of your daily blood glucose readings to show how well you are able to control your blood glucose. Use your record to keep notes about your diet, activity, and medication. This can help you and your healthcare team understand what may be causing your blood glucose to be high or low. And, it will help you decide what treatment changes — if any — are needed.

You can use several tools to help you track your daily blood glucose readings:

• Logbook. Many people use a paper logbook to record their daily readings and other important notes (symptoms, meals, exercise, and so on). You’ll get a free logbook with your glucose monitor.
• Meter memory function. Some glucose meters have enough memory to store several hundred test results. Some can even link the result with specific events such as meals, exercise, and illness. And some models with memory contain a data port that allows you to download your readings to your personal computer or share the information with your healthcare provider so you can both see your readings and understand patterns easily.
• Continuous glucose monitoring. A continuous glucose monitoring system (CGMS) is another way to track glucose. It doesn't replace testing with a glucometer, but it can be an important part of your care. A CGMS system automatically records glucose levels all day and night. It gives you readings every 5 minutes. It also helps you see trends in your glucose control. CGMS can help you and your doctor understand how food, exercise, and medication affect your glucose levels.

Call your healthcare provider for an appointment if you’re sick and …

• You’ve had a fever for a couple of days and aren’t getting better.
• You’ve been vomiting or having diarrhea for more than six hours.
• You have a fever higher than 101.5°F (38°C) or a fever that lasts for more than 24 hours.
• You have a fasting blood glucose level of 240 mg/dL or higher for more than 24 hours.
• You have moderate to large amounts of ketones in your urine. If you can’t reach your healthcare provider, or if you have large amounts of ketones in your urine, get emergency care.
• You begin to notice problems like confusion or dehydration (symptoms of dehydration include a decrease in urine output, dry mouth and skin, and dry sunken eyes).
• You aren’t sure what to do to take care of yourself.

You have high blood glucose (hyperglycemia), and …

• You can’t control your hyperglycemia, in spite of taking action to correct it.
• You have two or three readings in a row with results of 240 mg/dL or higher.
• You have more than two unexplained episodes of hyperglycemia in a week.
• You have repeated high glucose readings during a particular time of day.
• You have moderate to large amounts of ketones in your urine. If you can’t reach your healthcare provider, or if you have large amounts of ketones in your urine, get emergency care.

You have low blood glucose (hypoglycemia), and …

• You can’t control your hypoglycemia, in spite of taking action to correct it.
• You have two or three readings in a row with results of 70 mg/dL or less.
• You have more than two unexplained episodes of hypoglycemia in a week.
• You have repeated low glucose readings during a particular time of day.

Also consider calling if …

• You’re pregnant or thinking of becoming pregnant.
• You have questions about the goals of your treatment — for example, what your blood glucose numbers should be, or what a healthy cholesterol level is for you.
• You have questions or concerns about self-management. It’s important that you feel confident about how to monitor your blood glucose, plan your meals, take your medications, and do the other things you need to do to care for yourself.
• You have any of the following:

Repeated stomach or digestion problems (for example, constipation or diarrhea)

Foot problems such as sores on your feet, thickened or ingrown toenails, or cold, tingling, or puffy feet

Tooth or gum problems, such as bleeding or tender gums or persistent bad breath

Depression or other emotional difficulties

You just don’t feel right, and want to make sure your treatment plan is still working for you.

• American Diabetes Association
• Medline Plus
• Joslin Diabetes Center