Blood sugar testing: Why, when and how

If you have diabetes, self-testing your blood sugar (blood glucose) can be an important tool in managing your diabetes and preventing complications. You can use a device called a continuous glucose monitor (CGM). Or you can test your blood sugar at home with a portable electronic device called a blood sugar meter using a small drop of your blood.

Why test your blood sugar

Blood sugar testing provides useful information for diabetes management. It can help you:

• Monitor the effect of diabetes medications on blood sugar levels
• Identify blood sugar levels that are high or low
• Track your progress in reaching your overall treatment goals
• Learn how diet and exercise affect blood sugar levels
• Understand how other factors, such as illness or stress, affect blood sugar levels

When to test your blood sugar

Your health care provider will let you know how often to check your blood sugar levels. The frequency of testing usually depends on the type of diabetes you have and your treatment plan.

Type 1 diabetes

Your health care provider may recommend blood sugar testing 4 to 10 times a day if you have type 1 diabetes. You may need to test:

• Before meals and snacks
• Before and after exercise
• Before bed
• During the night (sometimes)
• More often if you're ill
• More often if you change your daily routine
• More often if you start a new medication

Type 2 diabetes

If you take insulin to manage type 2 diabetes, your health care provider may recommend blood sugar testing several times a day, depending on the type and amount of insulin you use. Testing is usually recommended before meals and at bedtime if you're taking multiple daily injections. You may need to test only before breakfast and sometimes before dinner or at bedtime if you use just an intermediate- or a long-acting insulin.

If you manage type 2 diabetes with noninsulin medications or with diet and exercise alone, you may not need to test your blood sugar daily.

What if you have a continuous glucose monitor (CGM)?

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Continuous glucose monitor and insulin pump

Continuous glucose monitor and insulin pump

A continuous glucose monitor, on the left, is a device that measures blood sugar every few minutes using a sensor inserted under the skin. An insulin pump, attached to the pocket, is a device that’s worn outside of the body with a tube that connects the reservoir of insulin to a catheter inserted under the skin of the abdomen. Insulin pumps are programmed to deliver specific amounts of insulin continuously and with food.

People living with diabetes, particularly those with type 1 diabetes, may also choose to use CGMs. These devices measure your blood sugar every few minutes using a sensor inserted under the skin. These sensors are typically worn for a week or two before they need to be changed.

The newest type of continuous glucose monitor has an implanted sensor that can detect blood sugar levels for up to three months. A transmitter worn on the body sends blood sugar information wirelessly from the sensor to a smartphone app.

Some devices show your blood sugar reading at all times on a receiver, smartphone or smartwatch, and an alarm goes off if your blood sugar is going up or down too quickly. Others require that you check your blood sugar by running the receiver over the sensor periodically.

Most of these devices still require finger-stick checks to calibrate the machine. Check your device's user's guide to learn if you need to check, and if so, how often you need to do it.

Certain medications, such as acetaminophen (Tylenol, others), albuterol (Proair HFA, Ventolin HFA, others) and lisinopril (Prinivil, Zestril, Qbrelis), may interfere with the accuracy of some CGM readings, particularly on older models of CGMs. Readings on newer CGMs don't seem to be affected by standard doses of acetaminophen (up to 1,000 milligrams for an adult).

If you need to take medications that may affect the accuracy of the readings, your health care provider may recommend double-checking your CGM results with a standard blood sugar meter. Check with your health care provider about using a CGM if you're pregnant, on dialysis or critically ill, as these conditions may affect the blood sugar readings from a CGM.

Know your target range

Ask your health care provider what a reasonable blood sugar range is for you. Your health care provider will set target blood sugar test results based on several factors, including:

• Type and severity of diabetes
• Age
• How long you've lived with diabetes
• Pregnancy status
• The presence of diabetes complications
• Overall health and the presence of other medical conditions

The American Diabetes Association (ADA) generally recommends the following target blood sugar levels:

• Between 80 and 130 milligrams per deciliter (mg/dL) or 4.4 to 7.2 millimoles per liter (mmol/L) before meals
• Less than 180 mg/dL (10.0 mmol/L) two hours after meals

But the ADA notes that these goals often vary depending on your age and personal health and should be individualized.

Some people will have slightly higher blood sugar goals, including people who:

• Are age 60 and older
• Have other medical conditions, such as heart, lung or kidney disease
• Have a reduced ability to sense low blood sugar levels (hypoglycemia unawareness)

How to test your blood sugar

Blood sugar testing requires the use of a blood sugar meter. The meter measures the amount of sugar in a small sample of blood, usually from your fingertip, that you place on a disposable test strip. Even if you use a CGM, you'll still need a blood sugar meter to calibrate your CGM device daily.

Your health care provider or certified diabetes care and education specialist can recommend an appropriate device for you. He or she can also help you learn how to use your meter.

Follow the instructions that come with your blood sugar meter. In general, here's how the process works:

1. Wash and dry your hands well. (Food and other substances can give you an inaccurate reading.)
2. Insert a test strip into your meter.
3. Prick the side of your fingertip with the needle (lancet) provided with your test kit.
4. Touch and hold the edge of the test strip to the drop of blood.
5. The meter will display your blood sugar level on a screen after a few seconds.

Some meters can test blood taken from an alternate site, such as the forearm or palm. But these readings may not be as accurate as readings from the fingertips, especially after a meal or during exercise, when blood sugar levels change more frequently. Alternate sites aren't recommended for use in calibrating CGMs.

Recording your results

Talk with your health care provider about how often you need to record your blood sugar results. The readings given by many devices can now be downloaded to a computer.

If you manually log your results, record the date, time, test results, medication and dose, and diet and exercise information. Bring your record of results with you to your health care provider's appointments.

Talk to your health care provider about what steps to take if you get frequent results that don't fall within the range of your target goals.

Avoiding problems with your meter

Blood sugar meters need to be used and maintained properly. Follow these tips to ensure proper usage:

• Check the user's guide for your device for instructions — procedures may vary from one device to another.
• Use a blood sample size as directed in the user's guide.
• Use only test strips designed for your meter.
• Store test strips as directed.
• Don't use expired test strips.
• Clean the device and run quality-control checks as directed.
• Bring the meter to your health care provider's appointments to address any questions and to show how you use your meter.