Blood sugar testing: Why, when and how

Blood sugar testing is an important part of diabetes care. Find out when to test your blood sugar, how to use a blood sugar meter and more.

By Mayo Clinic Staff

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 test your blood sugar at home with a portable electronic device called a blood sugar meter using a small drop of your blood. You can also use a device called a continuous glucose monitor (CGM).

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 doctor 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 doctor may recommend blood sugar testing four 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 doctor 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 dinner 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)?

People 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 users 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 doctor may recommend double-checking your CGM results with a standard blood sugar meter. Check with your doctor 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 doctor what a reasonable blood sugar range is for you. Your doctor will set target blood sugar test results based on several factors, including:

  • Type and severity of diabetes
  • Age
  • How long you've had 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. For example, Mayo Clinic generally recommends that healthy adults under 60 can aim for slightly lower blood sugar targets.

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 doctor or diabetes educator 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.

Related information

Slide show: Blood sugar testing

Recording your results

Talk with your doctor 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 doctor's appointments.

Talk to your doctor about what steps to take if you get 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 users guide for your device for instructions — procedures may vary from one device to another.
  • Use a blood sample size as directed in the users 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 doctor's appointments to address any questions and to show how you use your meter.
Jan. 21, 2020 See more In-depth

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