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Blood sugar testing: Why, when and how

Medically reviewed on May 4, 2018

If you have diabetes, self-testing your blood sugar (blood glucose) can be an important tool in managing your treatment plan and preventing diabetes complications. You can test your blood sugar at home with a portable electronic device (glucose meter) that measures sugar level in a small drop of your blood.

Why test your blood sugar

Blood sugar testing — or self-monitoring blood glucose — provides useful information for diabetes management. It can help you:

  • Judge how well you're reaching overall treatment goals
  • Understand how diet and exercise affect blood sugar levels
  • Understand how other factors, such as illness or stress, affect blood sugar levels
  • Monitor the effect of diabetes medications on blood sugar levels
  • Identify blood sugar levels that are high or low

When to test your blood sugar

Your doctor will advise you how often you should check your blood sugar level. In general, the frequency of testing 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, and sometimes during the night. You may also need to check your blood sugar level more often if you are ill, change your daily routine or begin a new medication.
  • Type 2 diabetes. If you take insulin to manage type 2 diabetes, your doctor may recommend blood sugar testing a few 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 twice daily, before breakfast and dinner if you only use 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 treated with insulin, particularly those with type 1 diabetes, may also choose to use a CGM. These devices measure your blood sugar every few minutes using a sensor inserted under the skin.

Some devices show your blood sugar reading at all times on a receiver, and an alarm will go off if your blood sugar is going up or going 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 guide to learn if you need to check, and how often you need to do so.

Continuous glucose monitor

A continuous glucose monitor (CGM) is a device that measures your blood sugar every few minutes using a sensor inserted under the skin.

Know your target range

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

For many people who have diabetes, Mayo Clinic generally recommends the following target blood sugar levels before meals:

  • Between 80 and 120 milligrams per deciliter (mg/dL) for people age 59 and younger who have no other underlying medical conditions
  • Between 100 and 140 mg/dL for people age 60 and older, or for those who have other medical conditions, such as heart, lung or kidney disease or reduced ability to sense low blood sugar levels (hypoglycemia awareness)

For many people who have diabetes, the American Diabetes Association generally recommends the following target blood sugar levels:

  • Between 80 and 130 mg/dL (4.4 and 7.2 mmol/L) before meals
  • Less than 180 mg/dL (10.0 mmol/L) two hours after meals

How to test your blood sugar

Blood sugar testing requires the use of a small electronic device called a glucometer. The meter reads the amount of sugar in a small sample of blood, usually from your fingertip, that you place on a disposable test strip. Your doctor or diabetes educator can recommend an appropriate device for you.

Your doctor or diabetes educator can also help you learn how to use a meter.

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

  1. Wash and dry your hands well.
  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. Gently squeeze or massage your finger until a drop of blood forms.
  5. Touch and hold the edge of the test strip to the drop of blood.
  6. The meter will display your blood glucose level on a screen after a few seconds.

If your meter can test blood taken from an alternate site, such as the forearm or palm, it's important to understand that these readings may not be as accurate as readings from the fingertips, especially after a meal or during exercise when glucose levels change more frequently.

Recording your results

Talk with your doctor about how often you need to record your blood sugar results. Many devices can now be downloaded to a computer.

When 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 all appointments with your doctor. Talk to your doctor about what to do and when to call when you get results that don't fall within the range of your target goals.

Avoiding problems with meter usage

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

  • Follow the user manual for your device — procedures may vary from one device to another.
  • Use a blood sample size as directed in the manual.
  • 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 appointments to address any questions and to demonstrate how you use your meter.

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