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Continuous Glucose Monitoring


There are different types of glucose monitors that you can use to check your glucose (sugar) level at home. These include:

  • A capillary blood glucose (CBG) meter that measures the amount of glucose in your blood at the time you check.
  • A real-time continuous glucose monitor (rtCGM) or an intermittently scanned continuous glucose monitor (isCGM). These measure the amount of glucose in the fluid between your skin cells, called interstitial fluid. The rtCGM and isCGM can help you learn about your glucose trends, especially overnight.

How does a continous glucose monitor (CGM) work?

A rtCGM has several parts. You wear one part—the sensor—against your skin. It has a tiny sensor that stays under your skin. The sensor is constantly reading the blood glucose level of the fluid between your cells (interstitial). The sensor sends this information to the other part of the monitor (a wireless receiver or smartphone app). Some insulin pumps include an rtCGM receiver that is built into the pump.

An isCGM also has several parts. You wear one part—the sensor—against your skin. It has a tiny sensor that stays under your skin. The other part is a wireless reader device that displays your glucose level when the sensor is scanned with the reader device or smartphone app. You can get a reading when you need it (on demand).

There can be a 5 to 15 minute difference between an interstitial glucose level and a blood glucose level. If your glucose levels are not changing quickly there may be little difference between your interstitial glucose level and your blood glucose level.

But when your glucose level is changing quickly, like after eating a meal or treating a low blood glucose, there can be delays in getting an accurate reading from your CGM.

At any time, you can look at the receiver or smartphone app and see your glucose level. You can see if your level is going up or down—and how fast. You can see the trends and patterns of your glucose levels. Some monitors use apps and websites to show you trends and patterns.

Some monitors let you add notes of when you eat, exercise, and take medicine. That way you can see how those activities affect your blood glucose levels throughout the day and night.

All this detailed information gives you and your diabetes care team a better idea of your treatment needs.

Some rtCGMs and isCGMs need you to poke your finger and use your CBG meter to confirm what the monitor is telling you. Anytime your rtCGM or isCGM does not display a value, or if the reading does not match your symptoms, it is recommended that you verify the rtCGM or isCGM result with your finger poke CBG meter to make sure your sensor is reading accurately.

What are the benefits?

An rtCGM or isCGM is constantly measuring your blood glucose levels. This information helps some people who have diabetes make decisions about what to eat, how or when to exercise, and how much medicine to take.

Some rtCGMs and isCGMs have an alarm feature to alert you if your blood glucose level is quickly going up or down, or if you have a blood glucose level out of your target range. This is helpful for people who have problems knowing when they have low blood glucose (hypoglycemic unawareness). Some monitors can be set up so that parents, partners, or caregivers can be alerted when your blood glucose is dropping quickly while you are asleep.

What are the drawbacks?

CGM technology is always changing and getting better. Here are some things to know about most CGMs. These may not apply to all systems.

  • Sensors need to be changed. Depending on the monitor, you may need to change the sensor every few days. Some monitors have sensors that last 10-14 days.
  • Some rtCGMs will require that you poke your finger to confirm the CGM's accuracy.
  • Acetaminophen (Tylenol) and other medicines can affect the accuracy of the reading displayed on your rtCGM or isCGM. Depending on the medicine, the readings can be higher or lower than they actually are. When taking some medicines, it is recommended that you verify your rtCGM or isCGM result with your finger poke CBG meter to ensure your sensor is reading accurately. Find more information in your CGM manual.
  • If you are having an imaging test (such as an MRI, radiofrequency, or some CT scans or x-rays), you may need to remove your sensor before the test. Learn more at Continuous glucose monitors and imaging tests (PDF).
  • If you are sick, dehydrated, or if the sensor is not inserted properly the results displayed on your rtCGM or isCGM, may report the readings higher or lower than they actually are. Use finger poke CBG results (not interstitial glucose level or CGM results) to make decisions about your care and treatment when you’re dehydrated, unwell, or sick.
  • CGMs can be costly without insurance coverage. Contact your insurance provider to see if a CGM is covered.
  • If you are interested in using an rtCGM or isCGM you should talk with your diabetes care team for more information.


Adaptation Date: 12/13/2023

Adapted By: Alberta Health Services

Adaptation Reviewed By: Alberta Health Services

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