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Learning About Continuous Glucose Monitors (CGMs)

Continuous glucose monitor.

What is a continuous glucose monitor?

A continuous glucose monitor (CGM) is a device that you attach to your body to read your glucose (sugar) level any time at home. CGMs measure the amount of glucose in the fluid between your skin cells, called interstitial fluid. If your glucose is too low and you don't notice it, some CGMs can alert you in time to get treatment. Your doctor may also recommend using a CGM to help you stay in your target glucose range.

How is a CGM used?

CGMs have several parts.

You wear one part—the sensor—against your skin. It has a tiny sensor that stays under your skin and constantly reads your glucose level. An rtCGM sensor sends this information to the other part of the monitor (a wireless receiver or smartphone app). Some insulin pumps include a built-in rtCGM receiver. An isCGM has 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 whenever you need it. You can view the stored data to help you identify trends in your glucose level.

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

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

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

  • Some rtCGMs or isCGMs have an alarm feature to alert you if your glucose level is quickly going up or down or if you have a glucose level out of your target range. This is helpful for people who have problems knowing when they have low glucose (hypoglycemia unawareness).
  • 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 to 14 days.
  • Some rtCGMs will require that you poke your finger with a CBG meter to confirm the CGM's accuracy.
  • Acetaminophen (Tylenol) and other medicines can affect the accuracy of the reading 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 check 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 sick, dehydrated, or if the sensor is not inserted properly, the results on your rtCGM or isCGM may be 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.

Where can you learn more?

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