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  • Writer's pictureDr. Mansur Shomali

How does a continuous glucose monitor (CGM) work?

Updated: Nov 24, 2020

Guest Post by Dr. Mansur Shomali

What, How and Who of CGMs

What are common CGMs?

The use of CGM systems has grown tremendously in the past couple of years. The newest technologies are more accurate and easier to use than ever. The most commonly used systems in the U.S. and Canada are:

· Dexcom G6 CGM System

· Freestyle Libre 14 Day System and FreeStyle Libre 2 System

· Medtronic Guardian Sensor 3

How do they work?

These systems work by measuring glucose in interstitial fluid (ISF), the fluid right under the skin. Though this is not blood glucose, the system converts the readings to blood glucose. To get a reading, you either can just look at the CGM reader or you can use an app on your compatible phone. A new data point is generated every minute or every 5 minutes, depending on the system.

One very useful feature is the trend arrow. Whenever you get a reading, you get a number and an arrow. For example, you might get a reading like this:


This means that your glucose is 81 mg/dL (4.5 mmol/L) and rising. When the arrow is sloped to the right it means it is rising slowly and no urgent action needs to be taken.

Or you may get a reading like this:

85 ↓↓

In this second example, the glucose is 85 mg/dL (4.7 mmol/L) but dropping very quickly as shown by the double arrow. Your glucose is dropping so fast that you should eat some fast-acting carbohydrates to avoid a serious low blood sugar reaction (hypoglycemia). See how useful the trend arrow can be!

The most important measurement is something called Time in Range. This is the percentage of time the glucose stays between 70 to 180 mg/dL (3.9 to10 mmol/L). For most people, Time in Range should be over 70%. The CGM system tracks this value for you so you can see how you are improving from day to day and week to week.

Who is CGM for?

CGM systems are useful for people with type 1 and type 2 diabetes who need the extra data to better manage. For example, knowing how high your glucose goes up after a meal can inform you to make better food choices. In addition, CGM can be very useful for improving insulin dosing and preventing hypoglycemia.

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