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Ask Dana: How accurate are continuous glucose monitors?


Dear Dana,

I have been wearing a continuous glucose monitor (CGM) now for several years but I still don’t always trust it. How do I know if my blood sugars are accurate? And how do I know when to change out my CGM sensor?

– Travis Pinkerton
Toronto, Ontario, Canada


Dear Travis,

I understand and totally relate to your concern about the accuracy of your continuous glucose monitor (CGM). Having reliable information about your blood sugar levels is crucial for managing diabetes, so it’s natural to feel uneasy or even frustrated if you question your device’s accuracy.

The good news is that all CGMs on the market have something called a MARD – Mean Absolute Relative Difference – which rates and assures the accuracy of a CGM system.  The CGMs on the market must prove that they are accurate and are verified against a reliable reference point.  At home though, you can also verify the accuracy of a CGM sensor; the reference point that you would use at home would be a blood glucose monitor.

While CGM devices test glucose values in interstitial fluid, glucometers test blood glucose.  This means that CGM devices are about 5 minutes behind in data compared to blood glucose meters available for home use.  In times of quickly changing blood sugars, it is still advised and necessary to use a glucometer in order to verify your blood sugar and CGM device.  We often see a large difference between glucometers and CGMs when people are treating a low blood sugar. The glucometer will determine that a blood sugar is back in normal range much faster than a CGM.  If you only use a CGM to watch low blood sugars respond to treatment, many times this causes over treating of those low blood sugars.

Despite the fact that CGM devices do not, for the most part, need to be calibrated by the user, it is still recommended to use a glucometer to calibrate a new sensor and verify your sensor is reading accurately.  When you begin to trust the accuracy of a sensor compared to an accurate glucometer, a CGM is indicated to be used as a guide in dosing insulin.

An easy way to compare your CGM readings to a traditional blood glucose meter is to use the “20/20 rule.” The rule states that if the difference between the two readings is less than 20%, then your CGM is considered accurate. It’s important to remember that occasional discrepancies are normal but consistent differences beyond the 20% range might indicate an issue with your CGM sensor.  However, make sure you are using a glucometer that also has a high MARD value or accuracy rating. Your healthcare team can help you with this information.

In addition to calibrating your sensor, make sure that your sensor is inserted properly under your skin. You may need to place an overlay tape over the sensor to make sure it remains in place.  There have been studies showing that the back of your arm is the best placement for accurate CGM readings. While wearing devices in alternative places on your body is perfectly acceptable and, in some cases, preferable, if you are having trouble with the accuracy of your sensor, try wearing it on the back of your arms.

Also of note, certain CGM manufacturers list potential medications that can interfere with a sensor’s accuracy.  These medications can be something you use on a regular basis as it includes acetaminophen and even vitamin C.  Double check on the sensor packaging if these are medications to avoid for your CGM system.

If you are still struggling to trust your CGM and receive reliable data, discuss this with your healthcare team or reach out to our team of Certified Diabetes Care and Education Specialists at Integrated Diabetes Services.  We can help troubleshoot or recommend another course of action for your diabetes management.

Don’t hesitate to reach out to the team here at Integrated Diabetes Services and enjoy the holiday season!

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