CGM Accuracy - CGM vs Finger Prick Glucometer

Overview: 

This article provides an overview of the accuracy of the FreeStyle Libre and Dexcom G6 CGMs in comparison to a finger prick glucometer.

In this Article

Difference in Readings - CGM vs Finger Prick Glucometer

It's common for people to compare their CGM readings against their finger stick to compare accuracy and notice that the values are different. I know that this can be disconcerting, but the key thing to know is that some variations in readings will occur and this is expected.

CGMs and finger stick glucometers measure glucose concentration from different sources within the body. Freestyle Libre and Dexcom G6 are factory-calibrated to estimate blood glucose based on  interstitial glucose - which is the fluid between skin cells. These numbers can lag behind capillary blood glucose levels (aka a finger stick readings) by ~15 minutes depending on how quickly things are changing. This can stack up especially during times of actively changing glucose to produce a delta between finger stick and CGM reading. If you're comparing readings, it's best to do so in a fasted state.

Error Margin - CGM vs Finger Prick Glucometer

The gold standard for accuracy is a blood draw measurement and both finger sticks and CGMs have error margins (MARD) to that standard. Finger sticks tend to be in the range of 5-10% MARD, while the Libre has a MARD of about 9.7% over 14 days, and the Dexcom G6 has a MARD of 9% over 10 days.

As well, blood is dynamic and always moving, so the enzymes even from sample to sample in the same blood droplet vary slightly one from the other, a simple way to test this is use two different fingers on two different strips with a finger stick glucometer.

Make the Most of Your CGM Readings

We find that focusing on the trends in glucose values before and after meals is the most valuable way to use CGM even if baseline readings are slightly off. Levels' Zone Score algorithm only weighs the change between pre- and post-meal glucose and glucose variability, rather than the absolute numbers at the beginning or end. The absolute numbers are factored into the Metabolic Score, but the relative glucose changes are most heavily weighted.

This all being said, unfortunately sensor errors do happen from time to time. These typically appear with readings that are extremely low or persistently far off a finger stick measurement over a couple of days. If it seems like this is the case, please email support@levels.com and we'll be happy to help out.

Further Reading:

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