Improving the accuracy of a CGM by wearing two CGMs and by transferring the accuracy of a glucose meter to the two-CGM system
Fig 1: Green solid and dotted lines are glucose readings of CGM1 and CGM2. They are replaced by using the CGM average glucose in a dotted red line and the gap marked by red arrows. The white line is the reconstructed ISF glucose reading, which has an offset with the CGM average glucose line. This offset is constant over the entire observation window and thus can be calibrated out using a glucose meter.
What is this post all about?
This post proposes a method to transfer the accuracy of a glucose meter to a two-CGM system by wearing two CGMs. This post may be redundant with the last post, but I am posting it because the emphasis of the last post is tracking of two CGMs, whereas the emphasis of the current post is how to improve the CGM’s accuracy.
Basis for the proposed method
The method employs a simple idea, which is to use an additional CGM to monitor the current CGM to see if the inaccuracy of the current CGM has changed. If not, leave it alone, and if yes, then perform a new calibration. As a result, the calibration is always valid, thus providing the same accuracy as the glucose meter, which is used for calibrating the CGM. Tracking of two CGMs is used for this purpose. If two CGMs’ glucose readings are otherwise exactly the same except differ by a fixed value (called “offset”) over an observation window, the offset (or gap) over time can tell us if the inaccuracy has changed.
What if the two CGM system does not work as expected?
It will not affect the operation of each of the CGMs. One can continue to use the CGM that has better performance or use both individually and not as a system.
What is the difference between the calibration between using a single CGM and two CGMs?
It is well known that the inaccuracy or error of a CGM changes over time. When using a single CGM, we don’t know when the inaccuracy or error of a CGM has changed sufficiently so that a new calibration is needed. The two-CGM system resolves this difficulty by using one CGM to monitor the other CGM.
CGMs currently can only be used as a trend indicator
It is well known that the Libre CGMs have an inaccuracy of ±20%, and worse yet, for values <80mg/dL, it is ±20mg/dL. This error forces users to use it as a “trend” indicator, that is, to show if the glucose is rising or falling. Whenever the users want to know the “true” glucose reading, for example when plunging towards the dangerous hypo zone, they would need to use a glucose meter to double-check the reading. To my knowledge, the most accurate glucose meter is the Contour Next One, which has an accuracy of ±8.4%.
Explaining the time behavior of the 20% error or inaccuracy
Before we begin, we need to understand the time behavior of the CGM’s 20% error. For simplicity, I assume the observation window to be 8 hours. According to the published literature, this error consists of a simple offset error a more rapid time-varying error. The offset error means that over time the difference between the ISF glucose and the CGM glucose is a fixed value, whereas the time-varying error is unpredictable. The sum of these two errors gives us the 20% inaccuracy. The glucose in the ISF causes a response from the CGM. When the first CGM is activated, its response includes these two errors: an offset error and a time-varying error. Similar errors occur when the second CGM is activated. All these four errors are not expected to have the same magnitude. In the next paragraph, the time-varying errors disappeared.
Results from a previous two-CGM experiment show that only the offset error is important because the time-varying errors are too small and can be ignored
The results of a previous experiment show that two CGMs not from the same lot, one on the left and one on the right chest, track each other well and with a constant gap between them over the observation window. These results indicate that the time-varying errors of each CGM are so small that they are negligible. What is left is the offset error.
Study of factory calibration for CGMs shows that the offset error will also change in time, but the change is slow (measured in days after insertion). Experimentally, indeed, this is what has been observed: the gap between the CGMs changes from ~10mg/dl for days 1-7 to ~25mg/dL for days 7-15.
Thus, we can use the amount of change in offset to tell if a new calibration is needed. For example, recalibrate if the offset has changed by 5mg/dL or more.
Replacing the glucose readings from CGM1 and CGM2 by using the average of the two CGMs’ readings
Using the readings of two CGMs is inconvenient. It will be more convenient if we use the average of the two readings instead because, as a result, we only need to deal with one variable instead of two. This is shown in the figure.
With the behavior of the error understood, instead of having to use the glucose readings of two CGMs, it is simpler to use the CGM average value (defined as G(t)=[G1(t)+G2(t)]/2)] of the two CGM values and the gap (gap(t)). This is shown in the figure. As far as the waveform is concerned, since CGM1 and CGM2 have the same waveform as the ISF glucose, the CGM average glucose also has the same waveform as the ISF glucose. In addition, the CGM average glucose also has an offset with the ISF glucose. If the offset between CGM1 and CGM2 is a constant, then the offset between the ISF glucose and CGM average glucose is also a constant. This is the rationale for why the ISF/CGM-average glucose offset can be calibrated out by the glucose meter because after calibration, the the CGM average glucose will be the same as the ISF glucose.
An earlier analysis shows that the CGM average glucose has improved accuracy, but not as much as offered by the current method.
How to get the highest accuracy from the Contour Next One?
u/SuspiciouslyDullGuy has a clever way to get the highest accuracy from the Contour Next One. The accuracy is listed as “95% of results within ±8.4%.” There is a study that used MARD, and the result for 70-179 mg/dL is MARD=4.8%, SD=3.5. “MARD represents the average percentage difference between .. glucometer’s readings and gold-standard laboratory measurements.” What u/SuspiciouslyDullGuy did was, for each CGM calibration, he painstakingly made several measurements using the Contour Next One. He would then throw away all the outliers and average the remaining measurements that had similar values. As a result, he got an accuracy similar to the MARD, which is ~ 5%.
u/Secret-Bobcat-4909 had also checked the accuracy of glucose results between using the Contour Next One and from blood tests using lab equipment. He often got within 5%, sometimes 10%, validating the accuracy of the Contour Next One.
Supportive evidence that improving the accuracy of a CGM is possible from another user
u/SuspiciouslyDullGuy has put in a lot of effort in calibrating the Dexcom CGM. “By testing a few times a few minutes apart when levels are stable, …. it's possible to identify the outlier readings and discount them. I then use the average of the 'good' readings to calibrate the CGM. …I have to recheck it every few days to be sure the calibration hasn't drifted. I'm not sure why that happens, but sometimes I have to repeat the calibration procedure every few days. Once calibrated in this way the correlation between CGM readings and Next readings can be remarkable, when levels are stable, much better than the percentages quoted on either the CGM or meter data sheets. By cherry picking probable good readings from a set of Next readings and using the average to calibrate the CGM high levels of accuracy are possible.”
I follow his calibration method. However, the proposed two-CGM method is an improvement over his single CGM method because it knows exactly when a new calibration is needed. That time is when the gap between the two CGMs changes by a predetermined mg/dL, for example, 5 mg/dL.
For the best accuracy, lag time must be taken into account in calibration
To get the highest accuracy from calibration, one must take into account lag time by making sure that the CGM’s readings are stable as well as use the CGM average glucose reading ~ 7 min after a finger prick (7 min is the lag time measured by using a radioactive glucose tracer.). This is to account for the lag time, which glucose takes to diffuse from capillary blood to the interstitial fluid.
Cost of putting on the additional Libre sensor
Higher accuracy will cost more. According to posts on this sub, in the US, the lowest price is Costco at ~$30/Libre; the next lowest cost is with a co-pay coupon from Abbott at ~$35/Libre. So, it will cost ~$70/month to improve the Libre accuracy from 20% to less than 10%, in addition to the cost of test strips.
Caveat: As pointed out by u/Equalizer6338, more experiments are needed to substantiate the tracking results.
Posted Feb 14, 2026, 5:52 AM UTC