Close tracking of two CGMs from different lots assures the CGMs are working correctly and the advantages of wearing two CGMs
In a previous tracking experiment, I used two Libre CGMs of the same lot and had adjacent serial numbers. Results that the two CGMs tracked each other within ±2 mg/dL and often just within ±1 mg/dL, except during fast-rising or fast-falling BG levels. The results suggest that the two CGMs have the same characteristics and performances.
In the current tracking experiment, two Libre CGMs (G1 and G2) not from the same lot were used. Further, G2 was activated 7 hours after G1 increasing the randomness between the two CGMs. This allows different interpretations of the experimental data based on probability.
Info on CGMs
For G1: Lot T60003484 SN 0PFAMMP3l expiration date 2026-03-31. For G2: Lot T60003761 SN 0R6G6H93E expiration date 2026-03-31. G1 was placed on left chest and G2 was placed on right chest.
Factory-calibrated CGMs
A CGM uses the current I(t) generated by electrochemical reaction to measure the interstitial glucose. This current needs to be converted to glucose level G(t) (in mg/dL). This conversion is complex. In one example the following equation (link) is used:
G(t)=a*I(t) –[b+c* Δ(t)] -w(t)
Where a*I(t) is referred to as the sensor gain, [b+c* Δ(t)] is the offset, and w(t) is white noise.
“Due to the changing environment around the sensor and other factors affecting sensor drift, this linear function is only accurate for about 24 h. This results in the parameters of the function needing to be adjusted every 12 h…”. “.. a calibration function, which corrects for sensor drift over the 10-day wear period by keeping track of the day since insertion and adjusting the calibration function, …The adjustments are hardcoded and based on how much an ‘‘average’’ sensor would drift. Additionally, rather than the near function used in previous models, which accounts for drift over 24h, the G6 uses a calibration function that has time-varying functions for sensor offset and gain, which can account for drift over 10 days.”
Given the complex calibration function, I did not expect two Libre CGMs from different lots would track each other. To my surprise, they did closely track each other over 15 days. The close tracking has an important usage: it can be used to assure that the CGMs are working correctly, to be explained below.
A behavior of the two CGMs system
I noticed a special behavior of the two CGMs system. For simplicity, I only care about the glucose in the interstitial fluid (ISF) and ignore the venous blood glucose.
Assume that we have the ISF glucose waveform given in red in Fig 1: It slowly declines in a straight-line manner, followed by having more time variation. The question is, what should the glucose waveforms of CGM1 and CGM2 look like?
The answer is it depends on the error of each CGM, which can be represented by an offset and a more rapid time-varying component. The inaccuracy spec is the often-quoted glucose value ±20%. Over the observation window W, if the rapid time-varying error of each CGM is very small such that it can be ignored, then the only error left is the offset. This situation is shown in Fig 1. In , the ISF glucose (red line) is faithfully reproduced by CGM1(blue line) and CGM2 (green line). Each has an offset from the red line as shown. The offsets caused CGM1 and CGM2 to have a gap between them, but the height of this gap does not change over the observation window W because the rapid time-varying error of each CGM is very small.
The next situation is we are wearing two CGMs. The waveforms of CGM1 and CGM2 are recorded on the iPhone Zukka app for example. As shown in Fig 2, we have the situation of the two waveforms are the same, also with a constant gap between them. The question is: what is the ISF glucose waveform, and where should it be located? If there is only one CGM, the answer is that the ISF glucose waveform is located within ±20% of the CGM waveform. But what will be the answer if the CGM is faulty and is an outlier? With one CGM, there is no easy way to determine if the CGM is faulty, or an outlier, except by using a finger prick. But the value of finger prick is only valid at a single time point, it does not tell us if the CGM is working correctly at other times.
However, using two CGMs and if they have the same waveforms with a constant gap between them, then, due to probability, we know for certainty, the two waveforms are the results of detection of the ISF waveform and this ISFwaveform is the same as CGM1 and CGM2. Furthermore, since CGM1 and CGM2 have the same waveforms with a constant gap between them, the time-varying error of each CGM must be negligibly small over the observation window. That is, we have the situation and relationship as in Fig 1. The ISF waveform is replica of either CGM1 or CGM2 and is also separated by an offset from each CGM.
To answer the question, where is the ISF glucose waveform located, a high-accuracy glucose meter such as Contour Next One could be used to calibrate the system. One expects the gap to change as the days go on. It is clear that recalibration is needed when the gap changes.
Results from wearing CGM1 and CGM2
I found that in general, the average error of (G1-G2) is ~ 10mg/dL within the first 7 days, ~ 20-25 mg/dL from day 8- to 15.
Some results are given in Fig3-6. They are self-explanatory. Fig 3-4 shows the close tracking of CGM1 and CGM2 with an approximately constant gap over 8 and 12 hours respectively. Fig 5-6 shows the tracking for falling and rising glucose levels.
My personal opinions on the advantages of wearing two CGMs are:
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I am sure that the two CGMs are working correctly if the waveforms are the replica of each other and with a constant gap.
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Can calibrate the CGM system against a given gap. The two CGM systems can have better accuracy than using a single CGM. (In a previous post, I had analyzed the error of the CGMs and showed that it has a better accuracy than using a single CGM.)
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Local BG fluctuation of readings of one CGM can be ignored if fluctuation only happens to one CGM.
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When the BG level is taking a cliff-dive towards hypoglycemia, when one CGM times out (for many hours), the other CGM may not time out.
The disadvantage is obviously cost and inconvenience. These above results and conclusion are just from one experiment and cannot be generalized.
Posted Feb 10, 2026, 4:01 PM UTC