Most people who have used a continuous glucose monitor for long enough will probably already know that there comes a point when the numbers stop. Glucose rises beyond the range the sensor can report and, instead of telling you exactly how high it has gone, your CGM can only indicate that it is very high.
It can be a frustrating limitation, particularly because this is precisely the point at which knowing more about what is happening could be useful. Now researchers have put some numbers around just how frequently it happens.
After analysing almost 47 million CGM readings from 948 people with type 1 diabetes, researchers found that between 93.5% and 100% of participants reached either the upper or lower measurement limit of their CGM at least once.
For some people, the missing information lasted considerably longer than a few minutes.
When a CGM reaches the end of its scale
Every measuring device has a range within which it is designed and validated to operate, and CGMs are no different. Once glucose moves outside that range, the CGM can indicate that the limit has been exceeded but cannot necessarily tell us what the true glucose concentration is.
The new study analysed data from four major international type 1 diabetes trials using older Dexcom G4 Platinum and Dexcom G6 systems. The upper measurement boundary in those datasets was generally around 22.2 mmol/L (400 mg/dL).
Once glucose exceeded that point, readings accumulated at the upper boundary rather than continuing upwards with the person's actual glucose. The researchers describe this as measurement "capping".
This was not particularly unusual. In three of the four datasets, more than a third of episodes reaching the upper limit remained there for at least an hour. For a typical participant, the longest episode lasted around two to three hours.
Perhaps more strikingly, for 122 participants, representing 12.9% of everyone included in the research, the upper boundary was actually their most frequently recorded CGM value.
That doesn't mean their glucose repeatedly settled at exactly the same level. It reflects the sensor reaching the point beyond which it could no longer quantify how high glucose had gone.
How does this compare with FreeStyle Libre?
The research becomes particularly interesting when compared with the CGMs people are using today.
Importantly, the study did not test current FreeStyle Libre sensors or Dexcom G7. It analysed historical datasets involving Dexcom G4 Platinum and G6, so the findings shouldn't be interpreted as a direct comparison of today's FreeStyle Libre and Dexcom systems.
However, the measurement ceiling isn't identical across CGMs, and this produces an interesting difference between the two major CGM brands.
| CGM | Lower reporting limit | Upper reporting limit |
|---|---|---|
| FreeStyle Libre | 2.2 mmol/L (40 mg/dL) | Up to 27.8 mmol/L (500 mg/dL)* |
| Dexcom G7 | 2.2 mmol/L (40 mg/dL) | 22.2 mmol/L (400 mg/dL) |
*Abbott documentation states that the upper reporting limit varies by system and market. For relevant FreeStyle Libre systems outside the US, a HI result can indicate glucose above 27.8 mmol/L (500 mg/dL).
The difference at the upper end can be substantial. A FreeStyle Libre user whose glucose reaches 24 mmol/L may still receive a numerical glucose reading on a system with a 27.8 mmol/L ceiling, while a CGM with an upper reporting limit of 22.2 mmol/L has already reached the end of its scale.
Ultimately, however, both systems encounter the same fundamental limitation. At some point the numerical reading stops and the CGM can tell us that glucose has exceeded its measurement range, but not whether it has gone slightly beyond it or considerably further.
Does this make CGM data unreliable?
Importantly, the researchers aren't suggesting that CGMs are unreliable. They found that measurement capping generally had little effect on familiar measures such as average glucose, glucose variability and time in range because most glucose readings remain within the measurable range.
The problem is specifically what happens at the extremes. A CGM can tell us how much time someone has spent above the standard target range, but once glucose exceeds the sensor's measurement ceiling it can no longer quantify the full severity of that particular excursion.
Two people might therefore each spend an hour with very high glucose, but if both exceed their sensor's measurement ceiling, their CGM data cannot necessarily show how different those two events really were.
Could this actually be fixed?
This is perhaps the more interesting question raised by the research, and there are really two different issues.
The first is whether CGMs could simply measure a wider glucose range. That may not be straightforward. CGMs are electrochemical biosensors and extending their measurement range would require manufacturers to demonstrate that sensors remain sufficiently accurate and reliable at much higher or lower glucose concentrations.
The study does not establish that Abbott, Dexcom or another manufacturer could simply remove the measurement ceiling with a software update.
But the researchers identify a second, potentially much simpler improvement: tell users more about what happens when the ceiling is reached.
A CGM already knows when it has reached its reporting boundary. Its software therefore has the information needed to record how frequently this happens and how long the sensor remains capped, even if it cannot determine the missing glucose values themselves.
The researchers suggest that the frequency and duration of capping could be reported alongside conventional CGM statistics.
That distinction is important. Providing this information wouldn't suddenly tell someone whether their glucose had reached 25 or 30 mmol/L, but it would distinguish a five-minute excursion beyond the sensor's range from one lasting two hours.
Despite increasingly sophisticated CGM apps, reports, predictive alerts and automated insulin delivery systems, this relatively simple piece of information generally remains invisible to users.
But glucose may only be part of the story
There is another reason the limits of glucose measurement are becoming particularly interesting now.
For people with type 1 diabetes, very high glucose can sometimes be associated with insufficient insulin, rising ketones and the risk of DKA ie. diabetic ketoacidosis. Yet a conventional CGM cannot tell us whether ketones are rising.
Two people could therefore have apparently similar CGM displays while something quite different is happening metabolically. One could have glucose above the measurement range with normal, stable ketones, while another could have a similar glucose display alongside elevated and rapidly rising ketones.
Glucose also doesn't always provide an early warning of DKA. Euglycaemic DKA can occur without exceptionally high glucose levels, one of the reasons researchers have been investigating the potential of continuous ketone monitoring.
Abbott is already looking beyond glucose
This isn't simply a theoretical possibility. In May 2026, Abbott announced that Libre Duo had received CE marking in Europe.
Unlike a conventional FreeStyle Libre sensor, Libre Duo is designed to continuously measure both glucose and ketones from a single wearable sensor, with readings every minute. Abbott says continuous ketone information could provide visibility of rising ketones that may lead to a DKA emergency.
This represents an important development because diabetes wearables have traditionally concentrated on providing increasingly frequent and accurate glucose information. The next generation of sensors could potentially provide a much broader picture of what is happening metabolically.
It also creates an interesting contrast with the new CGM research. The industry is now capable of developing a wearable sensor that measures an entirely different biomarker alongside glucose, while at the extremes of glucose measurement itself, information can still disappear.
What should the next generation of CGMs tell us?
Extending the glucose measurement range and adding continuous ketone monitoring are not the same solution.
A wider glucose range could tell us more about the severity of extreme hyperglycaemia. Ketone sensing could tell us something that glucose cannot tell us at all. Meanwhile, better software reporting could provide useful information about extreme events without requiring the sensor to measure any further.
Together, however, they illustrate how much more informative future diabetes wearables could become.
A future system might not simply display HI. It could potentially tell us that glucose has been beyond the measurable range for 45 minutes and, with dual-analyte technology, whether ketones are stable or rising at the same time.
CGMs have transformed diabetes management and this research doesn't challenge that. Instead, it highlights one of the remaining gaps in technology that has become remarkably sophisticated in almost every other respect.
For many experienced CGM users, discovering that their sensor has a measurement ceiling isn't news. What is surprising is just how often people reach it, how long they can remain there and how little information we're still given about what happens beyond it.
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