A quick recap from Understanding Risk Before Performance.
The only CGM Systems tested with a study design meeting basic criteria are:
- Accu-Chek SmartGuide
- Dexcom G6 and One
- Dexcom G7 and One+
- EverSense
- Freestyle Libre 2 and 2 Plus
- FreeStyle Libre 3 and 3 Plus
- Medtronic Guardian 3
- Medtronic Guardian 4
- Medtronic Simplera
Ideally, CGM accuracy should be assessed against capillary glucose rather than venous glucose. Capillary glucose represents the highest glucose levels that micro-vessels—such as those in the eyes, kidneys, feet, sexual organs, and nerves are exposed to, making it the key driver of long-term complications. In contrast, venous glucose tends to underestimate peak glucose exposure, which can lead to a misleadingly lower assessment of risk.
However, data comparing CGMs directly against capillary glucose is not consistently available for all systems. Given this limitation, I have initially summarised the accuracy of all the CGMs (except Accu-Chek SmartGuide) based on their performance against venous glucose, ranking them into two categories. Capillary comparisons including the Accu-Chek SmartGuide come after.
Top Band: CGM systems meeting the 15/15, 20/20 and 40/40 iCGM Performance Standards. These systems meet the highest accuracy standards, ensuring reliable performance in hypoglycemia (<3.9 mmol/L or <70mg/dL) target range (3.9-10.0 mmol/L or 70-180 mg/dL), and hyperglycemia (>10.0 mmol/L or 180 mg/dL)
- Dexcom G6 and One
- Dexcom G7 and One+
- EverSense
- Freestyle Libre 2 and 2 Plus
- FreeStyle Libre 3 and 3 Plus
Second Band: Approved CGMs (Non-iCGM). These CGMs do not meet iCGM criteria when assessed against venous glucose, but they are still accurate enough for treatment decisions.
- Medtronic Guardian 3
- Medtronic Guardian 4
- Medtronic Simplera
But the Real Question Is…
How accurate are the leading CGM systems against capillary glucose?
That’s what really matters because capillary glucose better reflects complications risk. If CGMs align with venous glucose, they may not provide the most meaningful insight for long-term health compared to CGMs aligned with capillary glucose levels.
Fortunately, the current generations have been tested against capillary glucose, so let’s take a look at them
- Accu-Chek SmartGuide
- Freestyle Libre 3 (Freestyle Libre 2 has the same sensor technology and algorithm)
- Dexcom G7 (Dexcom One+ has the same sensor technology)
- Medtronic Simplera
Before we take a look, here is an important note about the testing methodology:
- Freestyle Libre 3, Dexcom G7, and Medtronic Simplera were tested independently using the Dynamic Glucose Region (DGR) Challenge. The most rigorous approach evaluates performance under rapid glucose fluctuations, making it a more robust assessment of accuracy, but it will give the appearance of lower accuracy.
- Accu-Chek SmartGuide Study, although it did use meal and insulin challenges, the DGR Challenge was not used. While it provides valuable data, it has not been tested under the same conditions as the others, biasing it towards higher accuracy when compared to the others tested under DGR conditions. This is no different to all the CGM systems that achieved iCGM approval with a similar study design, they did not use a DGR challenge either.
This graph illustrates a DGR test, which evaluates rapid glucose rises after a meal without insulin, followed by a sharp drop after a high insulin dose during the same study visit. In contrast, when the meal and insulin challenges are conducted on separate visits, glucose fluctuations occur more gradually, and the CGM is not pushed to its limits.
Using a DGR is like using a car that’s been tested the way glucose sometimes does (like a rollercoaster!). For example:
- Food without insulin followed by several rage bolus’s in quick succession
- Very late bolus for food when using an AID System
- Undertaking exercise within 90 minutes of eating (high insulin on board, my nemesis!)
Performance Metrics
There are many accuracy metrics, each with its limitations. MARD, the most commonly used metric, provides an overall average error but does not account for the risks associated with extremely high or low glucose levels. A useful way to understand this is by asking:
“Would you cross a river if its average depth were 4 feet, even if you couldn’t swim?”
If your answer is yes, you may not fully grasp risk.
The extremes are where the most critical decisions happen!
Agreement rates indicate the percentage of readings that fall within an acceptable range, such as ±20%. This helps identify how often readings are accurate enough to avoid problems. However, it lacks clarity on the proportion of readings that may cause minor or major issues, making it less effective for understanding overall risk.
The iCGM measures are comprehensive and stringent. When a CGM meets these standards using venous glucose as the comparator, it is considered highly accurate. However, when a DGR test is used with capillary glucose as the comparator, the criteria become overly strict and virtually impossible to meet. Even top-performing CGMs like the Dexcom G7 and Freestyle Libre 3 fail to meet these standards.
This does not mean the sensors have suddenly become less accurate. Instead, it highlights that when a rigorous study protocol is followed using capillary glucose as the reference, meeting the iCGM Performance Standards becomes virtually impossible. Therefore, once CGM testing is standardised, appropriate performance metrics and cut-offs are required.
A set of standardised testing procedures, including the DGR challenge, from the IFCC working group for CGM have been tested and verified (Feb 2025).
So, is it time for DGR performance standards?
I hope I have made a strong case, as I believe so.
We need performance metrics that test candidate CGM systems in the five critical DGR Regions: BG low, Alert Low, Neutral, Alert High, BG High.
Why Five Regions?
This approach gives us a perspective on sensor accuracy when glucose levels are dropping toward hypoglycemia or rising rapidly to high levels. Therefore, the reported accuracy of sensors will reflect what we, as users, experience from time to time. This will help us understand when sensors are most reliable and when we need to be more vigilant.
Hint:
We must be especially mindful during rapid glucose changes, especially drops caused by exercise. This has been highlighted by Professor Othmar Moser and the wide range of experts who authored the CGM and AID international guidelines for exercise and T1D. It’s always wise to listen to a consensus when compiled by experts in the field!
However, the DGR metrics and standards are not quite ready for widespread use yet.
So, what do we have that gives us clear indications of clinical risk, has multiple zones, and visually allows us to assess risk and compare performance?
The DTS Error Grid
The Diabetes Technology Society (DTS) Error Grid is a tool designed to evaluate the clinical accuracy of blood glucose monitoring systems by assessing the potential risks associated with measurement errors. It compares the glucose readings from a device to comparator values and categorises the differences based on their potential impact on patient care.
The horizontal axis (x-axis) represents the true blood glucose values (in our case, capillary glucose). The vertical axis (y-axis) shows the glucose values measured by the device being evaluated (in our case, the CGM System values).
Zones: The graph is divided into different zones, each indicating a level of risk associated with the discrepancy between the CGM and comparator (capillary in our case) values:
- Zone A: Differences here are clinically insignificant, meaning the device’s readings are accurate enough for safe and effective treatment decisions.
- Zone B: Represents minor errors that would lead to benign or no treatment changes.
- Zone C: Indicates discrepancies that could lead to overcorrection or unnecessary treatment adjustments.
- Zone D: Errors here might cause a failure to detect and treat hypo- or hyperglycemia.
- Zone E: This signifies errors that could result in dangerous treatment decisions, such as administering insulin when it’s not needed, potentially leading to severe hypoglycemia.
Using the DTS Error Grid, we can compare the Freestyle Libre 3 (yellow), Dexcom G7 (green), and Medtronic Simplera (blue), using the data from the three sensors study that used capillary as the comparator and performed a DGR Challenge.
Also, the Accu-Chek SmartGuide Study used an older Consensus Error Grid with less robust meal and insulin challenges, but it’s still very informative and shows good accuracy.
What do the results mean?
The following systems have approximately 90% of their readings carrying no clinical risk, while the remaining 10% pose only a slight risk, with virtually no readings carrying high risk:
- Dexcom G7 (Dexcom One+ uses the same sensor technology, algorithm, and hardware as the G7.)
- Freestyle Libre 3 (Freestyle Libre 2 shares the same sensor technology and algorithm, so its accuracy is expected to be almost identical as only the hardware is different.)
- Accu-Chek SmartGuide
In contrast, the Medtronic Simplera underreports capillary glucose levels significantly, with only 66% in Zone A (no risk), which has important clinical implications:
- Hypoglycaemia Management – There is almost no risk of missing a true low glucose level (hypoglycaemia), but the system is likely to incorrectly indicate hypoglycaemia when the person’s capillary glucose is in a safe range. This may be very helpful for those with serious hypo issues but can lead to unnecessary treatments for low blood sugar when it’s not needed.
- Hyperglycaemia Management – The Simplera also underestimates high capillary glucose levels, which presents issues for individuals using Automated Insulin Delivery (AID) systems or those manually managing insulin with pens or pumps:
- AID users may miss important insulin corrections if their sensor glucose readings appear within range while their actual capillary glucose is higher. However, the SmartGuard algorithm in the 780G is particularly aggressive, providing approximately 5% more time in range compared to the t:slim X2 with Control-IQ. This increased time in range compensates for the underreporting of capillary glucose, ensuring that HbA1c levels remain equivalent between the systems. However, this equivalence is only achieved due to the 5-7% higher time in range.
- On the other hand, people using insulin pens or non-automated pumps may not adjust their insulin dosing if their sensor glucose appears in range, even when their actual capillary glucose is higher. As a result, achieving a 70% time-in-range with the Simplera sensor may not offer the same level of protection as a 70% time-in-range with a Dexcom, FreeStyle Libre, or Accurate-Chek SmartGuide system, despite users putting in the same effort. Interestingly.
- Medtronic has partnered with another CGM manufacturer to develop future sensors, so this may be different in the future.
Takeaway
If you are comfortable with venous glucose comparisons, an iCGM-approved CGM should be your first choice. However, don’t overlook other CGMs, as they are still more than accurate enough for clinical decision-making.
For the most accurate reflection of capillary glucose levels, what matters most for day-to-day management, CGMs like the Dexcom G6/G7/One+, Freestyle Libre 2 and 3 Plus, and Accu-Chek SmartGuide offer better alignment.
The Medtronic Simplera, which tends to underestimate capillary glucose levels, may be beneficial for those prone to severe hypoglycaemia. However, this could also lead to unnecessary hypo treatments and delayed responses to high glucose, potentially affecting HbA1c and future complication risks.
Now that you have a clearer understanding of accuracy, the next section explores all the bells and whistles, such as…
- Factory Calibration vs Manual Calibration (choose a blood glucose meter that aligns with your CGM System)
- Optional Calibration
- Alarms
- AI-powered systems
- Sensor wear duration
- Followers
- Hardware
- Integration with automated insulin delivery (AID) systems
- Age restrictions
