CGM: The Bells and Whistles

If you have jumped straight here, STOP.

Go back and look at;

Understanding Risk Before Performance

Assessing CGM Accuracy Performance

Or listen to the CGM Podcast Series

You now understand why we are only looking at a few CGMs. The ones that have been tested effectively and have published results in a peer-reviewed journal.

• Accu-Chek SmartGuide
• Dexcom G6 and One
• Dexcom G7 and One+
• Freestyle Libre 2 and 3 Plus
• Medtronic Guardian 4
• Medtronic Simplera

Choosing a CGM system can feel like picking the right smartphone; some features matter more than others, and the best choice depends on your needs. Having worked with over 1,000 people starting CGM and 250 using Automated Insulin Delivery (AID) systems, here’s a breakdown of the most important factors to consider.

• 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

Calibration: Factory vs. Manual
Calibration is like setting the time on a watch. Some watches adjust automatically (factory calibration), while others need manual input (manual calibration).

Factory Calibration (Automatic Adjustment)

All leading CGM systems are factory-calibrated, meaning they work straight out of the box without requiring finger-prick calibrations.

The Accu-Chek SmartGuide, however, still requires calibrating after 12 hours, but once that’s done, no further calibrations are needed.

Is this a big drawback?

It’s not ideal, but it’s not that big of a deal in my opinion.

Why?

As long as an accurate blood glucose meter is used to calibrate, and best calibration practices outlined below are followed, there shouldn’t be any issues.

Calibration: Glucose Alignment Matters (Capillary vs. Venous)

The capillary-aligned CGMs are: Dexcom Options, Freestyle Libre Options, and Accu-Check SmartGuide.

• If you’re using a CGM system aligned with capillary glucose, use a blood glucose meter that provides accurate capillary readings to avoid perceived inaccuracies. I’ve recently looked into this, and the most accurate and reliable blood glucose meters for CGMs aligned with capillary glucose, from independent research, are:
  • Best: Contour Next One and Accu-Chek Guide or Instant
  • 2nd best: Righttest GM700S

Venous-aligned CGMs (e.g., Medtronic Guardian 4 and Simplera)

• The most accurate and reliable blood glucose meters aligned to venous glucose from independent research are:
  • Best: Freestyle Freedom Lite
  • 2nd best: GlucoChek Gold

Optional Calibration: A Blessing or a Burden?

Some CGMs allow manual calibration, which may or may not be helpful for fine-tuning accuracy; the devil is in the details!

Your sensor measures glucose in your interstitial fluid (just under the skin), while a fingerstick test measures glucose in your blood. Because glucose moves from the bloodstream into the interstitial fluid with a slight delay, the two readings aren’t always identical, especially when glucose is changing quickly.

Optional calibration allows you to manually enter a fingerstick reading into the CGM system to help the sensor stay more aligned with your actual blood glucose. Useful if you notice a large difference between the sensor and a “reliable” fingerstick test!

This is only likely to happen 1-5 times per month, and you will only know if you test. Also, it’s only helpful if the test is performed using a meter that’s accurate and aligned with your sensor and if you are militant about your testing technique.

If required, and you can follow good practice, it’s a gentle “nudge” to get a derailed sensor back on track.

A bit like when the Sat Nav sends you down a dead end and you need to ask for directions.

Why Some Prefer Optional Calibration:

• Large inaccuracies happen 1-5 times per month. If you check your finger-prick during this time, it’s helpful to be able to get the CGM readings back on track.
• This helps to calibrate out of false low alarms that are especially annoying overnight- just ask my wife! This is why I like optional calibration; see an example of when I used it below.
• Useful for children who have multiple caregivers who finger-prick often! They need optional calibration to bring the sensor back on track and keep their confidence in the CGM’s accuracy. Whether this represents actual accuracy is irrelevant; it gives them psychological assurance, allowing them to get their mind off diabetes!
• If this is important to you, the FreeStyle Libre CGMs will not work for you. There is no optional calibration. This is by design for simplicity, which works for most, but not for those who sometimes cross-reference CGM readings with their finger-prick capillary glucose level.

Best Practice if using Optional Calibration:

• Always wash hands before testing, dry thoroughly, wipe away the first drop, and milk (not squeeze) the finger. Ensure strips are in date and the blood glucose meter is checked regularly against the control solution.
• Take two readings from the same blood drop and average them. However, if they differ by more than 10%, take a third and average the closest two. Think like you are an investigator from an accuracy trial to ensure the readings are accurate and helpful.

I am a freak when it comes to following calibration protocol. I use a ROCHE Accu-Chek Guide/Instant meter (the most accurate and reliable) that’s closely aligned with capillary glucose, just like my Dexcom sensor. For calibration samples;

“I wash my hands, dry them very well, milk (not squeeze) the finger, wipe away the first drop, take two tests, a third if they are not within 0.5 mmol/L (10 mg/dL) of each other, then average the results to for my final blood glucose value to calibrate my sensor with!”

Don’t calibrate unless you are prepared to be as militant as this! Otherwise you are likely to make the sensor less accurate.

Why Some Avoid It:

• User error (dirty hands, incorrect test strip handling) can make things worse.
• Can disrupt accuracy if bad data is entered.
• Hardly ever check finger prick, so it’s irrelevant
• If you cannot use best practices, DO NOT CALIBRATE; you will make things worse!
• If optional calibration offers you nothing, the simplicity of the Freestyle Libre options may suit you best.

Alerts & Alarms: Keeping You Informed Without Overloading You

All CGMs provide alerts for high and low glucose, but some are smarter than others.

Advanced Alarm Features:

• Urgent Low Soon (Dexcom G6/G7): Warns if glucose is predicted to drop below a set level within 20 minutes, allowing you to take action early.
• Delayed First High Alarm (Dexcom G7): Helps prevent unnecessary alarms by waiting before notifying you of a high reading after meals. As shown below in practice

Minimising Alarm Fatigue:

• If too many alerts are annoying, choose the Dexcom G7 system with its range of customisable alerts and alarms.
• For simpler alarms with fewer interruptions, Freestyle Libre 2 and 3 Plus may suit you better.

AI-Powered Predictive Insights

Accu-Chek Smart Guide:

• A 30-minute hypo prediction alert.
• Two-hour glucose projection, helpful for planning activities, driving, and meetings. This has the potential to give peace of mind for a longer period than the 10-30 minutes that trend arrows offer.
• A seven-hour overnight hypoglycaemia risk assessment is great for those using insulin injections at bedtime.
• These novel predictions, spanning 2 to 7 hours, have demonstrated over 85% accuracy when modeled using existing CGM data.
• Granted, this analysis wasn’t conducted in real time prospectively. However, this is based on retrospective data from real-world CGM use, combined with in-silico modelling. Together, they provide a test dataset covering more than 50,000 patient-days of CGM — a scale that far exceeds what could realistically be achieved in a randomised controlled trial. In this case, big data offers a clear advantage.

Who might this be helpful for?

Those on CGM and still having 4% or more hypoglycaemia (<3.9 mmol/L or 70 mg/dL). Those who want a crystal ball that predicts 30 minutes to 7 hours into the future, rather than just 20 minutes. For those who either don’t want or can’t access funding for an AID system, these predictive tools could support smarter, more informed decision-making.

Whether it flies in this highly competitive market will depend on the experiences of its first users when it launches in the summer of 2025! Watch this space! I will make it my business to give it a go and let you know.

Sensor Size & Wear Duration: Comfort Matters

Freestyle Libre 3 is the winner: It is the Smallest CGM available, with a 15-day wear time. If comfort and long wear time are priorities, Libre 3 is the best choice. The Freestyle Libre 2 Plus is also 15 days. The Accu-Chek SmartGuide is 14 days wear, the Dexcom CGMs are 10 days wear, and the Medtronic G4/Simplera are 7 days wear.

Remote Monitoring: Keeping Loved Ones in the Loop

Most CGMs allow data to be shared with family or caregivers. Therefore, there is no real difference between the CGM Systems. Particularly useful for children, partners, or those who need extra oversight, but not useful for those like me, where external support is not needed and very often not welcome!

Receiver vs. Phone App: How Do You Want to View Your Data?
Most CGMs only work via smartphone, while others offer the option of a separate receiver that is not connected to the World Wide Web.

• Dexcom G6, One, G7, and One+ provide an option to use a receiver, which is ideal for those who prefer not to rely on a phone or find a phone too cumbersome to carry all the time, especially for sports.
• The FreeStyle Libre 2 and 3 offer optional readers that also function as blood glucose and ketone meters, which can be quite useful.

AID System Connectivity: Which CGMs Work with AID Systems?

If you are going onto an AID system, compatibility matters. Best for AID Systems (as of March 2025 and country dependent):

• Dexcom G6 Tandem t:slim, Omnipod 5, and CamAPS FX.
• Dexcom G7: :t:slim & Omnipod 5,
• Freestyle Libre 2 Plus: Omnipod 5 and t:slim (not UK)
• Freestyle Libre 3 Plus: CamAPS FX
• Medtronic G4/Simplera: 780G

If AID integration is important, Dexcom G6 offers the widest compatibility in the UK. But in the US, and soon the UK, the G7 links to the most AID Systems. Abbott is also scaling their AID integration, so keep a close eye on this space!

If you are thinking about an AID system, this AID System Guide gives you all the important information.

Connectivity with Insulin Pens & Apps

Medtronic’s G4/Simplera works with the Smart MDI system that integrates with Medtronic’s connected insulin pen. This makes MDI simpler if you like using an app. Other CGMs are improving integration with insulin dosing apps, an area still evolving. I must confess that I am not up to speed in this loop area. I will get Amy Jolley (Diabetes Dietitian) from Salford on the Podcast to tell us more.

Age?

All CGM Systems discussed here are licensed for people aged 2 years and older, except the Accu-Chek SmartGuide, which is licensed for 18 years and older (April 2025).

Final Thoughts: How to Choose the Right CGM for You

Think of it like choosing a car; what features matter most to you?

All CGMs discussed in this section have undergone rigorous testing and offer solid accuracy. The best one is the one that fits your needs and helps you manage diabetes more easily and effectively.

Priority	Best CGM Option
Highest accuracy to capillary glucose (optimal glucose control)	Dexcom G6/One Dexcom G7/One+ FreeStyle Libre 2 Plus FreeStyle Libre 3 Plus
Optional Calibrations	Not a FreeStyle Libre CGM
Smallest & longest wear	Smallest: Freestyle Libre 3 Plus Longest wear (15 days): Freestyle Libre 2 and 3 Plus
Advanced alarms & predictive alerts	Dexcom G7
AI-driven glucose predictions	Accu-Chek SmartGuide
Strongest AID system interoperability	Dexcom G6 Dexcom G7
Connected Pen and Smartphone App	Medtronic G4/Simplera
Standalone receiver (no phone needed)	Not a Medtronic G4/Simplera

I hope this was helpful. If so, please share it with those who may benefit.

Now for 10 tips to master CGM

John