CGM: The Bells and Whistles

Choosing a CGM can feel like choosing a smartphone: plenty of features, lots of marketing, and it’s not always obvious what actually matters. Before we get lost in bells and whistles, two foundation points come first:

• Safety and risk: Understand the CGM Black Swan
• Comparability: why “70% time-in-range” does not mean the same thing across CGM devices

The lost art of finger-prick blood glucose testing

The CGM Black Swan has taught you;

“If in doubt, get the blood glucose meter out”

How to perform an accurate finger-prick blood glucose test

Be honest — how many of these do you actually do each time?

If a finger-prick result is being used to check a CGM, respond to an alarm, or calibrate a sensor, every step matters. If you don’t use good technique, the number is less trustworthy than you think.

If the finger prick blood glucose is very different from the CGM value (more than 20%)

• Take a second test
• If it’s more than 10% apart, take a third
• Average the closest two

False low scenario: CGM 3.4 mmol/L (60 mg/dL), BG 4.2 mmol/L (75 mg/dL) — what now?

This is an accurate finger-prick reading. But the consequence can be major: repeated alarms, sleep disruption, alarm fatigue, and behaviour changes.

In that situation, your practical options are:

• Option 1: Optional calibration (if your system allows it and your finger-prick technique is excellent).
• Option 2: If no optional calibration is available, a small amount of carbohydrate to increase the CGM value out of the hard-stop alarm zone.
• Option 3: Turn off the low alarm and “let it roll”. That is where asymmetric risk can build.

Please do not take option 3. There should always be a hard-stop low alarm active.

The same logic applies at the high end

The same argument holds for the need for a very high hard-stop alarm that should not be disabled. At that level, action is required:

• check blood glucose,
• check ketones,
• correct with insulin,
• and consider calibration or sensor replacement if readings do not make sense.

Turning off all high alarms and “letting it roll” is not benign when insulin deficiency and DKA are downstream risks. Again, this is not about frequency — it is about consequence.

Ok — we are ready to explore the features.

Feature checklist

Having worked with 1,000+ people starting CGM and ~250 using AID systems, here are the main features that seem to matter:

• AID integration and compatibility
• Senor Size and comfort
• Optional calibration
• Alarm customisation and prediction features
• Followers / remote monitoring
• Receiver vs phone

AID system connectivity: who talks to what?

If you are considering Automated Insulin Delivery (AID), CGM–pump compatibility is crucial. This changes frequently by region and update cycle. In the UK, Dexcom and Abbott options have the most integrations.

If you are AID-curious or AID-bound, the AID Systems Guide walks through your main options and which CGMs connect to which AID sytems.

Sensor size and wear duration: comfort matters

FreeStyle Libre 3 / 3 Plus is currently among the smallest CGMs and offers up to 15-day wear (model-dependent). Other systems vary by wear duration.

Optional Calibration: Blessing or a Curse

Some CGMs allow manual optional calibration. Whether that is a gift or a curse depends on your finger-prick blood glucose testing technique and your personality.

Your CGM measures glucose in interstitial fluid, while a finger-prick test measures glucose in capillary blood. Because glucose has to move from blood into the interstitial space, there is always a time lag. When glucose is changing quickly, the two readings will differ.

Optional calibration lets you enter a finger-prick reading to nudge the CGM back towards blood glucose. This can be useful when there is a consistent gap between the sensor and a reliable finger-prick result, or there is a false low or high alarm despite the CGM reading being accurate.

In practice, this can be beneficial in calibrating out of flase low and high alarms, which maybe 1–5 times per month, but only if:

• You use an ISO-standard blood glucose meter with strong accuracy performance.
• You are ruthless about good testing technique, as discussed above.

Alerts and alarms: informed, not overwhelmed

All CGMs can alert for highs and lows. Some are simply more nuanced about it.

Advanced alarm features

• Dexcom G6 / G7 – Urgent Low Soon: predicts if glucose will fall below a set level in ~20 minutes.
• Dexcom G7 – delayed first high alarm: delays high alerts after meals to reduce “you ate food” alarms.

AI-powered predictive insights

Accu-Chek SmartGuide introduces forecasting tools, including a 30-minute hypoglycaemia prediction alert, a two-hour projection, and a seven-hour overnight risk assessment. These predictions have been reported with >85% accuracy in modelling work using large real-world CGM datasets and in-silico simulations (see analysis).

Remote monitoring (Followers): keeping others in the loop

Most modern CGM systems allow “followers” — partners, parents, carers, or friends — to see your glucose data remotely. This is particularly useful for:

• children and young people,
• people with impaired hypo awareness,
• anyone who wants extra overnight reassurance.

For others, external monitoring may be unwanted. Functionally, the differences between CGMs here are often smaller than people think — this is more about whether you want the feature, and how easy the app experience is for your family/care network.

Receiver vs phone app: how do you want to see your data?

Some people love phone-only CGM. Others want a dedicated receiver (or a backup) so glucose data isn’t hostage to battery life, phone compatibility, or notification settings.

• Dexcom systems often offer receiver options depending on model/region.
• FreeStyle Libre readers can be useful for people who don’t want CGM on their phone and may double as BG/ketone devices (model dependent).

This space is evolving quickly and in 2026 we will have an updated guide.

Summary: which features matter most?

Use this table as a synthesis of the principles above — safety and education first, then features.

Priority	What to look for	Why it matters	CGM systems (examples)
Safety foundation	ISO BG meter + in-date strips + good technique	Your gold-standard backstop in doubt; enables safe calibration and sensible decisions	All CGMs (applies universally)
Hard-stop low alarms	Very low alarm that cannot be disabled (where available)	Reduces tail-risk exposure from silence/over-trust	Varies by device + region + app/receiver setup (check local labelling)
Optional calibration	Manual calibration available (and whether you’ll use it correctly)	Can rescue drifting sensors / false alarms; poor calibration can worsen accuracy	Available on: Dexcom CGMs. Medtronic CGMs; Eversense CGMs Not available on: Abbott FreeStyle Libre CGMs and Roche Accu-Check CGM
Alarm intelligence	Prediction alerts, customisation, sensible delay logic	Reduces alarm fatigue while preserving safety protections	Dexcom G6/G7: Urgent Low Soon; Dexcom G7: delayed first high alarm; Accu-Chek SmartGuide: predictive forecasting suite
Wear & comfort	Wear time, size, adhesion, site flexibility	Comfort drives adherence; adherence drives outcomes	FreeStyle Libre 3/3 Plus: Smallest
Followers	Easy sharing + reliable notifications	Critical for children, hypo unawareness, overnight reassurance	All CGM systems support sharing: Dexcom (Follow), Libre (LibreLinkUp), Medtronic ecosystems (CareLink / dependent services), Roche SmartGuide, EverSense
Receiver vs phone	Backup display option	Practical resilience when phones fail, misbehave, or aren’t wanted	All Dexcom, Libre, and Roche options Notavauilable for EverSense or Medtronic
AID compatibility	Which pumps/algorithms the CGM integrates with	Often the deciding constraint if you’re AID-bound	Highly region-specific; see AID Systems Guide

Next up: 10 tips to master CGM.

John