How to Choose a CGM

Guide parts

What is Continuous Glucose Monitoring (CGM)?

A Continuous Glucose Monitor (CGM) is a wearable device that tracks glucose levels day and night, providing real-time readings, trend arrows, and alerts for high and low glucose. CGMs are now central to diabetes management, giving insight far beyond traditional finger-prick blood glucose monitoring.

The picture below shows how CGM works and how trend arrows let you see into the future of your glucose levels.

For the basics, start with CGM Foundations — how continuous glucose monitoring can guide bolus insulin decisions — and then explore the GAME-SET-MATCH framework for dynamic glucose management strategies.

However, not all CGMs are created equal, and choosing the right one requires far more than comparing headline accuracy numbers and app features.

Adjunctive vs non-adjunctive CGM — why it matters

CGM systems fall into two broad categories:

• Adjunctive CGM — requires confirmation with finger-prick blood glucose (SMBG) before insulin dosing and hypoglycaemia treatment.
• Non-adjunctive CGM — approved for insulin dosing decisions and hypoglycaemia treatment without mandatory finger-prick checks.

This guide focuses only on systems with non-adjunctive approval.

Why risk assessment comes first

It is essential to first understand the risk profile of each device.

Think of choosing a CGM like scouting a football player. If someone claimed to have scored as many goals as Messi and Ronaldo, but at half the price, should you sign them immediately? Or would you first ask:

• Where did they score those goals? (Top leagues or Sunday league?)
• How competitive were the matches? (Training games or real fixtures?)
• How do they perform at the highest level, under real pressure?

Five key questions for CGM accuracy studies

The answers tell us whether the CGM has been tested across the full glucose range (typically 2.2–22.2 mmol/L or 40–400 mg/dL) and at the rates of change that occur in daily life.

Q1. Has the data been peer-reviewed or reviewed by the FDA?

High-quality CGM evaluations are reviewed either by reputable diabetes technology journals and/or regulators such as the FDA. That scrutiny covers study design, participant selection, test procedures, and outcomes. It allows clinicians, people with diabetes, and regulators to critique the evidence themselves.

We need this level of transparency considering CGM readings are used to make decisions about a drug (insulin) that carries one of the highest risks of any self-administered medicine.

Q2. Is there sufficient data on people with type 1 diabetes?

Data sufficiency exists to ensure that reported CGM accuracy reflects clinical risk. For insulin dosing, evidence must be independent, representative, and collected under conditions where treatment decisions are actually made. This requires coverage across the full glucose range (including hypoglycaemia and marked hyperglycaemia), across the full sensor lifecycle (early, mid, and late wear), and across a sufficiently large number of individuals to capture between-person variability and real-world failure modes. Data volume alone is insufficient if measurements are clustered within too few people or too narrow a set of conditions.

Participant number is therefore the primary anchor, with 50 participants (>70% with T1D) as the baseline, with >5,500 independent data points, or fewer than 50 wearing multiple sensors on the same site, where an equivalence calculation is required to show equivalence to 50 independent wearers with >5,500 data points.

Q3. Were meal and insulin challenges performed?

A robust accuracy study should deliberately trigger high and low glucose episodes, as per Performance Metrics for Continuous Interstitial Glucose Monitoring (POCT05).

• Food without insulin — rapid glucose spikes.
• Too much insulin without food — fast drops and hypoglycaemia (“rage bolus” territory).

Without these challenges, and without 70% having type 1 diabetes, the CGM may never be tested in the very scenarios where accuracy matters most.

Q4. What percentage of comparison readings were <4.4 mmol/L (80 mg/dL)?

At least 8% of paired CGM vs comparator readings should be below 4.4 mmol/L (80 mg/dL), as per Performance Metrics for Continuous Interstitial Glucose Monitoring (POCT05).

No one is aiming for 8% hypoglycaemia in real life. This threshold simply ensures enough data in the low range to meaningfully assess performance.

Q5. What percentage of comparison readings were >16.7 mmol/L (300 mg/dL)?

At least 5% of paired readings should exceed 16.7 mmol/L (300 mg/dL), to adequately test performance in the very high range, as per Performance Metrics for Continuous Interstitial Glucose Monitoring (POCT05).

If these extremes are not well represented, the CGM’s ability to reliably detect dangerous highs and lows is unknown. That uncertainty is not acceptable when readings are being used to treat hypos or to deliver large correction doses.

Which CGMs with insulin dosing approval have been tested robustly?

The most up-to-date DSN Forum Chart is hosted on their website. The first chart uses the five study-design questions above to identify which CGM devices have been tested using methods that meet basic international standards. It provides an overview of CGM systems currently available in the UK and Europe and gives each device a study design score out of 5.

Any CGM system with a study design score less than 5 does not provide enough evidence to understand the risk when using for insulin dosing. Specifically, the risk of using readings in the low and very high glucose ranges — exactly where clinical and insulin dosing decisions are made.

This remains true even if a CGM system has a CE mark for non-adjunctive use across a wide age range.

More recently, CE marking for CGMs used to drive automated insulin delivery (AID) without peer-reviewed published data has triggered serious concerns. These have been highlighted by paediatric endocrine societies and adult diabetes technology networks.

From this point forward, we only discuss CGM systems with a study design score of 5. They are:

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

To be clear, risk assessment always comes first, with no free passes. For a deeper explanation (not needed for most), see CGM Black Swan.

How much time in range you need varies by CGM system (70% or 75%)

Do all CGMs display equivalent glucose levels on their screens, pumps, or AID systems? Simply, no.

Each CGM shows, on average, glucose values that can sit above, within, or below the blood glucose your body is actually exposed to. This is pronounced when glucose levels are rising and go above 10.0 mmol/L (>180 mg/dL).

That matters when thinking about how much time in range (3.9–10.0 mmol/L / 70–180 mg/dL) to aim for. The same person using different CGMs — with the same biology — can end up with different reported time in range, by up to 10%.

This is the CGM Alignment Zone model, adapted from: International clinical opinion on transparency, standardisation, and calibration alignment in the performance evaluation of systems for continuous glucose monitoring.

Where current CGM systems sit

There are no known Zone A systems currently on the market, though with the lack of transparency in data, it is possible. All established CGM systems fall into Zone P or Zone B.

CGM systems reading in Zone P

On average, these systems read in line with physiological glucose exposure, particularly at glucose concentrations above 10.0 mmol/L (180 mg/dL). Meta-analysis data shows that, on average, approximately 70% time in range is needed to achieve an HbA1c of 7.0% for Zone P CGM systems.

CGM systems reading in Zone B

On average, these systems read below physiological glucose exposure, particularly at glucose concentrations above 10.0 mmol/L (180 mg/dL). Meta-analysis data shows that, on average, >75% time in range is needed to achieve an HbA1c of 7.0% for Zone B CGM systems.