CGM Guide Series – Step 1

The CGM Framework – Why CE Marking Is Not Enough

Regulatory approval tells you a device is safe to place on the market. It does not tell you whether the accuracy data behind it is good enough to make insulin dosing decisions. There is a difference, and it matters.

CGM Framework

Part of: CGM for T1D – The GNL Framework → Step 1 of 3

The problem with CE marking

A CGM can receive a CE mark and a non-adjunctive insulin dosing indication – meaning it is approved for use without a confirmatory finger prick – while the accuracy data behind it comes entirely from adults with type 2 diabetes. No children. No meals. No hypoglycaemia challenges. No high-risk populations.

That is not a hypothetical. Between 2018 and 2022, several devices entered the UK market with an indication for insulin dosing in children from the age of two, carrying data from adults with type 2. The CE mark said yes. The data did not say the same thing.

CE marking and UKCA marking are market authorisation standards. They assess whether a device is safe enough to sell. They are not the same as a clinical quality standard for insulin dosing accuracy. With blood glucose meters, ISO 15197 provides that standard. With CGM, there is no equivalent ISO or regulatory bar for study design, population, or sufficiency.

This framework fills that gap.

The five criteria

Developed with the DSN Forum UK nursing team and applied since 2024, the framework asks five questions about the accuracy data behind each CGM. A device must answer yes to all five to qualify for full inclusion. Each criterion reflects a real clinical situation where a low-quality answer creates real risk.

Criterion 1 – Were people with type 1 diabetes included?

Insulin physiology, glycaemic variability, and the risk profile of dosing errors differ between type 1 and type 2 diabetes. A device validated exclusively in type 2 populations cannot be assumed to perform the same way in type 1 diabetes, where insulin is the primary treatment and errors carry greater risk.

Criterion 2 – Was a meal challenge included?

CGM readings change fastest in the postprandial period – after eating. This is also when insulin dosing decisions are most frequent. A study that does not include a standardised meal challenge cannot adequately assess accuracy at the moment it matters most.

Criterion 3 – Was a hypoglycaemia challenge included?

The consequences of a CGM reading that is wrong when glucose is genuinely low are severe. A study that does not include a controlled hypoglycaemia challenge cannot tell you how the device performs at the glucose levels where a dosing error is most dangerous.

Criterion 4 – Was a venous or capillary reference comparator used?

Accuracy is meaningless without a reference. CGM measures interstitial glucose – not blood glucose. A well-designed accuracy study compares the CGM reading to a simultaneous venous or capillary blood glucose reference taken under controlled conditions. Studies using the CGM itself or uncontrolled comparators introduce circular errors.

Criterion 5 – Were the population and indications matched?

An indication for insulin dosing in children from the age of two requires data in children. An indication covering pregnancy requires data in pregnancy. If the claimed indication and the study population do not match, the approval is based on an assumption, not evidence.

How the framework is applied

Each criterion is scored as met or not met. A score of 5/5 means the device has data that adequately covers all five risk areas. Any score below 5 means there is at least one clinical situation where the evidence behind the device is insufficient to fully support insulin dosing decisions.

The score is not a ceiling – it is a floor. A device that scores 5/5 may still have limitations in specific populations or contexts. The framework is a minimum standard, not a full clinical endorsement.

The scoring is applied by John Pemberton in collaboration with the DSN Forum UK, using publicly available accuracy data and regulatory documents. It is updated when new evidence is published. Manufacturer input is invited to verify factual accuracy on device pages – it does not influence the framework assessment.

What happens next

The framework tells you whether a CGM has adequate study data. It does not tell you whether that data is sufficient in volume. A device can pass all five criteria with ten participants. The next step – data sufficiency – sets the minimum threshold for how much evidence is enough.

Step 2 – Data sufficiency Back to the hub

From the GNL Podcast

This framework was discussed in depth in two episodes of The Glucose Never Lies. Episode 36 – recorded with the DSN Forum UK nurses (Amanda Williams, Beth Kelly and Tamsin Fletcher-Salt) – covers how the framework was developed, why CE marking fell short, and the journey from features-first to evidence-first. Episode 35, with Professor Othmar Moser, covers the accuracy data behind the framework from an international clinical research perspective.

GNL Podcast episodes