Continuous glucose monitoring systems (CGM) are transforming diabetes management in the UK, offering real-time glucose data around the clock without routine finger-prick testing. By measuring interstitial glucose every one to five minutes, CGM devices provide a far richer picture of how food, activity, stress, and medication affect blood sugar levels. Recognised within NHS and NICE guidance, these systems are now routinely offered to people with type 1 diabetes and increasingly to those with type 2 diabetes on insulin. This article explains how CGM works, who qualifies on the NHS, which devices are commonly prescribed, and how to use them safely.

What Is a Continuous Glucose Monitoring System?

A CGM system automatically tracks blood glucose levels day and night at intervals of one to five minutes, providing real-time data and trend information without routine finger-prick testing.

A continuous glucose monitoring system (CGM) is a medical device that automatically tracks blood glucose levels throughout the day and night, providing real-time data without the need for routine finger-prick tests. Unlike traditional self-monitoring blood glucose (SMBG) methods, which offer only a snapshot reading at a single point in time, a CGM system records glucose values at regular intervals — typically every one to five minutes — giving a far more comprehensive picture of how glucose levels fluctuate in response to food, physical activity, stress, illness, and medication.

It is important to note that finger-prick blood glucose testing may still be required in certain situations — for example, when CGM readings do not match your symptoms, during periods of rapid glucose change, or before making significant treatment decisions. Your diabetes care team will advise you on when to verify a CGM reading with a finger-prick test.

CGM systems are primarily used by people living with type 1 diabetes, though their use in type 2 diabetes is growing, particularly where insulin therapy is involved. Use of CGM in inpatient settings varies by local hospital policy; patients admitted to hospital should follow the guidance of their clinical team regarding whether to continue using their personal CGM device.

There are two broad categories of CGM:

• Real-time CGM (rtCGM): Continuously displays glucose readings and can trigger alarms when levels fall outside a set range.

• Intermittently scanned CGM (isCGM): Also known as flash glucose monitoring, this requires the user to actively scan the sensor to obtain a reading, though some newer models also include optional alarms.

Both types have been shown to improve glycaemic outcomes and quality of life in clinical studies, and both are recognised within NHS and NICE guidance (including NICE NG17 and NG18) as valuable tools in diabetes management.

How CGM Systems Work and What They Measure

CGM sensors measure glucose in interstitial fluid via an electrochemical reaction, transmitting trend data wirelessly; readings lag blood glucose by five to fifteen minutes, so finger-prick verification is needed in some situations.

A continuous glucose monitoring system works by measuring glucose levels in the interstitial fluid — the fluid that surrounds the cells in subcutaneous tissue — rather than directly in the blood. A small, flexible sensor filament is inserted just beneath the skin, typically on the upper arm or abdomen, depending on the device. This sensor detects glucose concentrations using an electrochemical reaction involving glucose oxidase, generating an electrical signal that is converted into a glucose reading.

The sensor transmits data wirelessly to a receiver, smartphone app, or insulin pump, where readings are displayed in real time. Most CGM systems also generate trend arrows, which indicate whether glucose levels are rising, falling, or remaining stable — and at what rate. This directional information is particularly valuable for anticipating hypoglycaemia or hyperglycaemia before it becomes clinically significant.

It is important to understand that interstitial glucose readings lag slightly behind blood glucose levels — typically by around five to fifteen minutes. This discrepancy may be greater during very rapid glucose changes or during the sensor warm-up period immediately after insertion, when readings may be less reliable. For this reason, some clinical situations — such as confirming a hypoglycaemic episode before treating — may still require a finger-prick blood glucose test to verify the CGM reading.

Modern CGM systems also calculate metrics such as:

• Time in Range (TIR): The percentage of time glucose remains within a target range. A typical target for many adults with type 1 or type 2 diabetes is 3.9–10.0 mmol/L, but targets differ in pregnancy and for children and young people; your individual targets should be agreed with your diabetes care team in line with NICE guidance (NG17, NG18, NG3).

• Time Below Range (TBR) and Time Above Range (TAR): Indicators of hypoglycaemia and hyperglycaemia burden.

• Glucose Management Indicator (GMI): An estimate of HbA1c derived from CGM data. Note that GMI may not correspond closely to laboratory HbA1c in people with certain conditions, such as haemoglobinopathies or chronic kidney disease; laboratory HbA1c measurement remains important.

These metrics are increasingly used alongside HbA1c in clinical consultations to guide treatment decisions, as reflected in NICE NG17 and NG18.

Who Is Eligible for CGM on the NHS?

All people with type 1 diabetes are routinely offered CGM on the NHS under NICE NG17 and NG18; those with type 2 diabetes on insulin may qualify under NICE NG28, subject to individual clinical assessment and local commissioning.

NHS eligibility for a continuous glucose monitoring system has expanded significantly in recent years, driven by updated NICE guidance and NHS England commissioning policies. Eligibility and device choice may also depend on local commissioning arrangements and shared decision-making with your diabetes care team.

People with type 1 diabetes are routinely offered CGM on the NHS, regardless of whether they use an insulin pump or multiple daily injections (MDI). This applies to adults (NICE NG17) and to children and young people (NICE NG18, which covers both type 1 and type 2 diabetes in this age group). NHS England has confirmed that all people living with type 1 diabetes should have access to a CGM device, though local implementation timelines may vary.

People with type 2 diabetes may be considered for CGM in specific circumstances, in line with NICE NG28. This includes, but may not be limited to:

• Adults on insulin therapy who experience problematic hypoglycaemia or who have difficulty achieving glycaemic targets.

• Those who are unable to perform finger-prick testing due to physical or cognitive difficulties.

Access for people with type 2 diabetes is not universal and is subject to individual clinical assessment and local commissioning criteria.

Pregnant women with type 1 diabetes are specifically offered real-time CGM throughout pregnancy, as evidence shows it reduces the risk of large-for-gestational-age babies and neonatal hypoglycaemia (NICE NG3). For women with type 2 diabetes or gestational diabetes managed with insulin, CGM may be considered where specific criteria are met, in line with NICE NG3; this is not a universal recommendation and should be discussed with the obstetric diabetes team.

Eligibility is assessed by a GP, diabetes specialist nurse, or consultant diabetologist. Patients who believe they may benefit from CGM but have not yet been offered it are encouraged to raise this with their diabetes care team. Referral to a specialist diabetes service may be appropriate if primary care is unable to initiate CGM prescribing.

CGM Devices Commonly Used in the UK

Commonly prescribed UK CGM devices include FreeStyle Libre 2 and 3, Dexcom G6 and G7, and Medtronic Guardian Sensor 4; all are factory-calibrated and regulated by the MHRA.

Several continuous glucose monitoring systems are currently available in the UK. NICE issues guidance on the use of CGM in diabetes management, and NHS funding is determined by this guidance alongside local commissioning arrangements; NICE does not approve specific branded devices. Your diabetes care team will help identify the most appropriate option for your individual needs.

Devices commonly prescribed in the UK include:

• FreeStyle Libre 2 and Libre 3 (Abbott): Intermittently scanned CGM devices with optional real-time alarms. The Libre 3 offers continuous Bluetooth transmission to a smartphone. Sensors last up to 14 days. These devices are factory-calibrated and do not require routine finger-prick calibration. Both are widely prescribed on the NHS for eligible patients with type 1 and type 2 diabetes.

• Dexcom G6 and G7 (Dexcom): Real-time CGM systems with continuous transmission, customisable alarms, and integration with insulin pumps and closed-loop systems. The G6 sensor lasts up to 10 days; the G7 sensor lasts up to 10 days and features a smaller, all-in-one sensor-transmitter design. Both are factory-calibrated.

• Medtronic Guardian Sensor 4: Designed for use with Medtronic insulin pumps as part of a hybrid closed-loop system. Sensor wear duration is approximately 7 days. This device is factory-calibrated.

Other CGM systems, including longer-wear implantable sensors (such as Eversense), exist but NHS availability varies; discuss options with your specialist team.

The MHRA regulates CGM devices as medical devices in the UK, ensuring they meet safety and performance standards.

When selecting a device, relevant considerations include:

• Wear duration: Varies by model, typically from around 7 to 14 days for commonly prescribed sensors; always follow the manufacturer's instructions.

• Alarm functionality: Essential for those at high risk of nocturnal hypoglycaemia.

• Connectivity: Compatibility with smartphones, insulin pumps, or closed-loop systems.

• Calibration requirements: Most current devices prescribed in the UK are factory-calibrated. Always check the device's Instructions for Use for specific requirements.

• Potential interfering substances: Certain substances may affect readings on some devices. For example, high-dose vitamin C may affect some FreeStyle Libre sensors; hydroxyurea may affect some Dexcom sensors. Always consult the Instructions for Use for your specific device for a full list of potential interferences.

Patients should receive structured education on their chosen device, including how to interpret trend data and respond appropriately to alerts.

Using Your CGM System Safely and Accurately

Safe CGM use requires correct sensor insertion, awareness of interfering substances, appropriate response to alarms, and finger-prick verification when readings do not match symptoms or before significant treatment decisions.

To get the most from a continuous glucose monitoring system, it is essential to use it correctly and understand its limitations. Proper sensor insertion technique is the first step — sensors should be applied to clean, dry skin, avoiding areas with lipohypertrophy (hardened tissue from repeated injections), scarring, or excessive hair. Rotating insertion sites helps maintain skin integrity and sensor accuracy.

Key safety considerations include:

• Verify with a finger-prick test when symptoms do not match the CGM reading, during periods of rapid glucose change, or before making significant treatment decisions such as correcting a suspected severe hypoglycaemic episode.

• Be aware of interfering substances: Some medications and supplements can affect CGM accuracy on certain devices. For example, high-dose vitamin C may affect some FreeStyle Libre sensors, and hydroxyurea may affect some Dexcom sensors. Paracetamol (acetaminophen) affected older CGM models; most current devices have addressed this, but the degree of interference varies. Always check the Instructions for Use for your specific device.

• Respond to alarms appropriately: Do not silence low glucose alarms without investigating. A reading below 3.9 mmol/L should prompt treatment with fast-acting carbohydrate in line with your diabetes care plan.

• Maintain the device: Replace sensors according to the manufacturer's schedule. If your device uses a separate receiver or smartphone, ensure it is kept charged. Transmitter replacement schedules vary by device — some transmitters are disposable, others are replaceable; refer to your device's Instructions for Use. Ensure the app or receiver software is kept up to date.

When to seek urgent help: If you or someone else experiences severe hypoglycaemia — including loss of consciousness, seizures, or hypoglycaemia that does not respond to treatment — call 999 immediately. If glucagon has been prescribed, administer it as directed whilst awaiting emergency services. Repeated blood glucose readings below 3.0 mmol/L despite treatment also require urgent medical attention. Follow the NHS guidance on hypoglycaemia and your individual diabetes care plan.

Patients should also be aware that CGM does not replace clinical review. Regular HbA1c testing and consultations with the diabetes care team remain important. If you notice persistent unexplained glucose patterns or frequent sensor errors, contact your GP or diabetes nurse for advice.

Reporting device problems: If you experience a suspected adverse incident with your CGM device — such as a significant skin reaction at the insertion site, repeated inaccurate readings, or sensor failure — you should report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. You may also wish to inform your diabetes care team and the device manufacturer.

For those using CGM as part of a hybrid closed-loop system (sometimes called an artificial pancreas), additional training is required to understand how the system adjusts insulin delivery automatically. NHS England has expanded access to these systems in line with NICE technology appraisal guidance, and specialist diabetes teams can advise on eligibility and initiation.

Overall, a continuous glucose monitoring system is a valuable tool that, when used correctly and supported by appropriate education, can significantly improve glucose management, reduce hypoglycaemia risk, and enhance quality of life for people living with diabetes.

Frequently Asked Questions