Continuous glucose monitors (CGM): how they work and who can get one on the NHS

The NHS describes a CGM as a device for people with diabetes that lets you check glucose levels at any time, see patterns, and tell whether your blood glucose is too high or too low. It consists of a sensor, a small device you attach to your arm or tummy that senses the amount of glucose in the fluid under your skin (interstitial fluid), and a reader or receiver that displays the results, or a smartphone app.

The NHS says CGM can help you and your diabetes team see whether glucose is rising or falling, how it changes over time and what happens overnight. Because you have more information, you can act quickly. A high reading could be a sign of hyperglycaemia and a low reading could be a sign of hypoglycaemia.

There are two main styles. With real-time CGM, the NHS says the sensor sends results to a receiver or phone every few minutes, so you can see your levels at any time, and some types can send results to an insulin pump. With flash monitoring, also called intermittently scanned CGM, you need to scan the sensor with a reader or phone to see the result. The NHS says that the only type of flash monitor currently available is the Abbott FreeStyle Libre 2, which works in the same way as real-time CGM if used with a smartphone, and that the original FreeStyle Libre has been discontinued. Product availability changes, so check with your team.

Most CGMs, including FreeStyle Libre 2, have optional alarms to warn you if glucose goes too low or too high, the NHS says. NICE advises that when choosing a device, the person's needs and preferences should be discussed, and the lowest-cost suitable device offered if several meet those needs.

For type 1 diabetes, the NHS says CGM should be available to anyone, with children and young people usually offered real-time CGM and adults a choice of real-time or flash. NICE guidance for adults with type 1 diabetes recommends offering a choice of real-time or intermittently scanned CGM, based on preferences, needs and device functions.

For type 2 diabetes the criteria are narrower. The NHS says you should be offered a flash monitor if you take insulin at least twice a day and one of the following applies: you have frequent or severe hypos, you do not always recognise the symptoms of a hypo, you cannot monitor your own blood glucose using finger-prick tests, or you would need to do finger-pricks at least 8 times a day. The NHS notes that insulin pumps and hybrid closed loop systems are not currently available on the NHS for people with type 2 diabetes.

Local arrangements and supply may differ, so ask your diabetes team or GP practice. If you are pregnant with type 1 diabetes, NICE directs readers to its separate guideline on diabetes in pregnancy.

The NHS says interstitial fluid readings are a few minutes behind blood glucose. You will therefore still need to do finger-prick checks every now and then, particularly when you drive or when you have a hypo. NICE advises adults with type 1 diabetes who use CGM that they will still need capillary blood glucose measurements, though they can do this less often, to check accuracy and as a back-up, for example when glucose is changing quickly or if the device stops working.

NICE lists factors worth considering when choosing a device: accuracy, predictive alerts and whether they need to be shared with a carer, whether you need a smartphone, ease of use including with limited dexterity, fear and severity of hypoglycaemia, how often a sensor must be replaced and whether it needs calibration, skin sensitivities to the device, and body image concerns. It also says CGM should be included in structured education and that use should be reviewed as part of your care plan, with extra education and emotional support if there are problems using the device.

If a reading does not match how you feel, ask your diabetes team in advance when a finger-prick check is advised, and follow their guidance on alarms.

The NHS describes a hybrid closed loop, sometimes called an artificial pancreas, as technology that lets a CGM work together with an insulin pump. A computer programme on a phone or inside the pump uses the CGM data to work out how much insulin you need, and the pump gives it automatically. You still need to count carbohydrate in food and drink and enter it before meals. The NHS says hybrid closed loop can improve glucose levels, reduce the risk of hypos and make diabetes easier to manage.

The NHS says hybrid closed loop is being introduced gradually over five years from April 2024, starting with those who need it most. It is planned to be available to all children and young people with type 1 diabetes and eventually to adults who have an HbA1c of 58 mmol/mol or more, serious hypos despite using a pump or CGM alone, or who are pregnant or planning pregnancy. Speak to your diabetes team about whether you might be eligible.

CGM produces a lot of information. The NHS recommends reviewing it with your diabetes team to get the best out of it, looking at trends over days and weeks and at overnight patterns rather than reacting to every single reading. Many people find it helpful to look for repeating patterns, such as a rise after a particular meal or a dip after exercise, and to bring these to appointments.

The NHS pages describe CGM for people with diabetes. This guide does not cover the use of CGM in people without diabetes, and any decision to use a device out of the usual medical context is best discussed with a GP. If you have symptoms such as extreme thirst, passing a lot of urine or unexplained weight loss, ask your GP about a diabetes check rather than relying on a sensor.

On the NHS, CGM is prescribed or supplied through your diabetes team or GP practice, along with education on setting alarms and interpreting readings. If you meet the criteria, the sensors are supplied through the NHS.

People who do not meet NHS criteria, or who want a different device, sometimes buy sensors privately or arrange a private appointment with an endocrinologist or diabetes specialist nurse for help interpreting data. Prices vary and change, so ask for the full cost of sensors over a year rather than the first purchase, check that the device is intended for your type of use, and tell your NHS team so that your records are complete.