Continuous glucose monitoring (CGM): how it works and who can have it

A CGM system has three parts: a small plastic sensor inserted just beneath the skin of the tummy or upper arm and held by an adhesive patch, a transmitter on top of the skin, and a display such as a smartphone, an insulin pump or a handheld reader. Lab Tests Online UK explains that the sensor measures glucose in the space around cells, called the interstitial space, and that interstitial and blood glucose are not exactly the same but mirror each other closely.

Some devices send a reading to your phone every few minutes with trend arrows and alarms. Others, called intermittently scanned or flash monitors, store readings and show the latest value and recent history when you scan the sensor. Some CGM devices connect with an insulin pump; when your glucose rises after eating, the pump responds by delivering insulin, which is known as a hybrid closed loop system.

Lab Tests Online UK summarises NICE guidance: all adults with type 1 diabetes should be offered a choice of real-time CGM or flash monitoring, and real-time CGM should be offered to all children and young people with type 1 diabetes. For type 2 diabetes eligibility is restricted to people on multiple daily insulin injections who have recurrent or severe hypoglycaemia, impaired awareness of hypoglycaemia, a disability that makes finger-prick checks difficult, or who need to test their glucose more than 8 times a day.

The aim is to help people and their teams control glucose better and reduce the number of finger-prick tests. Sensors can also be bought privately; the same source suggests discussing this with your diabetes team first because you will need support to use the system and interpret the results.

No test preparation is needed. A healthcare professional or you apply the sensor, and devices are approved for specific periods and must be replaced as indicated by the manufacturer; they cannot be used beyond that time. Skin reactions to the adhesive can occur, so rotating sites and following the manufacturer instructions helps.

You should still carry means of finger-prick testing. NICE recommends that people using CGM keep checking capillary blood glucose to confirm the device is accurate and as a back-up if it stops working, particularly when readings do not match how you feel.

Glucose values from CGM generally correlate closely with blood glucose. Your target range is set by your healthcare professional and depends on daily activities, other conditions including heart disease, your likelihood of complications, and whether you have hypoglycaemia unawareness. CGM results can show variability across the day, identify trends, and warn when glucose is heading too low or too high.

Looking at patterns over several days can help you and your team adjust food, activity and treatment. Reports often include time in range, time below range and glucose variability, which give a fuller picture than an HbA1c alone because two people with the same HbA1c can have very different swings.

Treat symptoms of low or very high glucose seriously even if the screen looks reassuring. The NHS says diabetic ketoacidosis can be life threatening, and that if you have symptoms such as thirst, passing more urine, sleepiness, confusion or fruity-smelling breath you should check glucose and ketones and call 999 or go to A&E for high ketones. Ask your care team about how CGM fits into your sick-day rules.

Your diabetes team trains you to apply sensors, set alarms and interpret trends, and reviews downloaded data at clinic visits. If you do not qualify under the criteria but struggle with frequent hypoglycaemia or testing, ask the team whether your circumstances meet an exception.