Continuous glucose monitors (CGM): how they work and who uses them

A continuous glucose monitor, or CGM, is a device that estimates your blood glucose automatically through the day and night. Many people find it gives them a fuller picture than finger-prick checks alone. This article explains the basics based on the NIDDK, with related points from the NHS. A doctor decides whether a CGM is suitable for you.

What is a CGM?

The NIDDK explains that a CGM automatically estimates blood glucose around the clock. Users can check their level at any time and review trends over hours or days.

How does it work?

The NIDDK describes three parts:

• Sensor: a small, disposable device worn under the skin, usually on the belly or arm, that measures glucose in the fluid between cells. Some sensors are implanted instead.
• Transmitter: sends readings wirelessly to a receiver or app.
• Software: runs on a smartphone, an insulin pump or a separate receiver to show the data.

Types of CGM

The NIDDK lists three kinds:

• Real-time: readings are sent and displayed automatically.
• Intermittent-scan: records continuously, but you need to scan the device every few hours to see and store data.
• Doctor-download: collects data for a clinician to review later and is worn for a limited time.

Models differ in where the sensor is worn, how often it is replaced, warm-up time and how settings are adjusted. Some need finger-stick calibration. The NHS refers to continuous or flash glucose monitors in its advice on preventing low blood sugar and ketoacidosis.

Who uses a CGM?

The NIDDK says CGMs are used by people who need insulin for type 1 diabetes, type 2 diabetes or another form of diabetes. Adults and children can use them, and some models are approved for children as young as 2. Use may be continuous or limited to a few days, depending on the doctor's advice.

Possible benefits

Compared with a standard meter, the NIDDK says a CGM may provide:

• Better daily glucose management
• Fewer low blood glucose emergencies
• Fewer finger sticks
• Alerts and trend graphics showing how fast levels are rising or falling

The NIDDK says the greatest benefit comes from daily or near-daily use. It also says a CGM can sound an alarm when glucose drops, which is especially useful for people with hypoglycemia unawareness or frequent lows. The NHS says that for people with type 1 diabetes, a CGM can reduce finger-prick tests.

Limitations to know about

• Finger-stick checks may still be needed to confirm accuracy, especially before changing an insulin dose or acting on a warning alert, according to the NIDDK.
• Disposable sensors typically last 7 to 14 days, and some implantable sensors last up to 180 days. Transmitters may need replacing and devices sometimes need reconnecting.
• Skin redness or irritation from the adhesive patch can occur.
• A CGM costs more than a standard meter, although insurance may cover it.

What is time in range?

The NIDDK's managing diabetes page describes "time in range" as a CGM measure. For many adults, it describes 70 to 180 mg/dL as the range, with a goal of spending at least 70% of the time in it. The NHS describes aiming for about 70% of time in range for people with type 1 diabetes. These targets can differ for each person, for example if you are older, have heart disease or often have lows, so agree your own targets with your care team.

When to see a doctor

Ask your doctor or diabetes team whether a CGM may suit you, how long to use it and how to handle skin problems. Check coverage with your insurer or health service. Do not change insulin or other medicine based only on a sensor reading without following your care team's advice.

Seek urgent help for signs of severe low or very high glucose, such as confusion, fainting, fruity-smelling breath or trouble breathing. In an emergency, call your local emergency number (112 in India, 911 in the United States).

This article is for education only and does not replace advice from your own doctor.