What is a CGM and how does it work? A plain-English guide

A CGM, or continuous glucose monitor, is a small wearable device that tracks your glucose day and night. Instead of a single number from a finger prick, you get a live reading and a line showing where your blood sugar has been and where it’s heading. Here’s how it works, what it can and can’t tell you, and what the numbers mean.

What is a CGM (continuous glucose monitor)?

According to the U.S. National Institutes of Health (NIDDK), a CGM estimates your glucose level every few minutes and keeps track of it over time. It has three parts:

• A sensor: a tiny sensor inserted under the skin, often on the belly or arm, held in place with a sticky patch.
• A transmitter: sends the readings wirelessly.
• A display: software on your phone, on an insulin pump, or on a separate device called a receiver.

NIDDK says most disposable sensors are replaced every 7 to 14 days, depending on the model.

How does a CGM work, and why does it lag?

A CGM doesn’t measure your blood directly. The sensor sits in the fluid between your cells, called interstitial fluid, and estimates the glucose there. NIDDK notes this is very similar to the level in your blood.

Glucose takes a little time to move from your blood into that fluid, so the readings run slightly behind. The American Diabetes Association says CGM readings are about 5 to 15 minutes behind a finger-stick reading. Most of the time that doesn’t matter. When your blood sugar is changing quickly, such as after a meal, during exercise or while treating a low, it can.

Trend arrows and alerts

Two features make a CGM more than a meter that never stops.

• Trend arrows show whether your glucose is rising, falling or steady, and how fast. The ADA notes that trend arrows help you quickly see if your glucose is going up or down.
• Alerts can sound when your glucose goes too low or too high, according to Diabetes UK. This can be especially valuable overnight, and the overnight graph is also the best way to understand high blood sugar in the morning.

CGM vs. finger prick: do you still need a meter?

Usually less often, but yes, sometimes. NIDDK and Diabetes UK both recommend keeping a blood glucose meter. You may need a finger-prick check:

• if how you feel doesn’t match what the CGM says,
• if your CGM gives a warning alert,
• if you doubt the accuracy of a reading,
• or when changing your insulin dose, as NIDDK notes.

What is time in range?

A CGM gives you a new way to look at your glucose: not just single readings, but how much of the day you spend in a target range.

An international consensus published in Diabetes Care in 2019 defined time in range as the percentage of readings between 70 and 180 mg/dL (3.9–10.0 mmol/L). For most adults with type 1 or type 2 diabetes, it recommends:

Measure	Target for most adults
Time in range (70–180 mg/dL, 3.9–10.0 mmol/L)	More than 70% (about 17 hours a day)
Time below 70 mg/dL (3.9 mmol/L)	Less than 4% (under 1 hour a day)

For older people or those at higher risk of lows, the same consensus suggests a gentler target of more than 50% time in range. It also notes that every 5% increase in time in range brings meaningful benefits. Small improvements count. Your own targets are something to agree with your care team.

Who can get a CGM, including with type 2 diabetes?

• In the US, NIDDK says your doctor may recommend a CGM if you need insulin to manage type 1, type 2 or another form of diabetes. It may be covered by health insurance.
• In the UK, Diabetes UK says people with type 1 diabetes should generally be offered a CGM, and some people with type 2 may be too.

What a CGM doesn’t show you

A CGM tells you what your glucose is doing. It doesn’t tell you why, or how much insulin you’ve already taken that is still working, known as insulin on board. That context comes from your own records: when you took insulin, how much, and what you ate.

That’s the gap Insulin Pal is designed for. It shows an estimate of the insulin from your logged doses that may still be active, so you can look at it alongside your CGM graph. Like any estimate, it only knows what you log.

A word on smartwatches and rings

You may see ads for watches or rings that claim to measure glucose without a sensor. The FDA has warned that it has not authorized any smartwatch or smart ring that measures or estimates blood glucose on its own, and that inaccurate readings could lead to wrong insulin doses. A watch can display readings from a CGM, but it isn’t one.

The short version

• A CGM is a small sensor under the skin that estimates your glucose every few minutes.
• It measures the fluid between your cells, so it runs about 5 to 15 minutes behind a finger prick.
• Trend arrows and alerts are its most useful features, especially overnight.
• Keep a meter for checks when something doesn’t add up.
• Time in range (70–180 mg/dL) above 70% is the common target for most adults. Agree yours with your care team.