Medical technology
Continuous glucose monitoring: how CGM is changing diabetes care
For decades, people with diabetes checked their glucose with fingerprick tests — a snapshot a few times a day. Continuous glucose monitoring (CGM) replaced the snapshots with a moving picture, and in doing so changed how diabetes is managed. This guide explains how the technology works, what the numbers mean, and why it matters.
Education and reference only. This article explains how treatments work in plain language — it contains no doses and is not a substitute for advice from your doctor or pharmacist. Always discuss your own treatment with a qualified clinician.
How a CGM actually works
A CGM uses a tiny sensor filament sitting just under the skin, usually on the arm or abdomen, which measures glucose in the fluid between cells (interstitial fluid) rather than directly in blood. It reads glucose every few minutes and sends the values to a phone or reader. Because interstitial glucose lags blood glucose by a few minutes, CGM trends are slightly behind rapid changes, but the continuous stream — and the direction-of-travel arrows — is far more informative than isolated fingerpricks.
From single numbers to "time in range"
CGM introduced a more useful way to judge control than a single average. "Time in range" is the percentage of the day glucose stays within a target band (commonly 3.9–10 mmol/L). Alongside it, CGM shows time spent high, time spent low, and glucose variability. This matters because two people can have the same average glucose (or HbA1c) yet very different experiences — one steady, one swinging between highs and lows. Time in range captures that difference and is now a core treatment target.
Why it improves day-to-day life
The practical gains are large. Alarms warn of glucose heading too low — especially valuable overnight and for people who lose the usual warning symptoms of hypoglycaemia. Seeing the immediate effect of a meal, exercise or an insulin dose turns management into a feedback loop rather than guesswork. Many systems share data with family or a clinician, and some integrate directly with insulin pumps to adjust delivery automatically (so-called hybrid closed-loop or "artificial pancreas" systems).
Who benefits and what to keep in mind
CGM is now standard for many people with type 1 diabetes and is increasingly offered to people with type 2 diabetes on insulin, guided by national criteria that have widened access. It is a tool, not a cure: sensors need periodic replacement, readings can occasionally be affected by compression or certain substances, and fingerprick checks are still advised to confirm before acting on a reading that does not match how someone feels. Used well, though, CGM consistently improves time in range and reduces dangerous lows.
In short
Key takeaways
- CGM measures glucose continuously via a small under-skin sensor, replacing occasional fingerpricks with a moving picture.
- "Time in range" — the percentage of the day glucose stays in target — is a more useful measure than a single average.
- Low-glucose alarms and real-time trends improve safety and make management a feedback loop.
- Some systems link to insulin pumps to adjust delivery automatically (hybrid closed-loop).
- Access has widened, especially for insulin-treated diabetes; it is a powerful tool but not a cure.
Answers
Frequently asked questions
Does a CGM measure blood glucose directly?
It measures glucose in the fluid just under the skin, which tracks blood glucose closely but lags by a few minutes during rapid changes. A fingerprick can confirm a reading if it does not match how you feel.
What is a good "time in range"?
For many adults, a common goal is at least 70% of the day within 3.9–10 mmol/L, but targets are individualised — your diabetes team will agree what is right for you.
Can CGM replace insulin decisions?
CGM informs decisions and some systems automate part of insulin delivery, but it supports rather than replaces a person’s diabetes plan agreed with their clinical team.
Go deeper
Related guides
Sources
Where this is drawn from
- NICE NG17 — Type 1 diabetes in adults; NICE NG28 — Type 2 diabetes in adults
- Diabetes UK — Continuous glucose monitoring (CGM) and flash
- International consensus on time in range (Battelino et al., Diabetes Care)
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