How does a CGM measure glucose?

A small sensor with a fine filament under the skin measures glucose in the interstitial fluid (between cells), not directly in the blood, and sends readings to a phone, receiver or pump every few minutes. Because it reads interstitial fluid, it lags blood glucose slightly.

What is the difference between real-time and flash CGM?

Real-time CGM streams readings automatically and can alarm for highs and lows. Flash (intermittently-scanned) shows the reading when you scan the sensor, though newer flash sensors also offer alarms. Both show trends and history; your team helps you choose.

When should I confirm a CGM reading with a fingerstick?

When your symptoms do not match the reading, or when glucose is changing fast (during or after exercise, after eating or insulin, or when treating a hypo), because the CGM lags at those times. Trust how you feel and check with a meter when in doubt.

Do I still need to prick my finger with a CGM?

Less often. Many current sensors are factory-calibrated and need no routine fingersticks, while some need occasional calibration. You should still fingerstick to confirm when symptoms do not match the reading or glucose is changing fast. Follow your device’s instructions.

Continuous Glucose Monitoring (CGM) Explained

Q: Why do trend arrows matter?

They show whether glucose is steady, rising or falling and roughly how fast, so you can act early, for example taking carbohydrate when an arrow shows a fall, rather than reacting only once you are already low. Reading arrows alongside the number is a core skill.

Written & reviewed by the BestDoctors.sg editorial team · Last updated 28 Aug 2026 · General information, not medical advice

Continuous glucose monitoring, or CGM, has transformed day-to-day diabetes care by showing glucose readings and trends throughout the day and night from a small sensor worn on the skin, with far less fingerpricking. Understanding how it works, what its readings mean, and when to double-check with a fingerstick helps you use it safely and get the most from it. This guide explains CGM in plain terms.

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In a medical emergency, or for severe or sudden symptoms such as breathing difficulty, chest pain, fainting or stroke signs, call 995 immediately.

This is general information and does not replace your diabetes team’s advice. If your symptoms do not match the CGM reading, or glucose is changing fast, confirm with a fingerstick before treating. Treat an obvious hypo straight away, and call 995 for a severe low or signs of DKA.

What CGM is

A CGM uses a small sensor worn on the skin, usually on the back of the upper arm or the abdomen, with a tiny flexible filament sitting just under the skin. The sensor is held on by an adhesive patch and sends readings wirelessly to a phone app, a small receiver, or a compatible insulin pump. Readings usually update automatically every few minutes, giving a near-continuous picture of where your glucose is and where it is heading.

Interstitial fluid and the lag

An important point is that a CGM measures glucose in the interstitial fluid, the fluid between cells, rather than directly in the blood. Because of this, there is a short lag behind your actual blood glucose. The gap is most noticeable when glucose is changing quickly, for example after eating, after insulin, during or after exercise, or while you are treating a hypo. At those times the CGM number can trail what your blood is really doing, which matters for the safety advice below.

Real-time and flash monitoring

There are two broad types. Real-time CGM streams readings continuously and automatically and can sound alarms for high and low glucose. Flash, or intermittently-scanned CGM, shows the reading when you scan the sensor, though newer flash sensors also offer optional alarms. The line between the two has blurred as flash devices have added alerts. Both show recent history and trends, and your team helps you choose what suits you. Brand systems exist only as examples of these categories.

Trends and direction arrows

One of the biggest advantages of CGM over a single fingerstick is that it shows trends and a direction arrow, not just one number. The arrow indicates whether your glucose is steady, rising or falling, and roughly how fast. This lets you act early, for example taking carbohydrate when an arrow shows a fall, rather than only reacting once you are already low. Learning to read the arrows alongside the number is a core skill your team will teach, and it quickly becomes second nature. Over time you start to recognise your own patterns, such as how a particular meal or a walk after dinner tends to move your glucose, which helps you plan ahead.

Alarms

Alarms and alerts can warn you of highs and lows, and some devices give a predictive alert before you reach a low. This is especially valuable overnight and for people whose warning signs of a hypo have become faint. Alarms are a safety net, but too many can become tiring, so the thresholds are set sensibly with your team. Some devices also let readings be shared with family or your clinic, which adds support and helps at reviews.

Calibration and wear time

Some sensors need an occasional fingerstick to calibrate them, while many current sensors are factory-calibrated and need no routine fingersticks; follow the instructions for your specific device. Sensors are replaced on a set schedule that varies by system, and there is usually a short warm-up period after inserting a new one before it gives readings. Keeping to the schedule and looking after the sensor site helps the readings stay reliable. In Singapore’s heat and humidity, sweat can loosen a sensor adhesive, so an extra adhesive patch or an arm band can help a sensor last its full wear time.

When to confirm with a fingerstick

The key safety habit is simple. If your symptoms do not match the CGM reading, or if glucose is changing fast, confirm with a fingerstick blood test before you treat. Trust how you feel, and check with a meter when in doubt, rather than waiting for the sensor to catch up. A severe low or a very high reading with vomiting, drowsiness or signs of DKA still needs the standard urgent response whatever the sensor shows; in Singapore, call 995 for an emergency. A CGM does not make decisions for you, but it gives you far more information to make good ones. In Singapore, your diabetes team and Diabetes Singapore can support you in getting started with CGM.

Sources

Diabetes UK — Continuous glucose monitors: diabetes.org.uk
NHS — CGM and hybrid closed loop for diabetes: nhs.uk
American Diabetes Association — Devices & technology: diabetes.org
Diabetes Singapore: diabetes.org.sg

General information only. This guide supports, but does not replace, the advice of your diabetes team (your endocrinologist and diabetes nurse educator), who set your insulin and settings and train you on your devices. Never make big changes to your settings alone, and always keep backup insulin pens and ketone testing. An insulin pump uses only rapid-acting insulin, so if delivery stops, glucose and ketones can rise fast: unexplained high glucose means check the site, change the set and give insulin by pen. Call 995 for signs of diabetic ketoacidosis (high glucose with ketones, vomiting, tummy pain, deep or rapid breathing, drowsiness) or for a severe hypo (unable to treat yourself, fitting or unconscious). If you are struggling emotionally, you are not alone: the Samaritans of Singapore (SOS) are on 1767 and mindline.sg on 1771.

Continuous Glucose Monitoring (CGM) Explained (Singapore)