What Is Continuous Glucose Monitoring and How Does It Work?
Takeaways
A continuous glucose monitor (CGM) uses a sensor inserted under the skin to measure your sugar level every few minutes.
Its benefits over the traditional glucometer include continuous, almost real-time readings, the elimination of fingerpricks, and improved diabetes control.
CGMs measure your interstitial glucose levels, which rise and fall in response to your blood glucose levels, with a possible lag time of 5 to 15 minutes. However, this doesn't impact CGMs' usefulness as we're interested in overall glucose trends.
Your blood sugar target—the range you should aim for as much as possible—should be individualized to your goals. It should also be decided with your doctor.
Factors that can impact your glucose levels include food, exercise, stress, and medications. If your blood sugar levels are out of range, speak with your healthcare team. They’ll help you understand the causes and what you could do about them.
Making sense of the rich data your CGM provides by yourself can be challenging. That's where a healthcare professional comes in.
What is a CGM?
A Continuous Glucose Monitor (CGM) uses a sensor inserted under the skin to measure your sugar level every few minutes. It continuously monitors your blood sugar levels in more or less real-time and eliminates the need for a fingerprick. There are numerous manufacturers of these devices, and how each CGM works varies.
Some devices require calibration with a fingerprick reading daily. Newer devices are factory calibrated and do not require daily calibration. Some devices show your blood sugar reading at all times on a receiver, smartphone or smartwatch, and an alarm goes off if your blood sugar is going up or down too quickly. Others require that you check your blood sugar by running the receiver over the sensor periodically. Where the CGMs are sited can also differ. Most commonly, CGMs are sited either over the abdomen or the upper arm.
Why use a CGM?
It’s not an understatement to say that CGMs have revolutionized diabetes care. Unlike a traditional glucometer, which provides just a single glucose reading, CGM systems provide continuous, dynamic glucose information up to every minute. That equates to roughly 1,440 readings in a day (imagine measuring your glucose every single minute without CGM!).
What the CGM does is empower you. With the additional real-time data you are now able to access, you can unlock real medical and lifestyle benefits.
Firstly, you can literally see in real-time the effects of food and exercise on your blood glucose levels. This is a huge advantage over historic “static” blood glucose monitoring, which only provided a single glucose reading at a time. Using a CGM is like watching movie, whereas using a glucometer is like looking at photographs. Both tell you the story, but the CGM gives you a more complete picture, and a lot more detail. With the CGM, you can catch cases of high sugar levels and low sugar levels as they happen, avoiding the potentially dangerous consequences. You also can see how high and how fast your sugar goes up after a meal - something you will not be able to see on a glucometer where you only know the sugar level at the point of measurement.
There is also the convenience factor. CGMs can essentially eliminate the need for those regular fingerprick tests which require a lot more equipment and are more painful.
Furthermore, studies have shown that use of CGM can help improve diabetes control and lowers the number of hypoglycemic (low sugar) events, translating to better outcomes.
How are readings from a CGM different from capillary blood glucose (fingerprick) measurements?
CGMs do not actually measure your blood sugar (glucose) level; instead, it measures the amount of glucose in the fluid surrounding your body cells (known as interstitial fluid).
Glucose diffuses between the blood and the interstitial fluid. While they are strongly correlated, at times, blood glucose levels and interstitial glucose measurements may differ. Your glucose level in the capillary (a small blood vessel) will rise or fall ahead of the levels in the interstitial fluid. There can be a lag time of 5-15 minutes of delay in interstitial fluid glucose in response to changes in blood glucose. This can happen particularly during times when your blood glucose is changing quickly, for example after eating, taking insulin, or exercising. This lag time does not impact the usefulness of the CGM since we are interested in the glucose trends and patterns as well as the direction and speed of these changes. The only time that it is important to cross check the value seen on a CGM reading with a fingerprick, is when you have a low reading on your CGM and you wish to confirm that reading and initiate treatment.
This video explains more about the differences between interstitial and blood glucose:
What are the ideal blood sugar targets?
A blood sugar target is the range that you should aim for as much as possible. The target range should be individualized depending on your goals, and should be decided with your doctor. Factors that could affect the targets include:
Type and severity of diabetes
Age
Duration of diabetes
Pregnancy status
The presence of diabetes complications
Overall health and the presence of other medical conditions
In general, the recommended blood sugar target ranges for continuous glucose monitoring are:
| Measurement | Recommended Range |
|---|---|
| Before a meal | 4 - 7 mmol/L or 70 - 130 mg/dL |
| 2 hours after the start of a meal | < 10 mmol/L or < 180 mg/dL |
| Time in Range (TIR) | > 70% of readings between: 3.9 - 10.0 mmol/L or 70 - 180 mg/dL |
| Glycemic Variability (%CV) | < 36% |
Note: TIR and glycemic variability are measures specific to the CGM. They provide additional data, beyond what the traditional fingerprick and HbA1c measurements are able to give.
TIR represents the percentage of time that your glucose levels are within the target range. The higher the percentage, the better the control of your diabetes is.
Research has found that having large glucose fluctuations throughout the day may contribute to diabetes-related complications. Postprandial (post-meal) spikes in blood glucose levels, as well as hypoglycaemic (low sugar) events, can increase the risk for cardiovascular events such as heart attacks and stroke. Some individuals can have good HbA1c measurements (HbA1c measures your average blood sugar level over the past 2 to 3 months) but can have many high or low glucose readings in a day (i.e. high variability) which may result in poorer outcomes.
What is causing my blood sugars to be out of range?
There are many factors that can impact your glucose levels, such as:
Meals and snacks that you take
Exercise and other physical activity
Medication
Sleep
Stress
When interpreting your glucose levels from the CGM, it is important to look at them in the context of your health status, and the events surrounding their changes. For example, if you see a sudden increase in glucose level, it would be important to determine if it is related to a meal you just had, or if it is in relation to intense exercise you did. The action you and your healthcare provider would take would be very different in these two situations.
Common factors that contribute to changes in glucose levels include:
Food: Consumption of carbohydrates (starches and sugars) can cause your glucose to rise, especially if you consume a lot of refined carbohydrates such as white rice or white bread.
Exercise: If you exercise without an adequate snack beforehand, your blood glucose may drop, as your muscles are using up glucose.
Stress: Stress often raises blood glucose levels.
Medications: Diabetes medications lower blood glucose. In addition, certain other types of medications (e.g. steroids) can affect your blood glucose.
If your blood sugar levels are out of range, speak with your healthcare team to understand what may be the cause, and what you can do about it. Below are some common scenarios of less than ideal blood sugar levels.
High glucose levels before breakfast
Morning glucose levels are often referred to as fasting glucose levels. If these levels are out of range, you may find that it is difficult to keep your levels down for the rest of the day. Possible reasons why your blood sugar is high in the morning include:
Consuming sweet drinks (coffee/tea with sugar, fruit juice) in the morning before measuring your sugar levels
Forgetting to take medication
What to do if your blood sugar is high in the morning:
If this is the first time you notice it happening or if it only happens occasionally, try taking note of what you had the night before and the duration of sleep you had
If your blood sugar has been high in the mornings continuously for a few days, speak to your healthcare team about it
High glucose levels before meals
If your levels are out of the target range before your meals, think about whether you had anything to eat or drink within the last two hours. If the answer is yes, your glucose levels may have been caused by this. Perhaps you had a chocolate bar on the way home, or a sweet drink before dinner? If you have not consumed any food or drink, and your glucose levels are high before your meals, speak with your healthcare team as you may need to adjust your medication.
High glucose levels after meals
If your glucose levels rise very quickly after your meals, take a look at your food intake immediately preceding the raised level. Possible reasons for raised glucose levels after eating include:
Consuming refined carbohydrates such as white rice or noodles, pastries or any other sugary foods
Consuming soft drinks or other sweetened drinks such as herbal tea with sugar added
Large portion sizes
Refined carbohydrates such as white rice and noodles are digested quickly and can cause your glucose levels to spike rapidly. In addition, carbohydrates in liquid form (such as soft drinks) are absorbed more quickly than those in solid form, as your body does not need to break them down. Sweetened drinks can therefore also cause your glucose levels to spike.
Choosing healthy complex carbs, such as whole vegetables, whole grains, beans and lentils, can prevent your glucose levels from swinging as they are less processed and your body needs more time to break them down. They also contain fibre which doesn’t convert into sugar. Eating soluble fibre can slow the absorption of sugar and improve your blood sugar levels. For example, drinking a glass of orange juice can increase your blood sugar rapidly as the orange has already been broken down. If you eat a whole orange instead, it will be broken down more slowly by the digestive system as it is high in fibre.
Make sure you also have sufficient protein (e.g. lean meats, beans, pulses) and healthy fats (e.g. salmon and tuna) in your meal, as these help to lower glucose spikes and keep you feeling full for longer.
Note that rapid glucose spikes and subsequent sharp drops can often leave people feeling lethargic, tired or have trouble concentrating – yet another reason to avoid foods which spike your blood sugar!
Hypoglycemia (Low Blood Sugar)
If you have any glucose levels below 4 mmol/L and you are not on medication that lowers glucose levels, you need not be concerned as your body is regulating itself.
If you are on medication which can cause low glucose levels, such as insulin or sulphonylureas (e.g. gliclazide or glipizide), speak to your healthcare team as some alterations may be needed.
Making the most of your data from your CGM
It can be difficult to make sense of the rich data from your CGM - especially when you are new to it. While it is exciting to learn how your body responds to your daily habits and figure out ways to improve your blood sugar control, the process could sometimes be frustrating if you don’t know what the data means. We encourage you to have a discussion with your healthcare provider before you start monitoring so that you get the appropriate support and guidance throughout the process. You should also share your data with your health care provider to better understand your condition, and to get personalized advice on how to better manage your condition.
Feel free to reach out to us to get your CGM data interpreted and explained to you too! Our doctors and nutritionists enjoy the process of optimizing someone’s daily habits and we would love to go through your data with you through an in-person or virtual consultation.
This publication does not constitute an advertisement, and is intended solely for educational purposes. Please seek professional medical advice as to whether a health product would be appropriate for you.
