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  • Writer's pictureAnne Daly

Continuous Glucose Monitors and Flash Glucose Monitors – do I need one?

Updated: Sep 22, 2023

Continuous glucose monitors (CGMs) and Flash Glucose Monitors (FGMs) are small disk-shaped devices with thread-like sensors which is inserted under the skin. It is worn on the

arm or abdomen to monitor glucose in interstitial fluid (fluid that surrounds all our cells). They first came on the market for use by people with type one diabetes but are now used by people with all types of diabetes and others who just want to monitor their glucose levels for a short time.

Once in place on the arm or abdomen CGMs remain active 24 hours per day, sending

glucose measurements to a smartphone, smartwatch or insulin pump.

FGM are similar but they don’t automatically send glucose readings. The device must be scanned by an smartphone app each time the user wants to check their glucose levels. These are possibly the easiest for people to purchase privately.

How long do they last?

Depending on the brand, 7 to 14 days. If they are accidentally knocked off (it is easy to knock them off – I speak from experience) they have to be replaced with a new one. It’s impossible to reuse the one that fell off even if it is only a few hours in place. Most can be anchored with extra tape to reduce the risk of dislodgement.

Why can’t they measure blood glucose?

An important point is that CGMs measure glucose in interstitial fluid, not blood glucose. This causes confusion when users expect finger blood tests to be exactly the same as their CGM or FGM.

How do CGMs and Flash Glucose Monitors work?

They work by detecting glucose in interstitial fluid, which is the fluid that surrounds all tissues and cells in the body. It plays an essential role in maintaining homeostasis/balance (distributing oxygen, hormones and nutrients, removing waste products) and as a communication system between cells and organs. In this instance, it carries glucose from the blood to where it is needed. E.g., when you get up in the morning, it sends glucose to your limbs for energy.

In layman’s terms: Our circulating blood must offload excess sugar. It offloads it into interstitial fluid, distributed around the body to fuel organs and cells. Problems arise if interstitial fluid cannot get cells all over the body to take the glucose (insulin resistance/high blood sugar/diabetes). CGMs identity this problem by checking glucose levels in interstitial fluid every few minutes.

Do they replace finger blood tests?

Technology in this area is improving all the time. Some diabetes-user CGMs require a finger prick blood test calibration to determine blood glucose, which is submitted to the CGM to try to align them both. A blood test is required when an insulin user feels symptoms that aren’t in line with their CGM or for other reasons.

Driving - for insulin users: This includes insulin managed gestational diabetes. Until recently, a finger blood test had to be done to check that blood glucose was 5 mmol/l or above before driving in Ireland (driving licence/insurance regulations), but CGM readings of 5+ mmol/l are now acceptable. In the case of an accident, you may be asked to provide proof that you checked before driving.

Advantages of CGMs or FGMs

Beneficial for people with diabetes but with increased research and information on the impact of high blood glucose levels leading to many health risks, it is a good idea for everyone to check their glucose occasionally.

They show trend lines and arrows indicating glucose changes and alarms when glucose goes above/below targets (insulin users). Upward, downward and side arrows indicate fast rise or fall or steady glucose level. This is fantastic for insulin users and helps us deal with potential hypos (low blood sugar) or hypers (high blood sugar) earlier. Over time wearers notice things that affect their glucose levels, including food types, stress, exercise and sleep (or lack of it!).

Can be integrated into insulin pumps or stand-alone. They are also used alongside insulin injections and provide excellent information to help with insulin dosing decisions.

I have used a CGM for five years to help me with my type one diabetes

and it has transformed my life, as has each new development in the past 50+

years. Changes have been slow to come on stream, but I have grabbed them with

both hands when they do. New technology combined with training to become a

nutritional therapist has greatly benefited my health. However,

monitoring and reacting to CGM readings 24 x 7 can be exhausting, especially when

there is a lot of other ‘life stuff’ going on – this applies to people on insulin


CGMs provide glucose analysis on an app, which the user or medical team can review remotely or in clinic and take appropriate action. If a client's graph shows a steep rise in glucose at 11am everyday, I would investigate if there any differences on days off,

food, stress levels, sleep, exercise, supplements and medication. Armed with all information, we work on a suitable plan for them to try. The beauty of a CGM is that the client can see whether the change has been beneficial after a couple of days. If not, we try something else until satisfactory glucose levels are achieved. The client learns what triggers high glucose and what helped.

Next steps if you are interested in your glucose levels

You can ask your GP to check your levels or purchase a blood finger prick

kit (less than €30, but you have to pay for strips) or a CGM or FGM. You could

do this as a once-off or purchase every year and wear it for a few weeks.

If you have used a CGM but had no idea what the readings meant or what to do about them, then why not try again but this time, book a 60 minute consultation with me. I will review your CGM report, explain the readings and give you tips to reduce your glucose levels if necessary. A deep dive into your readings can be hugely beneficial. It sets you on the path away from prediabetes and type two diabetes. It is also useful when planning a pregnancy.

Reasons not to use a CGM or FGM

If you feel that you may become anxious about each reading, then it’s best not to go there.

Remember, prolonged stress and anxiety promote elevated glucose levels. Try

relaxation techniques and consider using a CGM at a later stage.

Availability and funding

Every country has different criteria for providing free CGMs to people with diabetes.

In Ireland FGMs are usually provided free for children under 21 years of

age. Unfortunately, that stops when a person reaches 21.

CGMs are only available in diabetic clinics who provide CGM training, monitoring and

specialised clinics. CGM training is essential for people with type one diabetes because you must understand how the body responds to insulin, food, stress and exercise to benefit from a CGM. They are sometimes but not always prescribed with an insulin pump but work well with insulin injections too. They are not routinely offered. Your Endocrinologist has to prescribe them or you have to put forward a case for needing one. Part funding is sometimes offered.

Buying a CGM or FGM

May be purchased whether you have diabetes or not. If you have diabetes you

don’t need permission to use one. May be purchased directly from suppliers, local or online pharmacies.

You can claim back VAT and add the remaining cost to your annual health expenses form. Health insurance may cover some of the cost.

You will have to purchase one if you have gestational diabetes or simply want to monitor your glucose level for a few weeks. Price is approx. €69 for an Abbot FGM. Look out for free trials. I think it’s a good idea to try using one for a few weeks to see trends and get an accurate picture of your glucose levels. It does prompt real action as you can’t ignore the data!

Alternatively book a free mini consultation and find out if working with a nutritional therapist alongside trying a CGM would benefit you. Some health insurers cover half the cost of the consultations, so it’s worth checking.

Hba1c blood test = blood test which estimates blood glucose over the past two to three months. I often ask clients who don’t have diabetes to get this test as it is a better indication of glucose levels than a once-off blood test. It has its drawbacks, especially concerning insulin-dependent diabetes. A ‘good’ Hba1c level can result from many hypos (hypoglycaemic/low blood sugar) and some hyperglycaemic (high blood sugar), which isn’t ideal. Doctors set Hba1c target levels, factoring in age, life stage, other health conditions, etc. I advise clients not to compare their targets and results with other people. The only target that matters is your own.

Frequent hypoglycaemia impacts brain health as it starves the brain of food/fuel until blood glucose levels return to the normal range (4mmol or above). Anyone who takes insulin is susceptible to this. Time in range and trends can easily be investigated using CGM data.


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