Practical Experiences, Advantages and Limitations of Measuring Blood Glucose using a Freestyle Libre Continuous Glucose Monitor (CGM).

For a two week period between November and December 2019 I used a Continuous Glucose Monitor (CGM) to measure my blood glucose. You can read about the concept and the experiments I planned in the blog I wrote at the time.

In this first blog of a series, we’ll explore the practicalities, advantages and limitations of having a CGM placed on your arm or tummy. Is the data worth the cost, time and hassle? 

The Freestyle Libre CGM Device. 

• A small disc, the size of a 50 pence piece. 
• Thin flexible needle or filament, maybe 4 millimetres in length. 
• Being medical tech rather than a Fit-Bit, the device has its serial code printed on the outside. This digital identity lends itself well to the bionic man novelty. 
• Expensive if bought privately. Even if you are diabetic, they can be difficult to get hold of in some UK areas, as their provision by GPs is governed by the policies of the local clinical commissioning group (CCG).

The CGM device after being removed from my arm. The needle is tiny!

Implanting the CGM (and the trials of keeping it in).

The device came placed in a neat spring-loaded pod and with a skin-cleaning alcohol wipe. When positioned against the skin on the back of my left tricep, the pod neatly inserted the device without any fuss. The device itself had an adhesive backing that easily stuck onto the skin – a glue so sticky that even when the device was removed two weeks later, I had a residue that was both resistant to showering and attractive to fluff. 

Discomfort from the CGM filament.

As you can see from the photo, the CGM had a small and flexible needle. My arms, however, must be more sensitive than those of others because I persistently noticed the needle’s presence. Despite being placed on the tricep, where there should be a layer of fat and some extra skin, the needle felt like it was staking together superficial layers of tissue that would previously have been able to slide over each other. It felt weird to use my arms in certain movements or at extremes of range of movement, especially when working out – push ups, over-head press and pull-ups became even more uncomfortable than they already were. 

CGM vs Glucometer

Maybe my tricep fat content is just too low, or more likely, I am a delicate person that doesn’t like needles. Whatever the case, had the CGM been pain-free, it would be an advantage over the alternative of finger-prick testing using a cheap glucometer. A finger-prick stings at first but is quickly forgotten. As the reality of using a CGM means that there can be persistent discomfort or pain, especially when moving during exercise or when lying down and sleeping, this potential benefit between CGM and a glucometer is nulled. 

Getting Sweaty. 

The neat Freestyle dressings dealt well with showers but came off quickly with the first suggestion of sweat. I had to place a large medical dressing over the CGM, reinforced with some redundant K-tape and a hefty dose of hope. I also chose to pick my battles – I skipped going to Ju Jitsu for the experiment as rolling around was uncomfortable and I was worried I was going to leave the CGM stuck on the mat or on the other person.

Diabetes Awareness

As an aside, after applying the device and trying to keep it stuck on, I definitely noticed more people with them in the gym and in life. Some people I saw had them applied to their stomachs – perhaps a better place for it to live if you too suffer from extra-lean triceps, however, this placement does not escape the pitfalls of discomfort if you sleep on your stomach or the risk of it being knocked off if you decide to crawl around or other such belly-based activities. 

Experiences Measuring Glucose. 

Using NFC on my phone to measure glucose was addictive and easy. It worked through clothes, so I could sample whenever and wherever. I didn’t need to as the device was supposed to record and store my glucose data at 3 minute intervals, but the novelty of melding man and machine was too much to resist. It seems I am very susceptible to this novelty – looking back at the data, I have made readings at odd times such as 4 AM when I really should have been sleeping. 

Device unreliability and failure.

Towards the end of the 2 week period, after syncing the device with my phone in the morning I would see that there were long periods overnight where the device had stopped recording.  It could have been due to sensor or memory failure due to damage, age or failing battery life; I do not know why this was as no error message was shown.

Differences between devices.

There were no readings that appeared anomalous in my data, implying that the machine was not wildly inconsistent in its calibration and measurement. I have heard that the Freestyle Libre tends to underestimate its readings, while its competitor, the Dexcom G6 may overestimate, suggesting that the entire dataset of readings collected may be ‘off’ in comparison to true serum glucose. 

The Freestyle User Interface – the App and Web Platform.

Generally neat and well presented, I enjoyed the ease of use of the mobile app. It was easy to measure and time-stamp data, but it was clunky to input what I had eaten, the type and degree of exercise I had done. The system would benefit from being able to upload photos or to have integration with food-tracking apps such as MyFitnessPal for easier input of carbohydrates and macros.

The online data viewer ‘Libreview’ can be synced with your healthcare provider for ease of communication. It presented the data in an easily consumable way, with important figures for people with diabetes (averages, highs, lows and estimated HbA1c) easily displayed. 

Is The Data Useful?

Doing some background reading and podcast listening, I learned that there are limitations to the accuracy of the data the CGM produces. 

For example, although we may infer that having a machine that continually samples glucose gives a real-time level of blood glucose, the data provides only a proxy of blood glucose: it samples the interstitial glucose concentration of the ‘interstitial’ fluid that sits between our cells. 

A CGM does not/cannot measure real time blood glucose.

Interstitial glucose can reflect the general trend of blood glucose, but there is a difference between the two due to the diffusion of glucose from the blood into the tissue. Interstitial glucose data might mis-represent blood glucose data with respect to timing and to the height of peaks and troughs of data. 

Being alerted to high blood glucose is a key valuable safety function. 

But, if the device tells you that it reads a measurement of 21 mmol/L, the degree of accuracy isn’t important here. You might be actually at 21.45mmol/L but the important message is that this is high and you should take intervening steps to bring the glucose down safely.

Hence, the CGM device is a very useful safety barrier for people with aberrant glucose control (e.g. people with diabetes) as the sensor will detect and alert the user to dangerous glucose levels that are very high or very low. 

Is it useful for people without diabetes?

For people who have normal glucose regulation, the inaccuracy of the CGM’s sensor in the normal range of glucose means that the minute-by-minute readings probably aren’t useful. A change of 0.1mmol/L between 5 minutes might just resemble a sensor error or variability. 

After I realised this, the absolute numbers contained on my elaborate spreadsheet of glucose numbers did not seem so important. I can’t focus on the minutiae of the data or one-off occurrences that have no comparison event to view a trend with. What is more important are the trends or repeated patterns of glucose excursions that are observed. 

Is Two Weeks Enough Time? 

For people with swinging glucose, two weeks is enough to detect these swings and to start implementing measures. This detection and alert system, paired with information about diet and exercise, is great for people who want to control their diabetes with lifestyle measures.

For people with normal glucose, where the pattern of glucose throughout the day in response to external stimuli is more subtle, two weeks might not be enough to gain meaningful insight. 

Does using the CGM for longer mean better results?

To explore these subtle relationships further, it might be worth extending the time course to a month, to allow repeated events (e.g eating the same meal again and again) to be compared. On the other hand, the relative inaccuracy of ‘normal range’ glucose measurements limits the value of any data collected, hence limiting the value (and point of) any subsequent extended experiments if you intend to delineate fine details of glucose control. 

Instead, perhaps two weeks is the sweet spot for deciding whether you do or don’t need to pursue further CGM – if you manage two weeks without any major shock findings, you could just accept that you’re doing just fine and can continue to enjoy your life as normal.

If you are really set on improving your already good glucose control, I’ll preach the normal stuff: continue eating well, building and maintaining muscle mass, staying active and getting good sleep. You can always re-evaluate your need for any subsequent testing at a later time. 

But Can We Use The Data?

It takes 2 seconds to stick a CGM on your skin. It takes hours to analyse the data.

The amount of data collected was immense. I created a huge spreadsheet, annotated with what I ate, when I ate, how I felt, what exercise I did and so on, in a premeditated effort to go back and mine my data.

The truth, however, is that life moves fast. We work a lot. We haven’t got that much free time. When we do have free time, drowning in a sea of numbers isn’t exactly the most appealing thing to do, especially if it’s sunny outside (actually, if there is anything outside more interesting than a spreadsheet).

The data is only as valuable as its interpretation. I struggled to dedicate the time needed to clean and interpret the data to come to any meaningful conclusions. Apps and algorithms may help us in the future to cut through this analytical quagmire, but until then, I think deriving information other than detecting hyper- and hypoglycaemic events will be limited by the interested individual’s time and will!

Summary:

• CGMs are geared towards those people with dysregulated glucose regulation and are sensitive to high and low readings but not necessarily reliably accurate. They do not provide accurate real-time blood glucose data.
• Glucose trends are the useful derivative of these experiments. 
• If you want to collect data about your glucose, use a traditional glucometer. They sample the right stuff (blood) and are cheaper.
• The Freestyle Libre app is good for viewing trends; you don’t need try to wade through a spreadsheet like I did.
• I maintain that for people with diabetes, especially type-2 diabetes, having an awareness of what activities and foods cause ‘bad’ glucose trends is an important tool for awareness and lifestyle change. 
• Two weeks of CGM is enough time to know if there’s something seriously wrong.
  • But, glucose isn’t the whole story – to understand glucose, we have to understand the importance of Insulin.

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