Is Flash a Good Match for My Family?
Pros & Cons of the Freestyle Libre system
I wore a sensor to try out the system (though I don’t have diabetes myself) and I was impressed by how convenient it was. My 10-year-old son (who does have T1D) was resistant about trying it out at first… he’s fed up with all things diabetes right now and didn’t want to give another d-tech device the time of day… until he saw me scan once. Then he was hooked.
So what is it that makes the Libre a worthwhile addition to our family’s diabetes management toolbox?
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Background reading on what Flash is and how it works:What is Flash Glucose Monitoring?
Top 3 Benefits of Freestyle Libre:
1. Libre Replaces Lots of Fingerpokes
If you have used a Continuous Glucose Monitor (CGM) then you’re familiar with the simplicity of knowing your current glucose level at a glance. Flash Glucose Monitoring offers the same ease: glucose checks are quick, convenient and painless; all you need to do is pass the reader over the sensor to “scan” it, wait 1 second for processing, and your current glucose reading pops up on the screen. For most of the glucose checks done in a day, Flash Monitoring eliminates the need to wash hands, get a strip, put the strip in, wait for the meter to be ready, apply the blood drop, and wait 4-6 seconds for the meter to process the sample and display the result. In addition, you don’t need to calibrate the system with a fingerstick meter check, like you do with CGM systems. In fact, with Flash Glucose Monitoring there’s no way to enter an external number, calibration is not even possible. (I can imagine that this could create an issue when the system is not reading accurately for an extended period, as there’s nothing you can do to correct it, but our family hasn’t yet come across that problem.) There are situations in which a fingerstick BG check is still recommended (see Limitations of Libre, below), but the bottom line is that Flash replaces more fingerstick checks than any other glucose-checking method.
In addition to telling you the current glucose reading at the moment that you scan the sensor, the reader also displays a graph of the glucose readings for the past 8 hours, whether or not you scanned the sensor previously during that time. This is because the sensor continuously measures glucose, automatically storing the reading every 15 minutes*. The data then becomes available to you when you scan the sensor with the reader.
This is useful for a few reasons:
First, you don’t have to think about it; the sensor is taking and storing the readings 24/7; as long as you scan at least once every 8 hours, you get a full, continuous data set, without having to remember to check repeatedly during that time.
Second, this continuous data set means that you not only know where you are in terms of your glucose level, you also know where you’ve been, and the path that got you there. For example, when my son’s BG is, say, 16.5, the first question in my mind is, “Why?” If I look at back-data and see an in-range plateau in the morning, followed by a steady climb in the two hours after lunch, then I may suspect that the lunch insulin-to-carb ratio is too weak. If, on the other hand, I see an in-range plateau in the morning, followed by a sudden drop after lunch, followed by a steep climb, then I may suspect that the lunch insulin-to-carb ratio is too strong. These very different scenarios will lead me to take opposite actions to prevent similar high BG tomorrow, and I don’t know which one is the better strategy unless I have that back-data to inform my decision.
In addition, this back-data allows for in-the-moment trend information. That is, when you see a reading of 5.5 mmol/L, you may wonder, “Will my glucose stay steady at 5.5? Will it drop over the next several minutes? Or will it rise?” These different glucose trends will lead you to different actions. If glucose is falling, you may decide to eat to prevent a low. If glucose is rising you may want to avoid eating and check back in a half hour to make sure it hasn’t skyrocketed. If glucose is steady, you may relax, knowing you don’t need to take any action right now. In each case, your response is influenced by the trend information made possible by the back-data.
(*In case you’re curious, in terms of back data, the system stores the single reading that occurs at the 15-minute point, not the average of all the readings over that time period. For trend information, however, the system uses all the data from the previous 15 minutes. And for the current glucose reading, the system displays the actual reading at the moment that you scan, in real-time, NOT the reading at the beginning of the current 15-minute interval.)
Accuracy is a bit of a complex issue when it comes to Flash Glucose Monitoring (or CGM, for that matter). How do we assess accuracy? To start, what external measure do we use to judge this? Research studies on CGM accuracy often use a lab analyzer blood glucose value as a comparison, and measure how well a collection of sensor readings match those lab BG values. I haven’t been able to find similar research published in Canada on Flash Glucose Monitoring. However, I understand (anecdotally) that in order to receive Health Canada approval, data from the Libre system would have had to be compared with a device with proven accuracy: in this case that’s the Yellowsprings instrument, a frequently used and well-established glucose analyzer. In addition, there are published studies out of the United States which support its accuracy, including one sponsored by Abbott Diabetes Care (manufacturers of the Freestyle Libre) which indicates that “interstitial glucose measurements with the FreeStyle Libre system were found to be accurate compared with capillary BG reference values, with accuracy remaining stable over 14 days of wear” ¹
The nitty gritty details from that study: “The accuracy of the results was demonstrated against capillary BG reference values, with 86.7% of sensor results within Consensus Error Grid Zone A” (Zone A is defined as having “no effect on clinical action”) Further, “the overall mean absolute relative difference was 11.4%.”
Translation: When compared with capillary BG reference values – ie. Fingerstick results using a blood glucose meter – over 86% of the Libre test readings were close enough to the meter readings such that the user would not have taken a different action based on the Libre results than they would have taken based on the meter results. Also, the average difference between the Libre readings and meter readings was about 11%.
A similar study from the UK ² supported the accuracy of the Libre for children and teens (age 4-17 years old): “Clinical accuracy of sensor results versus BG results was demonstrated, with 83.8% of results in zone A and 99.4% of results in zones A and B of the consensus error grid. Overall mean absolute relative difference (MARD) was 13.9%.
What does this mean? Does this mean that the Libre was more accurate than the meter, or that the meter was more accurate than the Libre? We have no real way of knowing, as meters can also be “out” by up to 15% and still be considered accurate by current standards. Further, we need to keep in mind that the 2 readings actually measure different things: the sensor measures the glucose in the interstitial fluid, while the lab result measures the glucose in the blood. In truth, these measures may not match at a moment in time, and further, they don’t have to match for each of them to be correct in their own right. But does that matter in our real-life use of diabetes tech?
Maybe the real question from real people with diabetes is: how can I be confident that the sensor is accurate?
Or in other words… Can I trust the sensor?
How do we measure accuracy?
For my own information, I used our tried and true, lab-verified blood glucose meters as a basis of comparison. After all, the clinical recommendations offered by Diabetes Canada and by our son’s endocrinologist, as well as our experience with diabetes over the past several years, have all been based on blood glucose values. The treatment decisions we make are based on blood glucose. So for the Flash Glucose system to be useful in real-life, it should at least approximate those blood glucose values, even though it’s getting there via a different measurement path.
So far, for us, the Libre system seems to be doing that very well: the sensor results resemble the meter results most of the time, and we haven’t had any problems by acting on the the sensor results. In short, we trust the Libre.
What about Lag time?
Glucose shows up in the blood first. Via the blood, it moves around the body and out into the surrounding tissue and organs. This process of movement into the tissue takes time to occur, creating a lag between blood glucose and the glucose in the interstitial fluid where the sensor measures glucose. This lag time occurs both when glucose is increasing (such as when we eat, or when the liver releases stored glucose) as well as when it’s decreasing (when insulin is active, or as a result of exercise, when glucose is drawn out of the blood to feed the muscles).
From the Freestyle Libre website:
“Glucose levels can be measured from the bloodstream, or from the interstitial fluid (ISF), the fluid which surrounds the body’s cells, as glucose freely diffuses from capillaries to the interstitial space³. ISF glucose and BG measurements taken simultaneously won’t always match, and, in fact, are likely to be different. There is a 5- to 10-minute delay in ISF glucose response to changes in blood glucose 4 which is unlikely to impact routine day-to-day treatment decisions. The average lag time of the FreeStyle Libre system is approximately 5 minutes 5 ”
What Does Lag Time Mean in Real-life?
- when glucose is rising, the sensor will read a glucose level that is lower than the current blood glucose level (similar to the level of glucose in the blood 5-10 minutes ago).
- when glucose is falling, the sensor will read a glucose level that is higher than the current blood glucose level (similar to the level of glucose in the blood 5-10 minutes ago).
So if you feel the symptoms of a low, but the sensor says your glucose is in-range, it’s wise to double-check using a blood glucose meter.
Other Benefits of Libre:
Limitations of Libre:
Whether you consider a particular feature (or lack thereof) a drawback of a particular technology depends on your perspective, and the Libre is no different in that regard. A “good” tool is one that matches your needs and objectives. The following qualities of Flash Glucose Monitoring may make it a mismatch for some people:
1. No Alerts
If you’re looking for a glucose monitoring system that lets you know when your glucose is low (or high, or dropping/rising quickly) then Flash Glucose Monitoring may not be the tool for you. Like CGM, Flash Glucose Monitoring measures your glucose levels continuously without you having to think about it, but unlike Continuous Glucose Monitoring, Flash Glucose Monitoring doesn’t give you feedback about your current glucose level until you scan the sensor. So if your child doesn’t recognize his own lows, or if playing on the iPad distracts him from feeling the symptoms, then flash monitoring won’t provide a safety net for catching undetected low glucose.
2. Possible Skin Reactions
Although most people do not have problems with the materials that make up the sensor housing or adhesive patch, some people do. And if you’re one of those people who experiences a rash, itching, even blistering at the insertion site, it can be uncomfortable. For some people, the reaction is significant enough that they can no longer use this tech tool.
3. Financial Cost
If you compare the price of a sensor to that of test strips for the same usage period (14 days), and you are routinely doing less than 8 glucose checks per day, then some people may feel that the cost of using the Libre is high. (Assuming a sensor cost of CA$89 each (as of January 2018), and an average test strip cost of CA$0.75/strip). But once again, it’s all about perspective…
If you usually check BG more than 8 times per day (as our son does), using the Libre will actually cost less than test strips (assuming you use it to the end of the sensor life, or 14 days). Additionally, if you factor in the potential to perform countless glucose “scans”, plus access to over 1300 stored glucose measurements over that period (based on the fact that the system stores the glucose reading every 15 minutes, or 96 times each day), then you may consider the cost of using the system to be quite low. Finally, compared to the cost of using Continuous Glucose Monitoring (with a greater initial cost for the CGM receiver, and CGM sensors priced approximately the same but lasting about half as long), Flash Glucose Monitoring comes in at a lower operational cost. Once again, the cost-benefit analysis creates a different “right” decision for different people. My hope is that by reading this information, both pros and cons, you will be in a position to make a decision that fits for your family.
Other Possible Limitations of Libre:
When might you still need to do a fingerstick test?
According to the Freestyle Libre User’s Manual, you should use a blood glucose meter to confirm the sensor reading:
ο when BG is rising or falling quickly (↑ or ↓ trend arrows), as the interstitial fluid levels don’t accurately reflect blood glucose levels at that moment because of lag time.
ο when glucose is low, or headed for a low… it’s wise to confirm the reading before treating.
ο when your symptoms don’t match the flash glucose monitor reading.
1. Bailey, T., Bode, B. W., Christiansen, M. P., Klaff, L. J., & Alva, S. (2015). The Performance and Usability of a Factory-Calibrated Flash Glucose Monitoring System. Diabetes Technology & Therapeutics, 17(11), 787–794. http://doi.org/10.1089/dia.2014.0378
2. Edge, J., Acerini, C., Campbell, F., Hamilton-Shield, J., Moudiotis, C., Rahman, S., … Trevelyan, N. (2017). An alternative sensor-based method for glucose monitoring in children and young people with diabetes. Archives of Disease in Childhood, 102(6), 543–549. http://doi.org/10.1136/archdischild-2016-311530
3. Rebrin K, Steil GM. Can interstitial glucose assessment replace blood glucose measurements? Diabetes Technol Ther. 2000;2(3):461-472.
4. Rebrin K, Sheppard NF Jr, Steil GM. Use of subcutaneous interstitial fluid glucose to estimate blood glucose: revisiting delay and sensor offset. J Diabetes Sci Technol. 2010;4(5):1087-1098.
5. Data on File, Abbott Diabetes Care Inc, Clinical Report: Evaluation of the Accuracy of the Abbott Sensor-Based Interstitial Glucose Monitoring System 2014.
Freestyle Libre Flash Glucose Monitoring System User’s Manual, 2017
This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional.
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