My Trial: The Dexcom G4 CGM vs Freestyle Libre FGM

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This is how easy the Freestyle Libre Flash Glucose Monitor from Abbott is to insert. 

Recently I conducted a device trial comparing my Dexcom G4 continuous glucose meter to a new flash glucose meter called the Freestsyle Libre from Abbott (currently only available in Europe and the UK). They call it “flash” because you don’t see where your blood sugar is until you scan the sensor with what’s called a reader, but for my trial, it didn’t matter.

The Freestyle Libre is very popular – it was out of stock for 3 months after its release. Here is where having a Dutch sister in law who lives in the Netherlands comes in especially handy. But I should tell you the trial made me wonder: how do we know how much we can trust our devices?

I wore both the Dexcom and Freestyle for a week. Dexcom’s sensors are approved for 7 day wear. The Freestyle Libre’s for 14 days. Here at a glance are the dominant feature differences between the two.

Features Dexcom G4 CGM Freestyle Libre FGM
Sensor approved for wear 7 days 14 days
Calibration 2 x day None
Alarms Yes No
Range 20 Feet Not appropriate
Insertion Cumbersome Easy
Body location Stomach Back of upper arm
Sensor size Somewhat bulky Thin

The Freestyle Libre pretty consistently gave me blood sugar numbers 20-30 points lower than my Dexcom. It was unnerving. I emailed a friend in Sweden who has also used both and he reported similar results with a comment, “The worst part is how do you know which to trust?”

Make no mistake, I would love Freestyle’s 14 days of not having to prick your fingers since the Freestyle Libre requires no calibration. But without calibration is it really reading where I am correctly?


My Dexcom is on the left. The Freestyle reader in the middle. Since it has the European system of mmol/l, 6.6 mmol/l as you see on the screen, is equivalent to 119 mg/dl. My meter is on the right. At 119 the Freestyle is way below my meter and Dexcom.

Over the years I’ve learned the numbers on your meter are not precise. They’re an approximate of where your blood sugar is. If you check your blood sugar two times in a row you’ll usually get two different numbers. Sometimes they’re close, sometimes not.

I also know continuous glucose monitors are measuring glucose from interstitial fluid, not blood. As such, their numbers lag about ten minutes behind your blood sugar meter reading and sometimes when your blood sugar is going up or down quickly they get a little confused.

All that said, it’s disconcerting when two devices are giving such a different reading. Abbott hopes to make the Freestyle Libre available in the US next year. And I’d love to use a glucose monitor that’s as comfortable and finger stick free as Abbott’s. I just want to know can I trust it?

Just as we keep fighting for greater meter accuracy, a twenty five point spread between two glucose monitoring devices is just too much.

For more on this and the emotional cost of devices, see my Huffington Post article here.

25 thoughts on “My Trial: The Dexcom G4 CGM vs Freestyle Libre FGM

  1. Hi Riva,
    Best information about CGM’s I have seen – congratulations! I’m using your comments, as a guide for development, of a next-generation CGM.
    To have a peek, at the future, see me on YouTube: Best-in-Class Glucose Monitor, and at, key ‘Explore’, on the top bar, then, on the next page, scroll down to ‘Diabetes’.
    Please send me your email, and I’ll send you a Picture of the DexCom and Medtronic ‘insertion’ needles vs the Company’s ‘working’ micro-needle. Even easier than the Abbott device, being used in Europe.
    NO inserter needed.
    Keep on Truck’n,
    Albert Kretz, Ph.D., MBA
    Advanced BioSensors-Ohio, LLC

  2. #freestylelibre genuinely awful service from @AbbottDiabetes. An innovative idea with no competitors is no excuse for faulty products and appealing customer service.

    Went through 3 meters, 5 sensors and hours of telephone calls do get this sorted and what do Abbott Diabetes Care offer me? Not so much as an apology.

    Their call centres feel like a bunch of people with exceptionally poor English going though a checklist and not actually listening. Seems like if its not on the form they don’t understand it.

  3. I’m also from Sweden and I’ve been using this system for a couple of months. I’ve had one faulty sensor, and Abbott replaced it without any discussion. The Libre always says 0.5-1 mmol/l lower than my ordinary meeter, but the difference is less than the difference between brands or even the same meeter if you were to try your BG two times in a row. I love it!

      • Being a diabetic (with an extreme tendency of being hyper a lot), I’m actually interested in better devices for managing my condition. The constant finger pricking and daily insulin injections notwithstanding, I’m actually anxious to see if the FDA will at some point approve this device in some usable form here in the States.

        The last I heard from the trades was sometime in the middle of last year where there some consideration of FDA approval for a “Pro”/blinded version for use by doctors and medical professionals in a controlled/facility setting. I think the hesitancy to use at the consumer level is that Medicare is being a bit more cautious about allowing more independence for patients and the way their funding works.

        Personally, as an able-bodied adult, I’d be up for something where I can gauge my own progress in making the relationship with my own pancreas a better one.

  4. Thanks for your perspective, Riva. I wonder if it will be available to people on Medicare. But if it’s that far off from the meters which we are told to rely on rather than CGMs, then that may be a moot question for me!

  5. Interesting comparison we have now used the Freestyle Libre for my son for a year and loved it, 3 months ago moved to Dexcom G5….what I really want is the insertion of the Libre, the sensor size of the libre and the technology of the Dex G5….maybe one day.

  6. Might be helpful to have results from finger-prick tests to see where the truth lies.

    I’ve been using the Freestyle Libre for around 18 months; typically it’s quite accurate in the normal range, but under-reads above it.

    Sometimes sensors aren’t accurate or drift, although the trend is usually correct. Abbott will replace faulty sensors.

    The Gliapp android app can read the Libre sensor with an NFC-enabled phone, and calibrates the sensor’s values with finger-stick tests which improves accuracy and corrects drift.

  7. Thank you so much for your post and you are right such a variance between Glucose meters is unacceptable but what I have learnt with my diabetes is that sometimes we look at things with only one lens. What you have not realised is that this device “may” not be providing the readings of the standard meter BUT what it WILL do is provide you with your CURVE – what your sugars look like over time even with a +/- variance of a few mmol/L … the shape of the curve will enable you / give insight into how the food is reacting in your body between finger pricks.

  8. I think this is a great summary, so I am new with the CGM, I never used any, so I am looking something practical to start and of course in low budget. So I was convinced to buy the Freestyle Libre, but after reading this I am not longer convinced.

    So based on this what is your recommendation? Dexcom over Freestyle? or Frestyle can be used and adjusted with some random checks (Calibration as Dexcom) to see the gap and only sum up the 20/30 points differences? So in example I use the freestyle and my Glucose is 120 so I know will be 150?

    Thanks for your feedback

    • Gustavo, the Freestyle Libre is not yet available in the US. You would have to purchase it in Europe. You cannot calibrate the Libre at all. I recommend the Dexcom as it works better for me.

      • Thank you Riva, Actually I live in Slovakia, I can buy the Freestyle Libre from Germany or UK without any problem is why I was thinking to get it.

        I was ready about the Freestyle Libre, I know we can not calibrate, but with “Calibration” I meant at least to have estimation how much variance we have from finger test, and used this variation as standard, so for example if my Freestyle result is 6.0 (110 aprox.) and my finger print is 7.2 (130 apox). So every time I get my freestyle Libre results I know I need to sum up these value to have an accurate figure, but not sure if this variance is always the same and this logic will work.

        What makes me think about the Dexcom is the price. =-(

      • Gustavo, Riva, Abbot’s sensor and software don’t calibrate the Freestyle Libre, but the Gliapp Android app does. My experience is that most sensors are usable out of the box and I’ve found that even the odd sensor which has drifted unacceptably is perfectly usable with Gliapp. In any case Abbott will replace faulty sensors.

        You should also consider the accuracy of finger stick tests. Not all are as accurate as the Libre.

      • Further to the issue of accuracy, you might be interested in reading the evaluation of 43 BG meters from the Journal of Diabetes Science and Technology (Volume 6, Issue 5, September 2012) here

        The tables on pages 8 and 9 of this article shows that some meters are significantly more accurate than others. The scatter graphs a couple of pages on show the spread of readings – FWIW the Freestyle Lite seems to have one of the tightest spreads and no bias, particularly around the 70-150mg/dl range.

        Most of the readers aren’t significantly biassed (see page 14), i.e. consistently high or low, so you can check the accuracy of your meter by comparing multiple BG tests using strips from different batches at the same time.

        Bottom line is that some of the finger stick BG meters tested are giving less accurate results than CGMs with or without calibration, and the accuracy of CGM calibration will depend on the accuracy of the meter used to calibrate it.

        Therefore a comparison of CGM accuracy requires independent BG values, which as a minimum could be taken from multiple readings from an accurate BG meter.

  9. Ah, afraid I cannot advise re: creating your own variation replacement. If Dexcom is too expensive, I’d get the Freestyle and try it. Since diabetes is so individualized, it may work really well for your body. Others have liked it.

  10. Riva

    Thank you for an interesting and insightful article that helps me. I am Hypoglycemic following a Transchiatal Oesophagectomy for Oesophageal Cancer. The prognosis is that I will have hypo’s for life as my digestive system cannot tolerate sugar.

    I have tried the Accu-Ceck Mobile. But it is cumbersome and the results are not always accurate. I need a predictive system that both the the Dexcom and the Libre appear to provide. I have reached out to both companies for help and I am about to start using a Libre and am hoping for good results.

    The drawback here is the Libre technology is not compatible with Apple Technology. Small change to some but it does restrict the market on what (on the face of it) appears to be the better product. I love the idea of 14 days of no finger pricking so am drawn to the Libre.

    Thanks again for your article.

  11. Hi, live in New York and am a Type 1 Diabetic for 51 years. I’ve been using Omnipod system for last 7 years.

    Last week at my usual 3 month Endo apt, I was hooked up to Freestyle Libre. I had read about it and looked forward to my first experience with it. All I was told was to come back in two weeks to obtain results. Lol

    Right before retiring, I was approved for Dexcom 4 and received the whole package including sensors to last three months. My employer insurance covered it all.

    Bottom line, I never got to use it. I had a big move and, when I realized Medicare wouldn’t cover it, I donated it to a Diabetes Foundation for a tax write off.

    I’m very interested in maybe one day obtaining the Libre. A friend of mine has relatives in Germany…just thoughts right now.

    Would anyone be able to guess the
    approximate price in US dollars for this unit and a years worth of sensors?

    Thanks for info anyone may be able to supply.

    • Joedy, the Freestyle Libre Flash monitor isn’t yet available in the States for patients, only for HCPs. Maybe by the end of the year. I don’t know if anyone yet knows what it will cost.

  12. Dears,
    Thanks for all your suggestions and comments regarding the Libre and the other device.
    One thing to tell you. My daughter had the FSL since July last year … this month the had a rash … allergy to the glue of the sensor. We replaced it and Abbott sent us a new one.because the sensor was telling us LO and she was on finger test 240 … High … we tried once again to fix a new sensor and after 3 days same issue not able to read the results. I called Abbott again and they suggested to make tests on finger during 2 hours and then if sensor do not work, replace it … when I told her that my daughter had allergies and was taking cetirizine caps + antiallergy on the skin she told me to stop using the FreeStyle Libre … I cannot tell you the disappointment of my daughter. So I decided to call her diabetologist and to find out a solution to this issue. We’ll try the one of Medtronic but she will have to make 2 blood tests on fingers per day … better than 7 but ….

      • Thanks a lot for your reply. We have an appointment with the diabetologist next Tuesday and we’ll ask her how to change into D’excommunication.

  13. Pingback: FreeStyle Libre Flash Glucose Monitoring System Approved! | Diabetes Stories

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