Since my first detailed report comparing the various Continuous Glucose Monitoring Systems (CGMs) back in 2014, a lot has happened. And not much has changed.
The systems have improved in terms of accuracy, features and ease of use, but the main players remain the same (Medtronic and Dexcom). Access via insurance coverage and professional loaner systems has grown exponentially, yet less than 20% of those eligible for CGM are currently using them. In many cases, insurers make the process of receiving coverage onerous and needlessly complex. This doesn’t even touch on Medicare, which continues to sit idly by with its head up its proverbial butt while older Americans suffer needlessly from dangerous glucose swings. New and improved software programs (plus a brilliant new book called “Practical CGM”) provide guidance on how to interpret/analyze CGM reports, yet few patients bother to look at their own data, and very few healthcare providers have the expertise to convert the reports into useful therapeutic insight to help guide their patients.
So let’s get down to business. How do the latest Medtronic and Dexcom CGM systems compare?
Dexcom’s latest and greatest, the G5, features a transmitter that sends data directly to either a handheld receiver or a mobile phone. Dexcom’s G5 Mobile App displays data on the phone and generates the various alerts; Dexcom’s Clarity App generates reports for retrospective analysis. G5, as well as G4 Platinum, utilizes Dexcom’s up-to-date 505 algorithm for translating subcutaneous electrical impulses into glucose values. Why call it 505? My best guess is that Medtronic copyrighted every other number below 1000. (for some reason, they skipped 505 when naming their various pumps)
Medtronic’s latest CGM features their new-generation Enlite sensor, which has undergone a series of manufacturing improvements in order to provide better adhesive, less trauma upon insertion, and better/more consistent enzyme chemistry. The pump that receives a signal from the Enlite sensor has the ability to react in an automatic fashion. The 530G pump (in the United States) ceases basal insulin delivery when glucose levels drop below a user-defined threshold. Medtronic has also introduced a cell phone app which can display sensor data in real-time with the aid of a Bluetooth transmitter called “Connect”. The 640G pump (in Europe) takes things a step further by temporarily turning off basal insulin delivery when hypoglycemia is anticipated — thereby preventing the low rather than waiting until it has already occurred.
So, which CGM system is best?
Being the fair and impartial (ahem) guy that I am, I spent the past eight weeks putting each company’s latest system to a head-to-head test. Keeping with common practice, I wore each sensor for as long as it was functioning and performing reasonably well. Other than that, I followed all of the rules and regs that each company provides in its Instructions for Use. Simultaneous calibrations were performed two to four times daily prior to meals, when glucose levels were in a relatively steady state. The sensors were stored properly and placed in appropriate/adjacent sites. The sites were prepared appropriately and rotated consistently. Extra adhesive was applied over the sensors at the first sign of loosening. I also avoided using acetaminophen (the pain reliever found in Tylenol products), as both sensors can produce falsely elevated readings when exposed to acetaminophen.
Below are my findings.
Medronic 530G with Enlite
Sensor Insertion & Comfort
Both systems use very thin introducer needles and automated insertion devices. However, insertion of Dexcom is a simpler process requiring fewer steps and less equipment. Dexcom’s built-in adhesive eliminates the need for extra tape for most people
Both Sensors function well past their “approved” life-cycle (7days for Dexcom and 6 days for Medtronic) with a bit of ingenuity (and some extra tape). Based on personal experience and that of our clients, I’ve found Medtronic Enlite sensors last an average of 8-10 days; Dexcom sensors last an average of 12-14. However, that doesn’t mean the sensors perform well at the later stages of use. Towards the end of this report, you’ll find specific data showing how Medtronic’s sensor accuracy falls significantly after 6 days of use, while Dexcom’s performance is fairly stable through day 14 (sometimes beyond).
Both systems offer High & Low alerts as well as rate of change alerts, albeit Medtronic’s rate of change alerts are much more customizable. Only Medtronic offers predictive alerts which provide an earlier warning of potential hypoglycemia (and hyperglycemia). Medtronic also allows th euser to vary the alert settings by time of day – a potential value for those wanting more aggressive or fewer alerts while working or sleeping (or sleeping at work).
Both systems can beep and/or vibrate to alert the user of a potential problem and both have adjustable settings, but the vibrate mechanism on Dexcom’s receiver is considerably stronger, and it can beep much louder (when desired). The Dexcom mobile app has the added benefit of allowing the user to choose from nearly two dozen audible patterns for each type of alert – making it nice to hear somthing other than a “BEEP” for an alert.
As an independent company, Dexcom has done a decent job of “integrating” with a number of devices, including insulin pumps (Animas Vibe, Tandem t:slim G4), cell phones, and smart watches (Apple Watch and Pebble). However, in all cases, integration only replaces the traditional Dexcom receiver with the display of data on these other devices. Medtronic takes things another important step: automated therapy adjustments. Yes, Medtronic’s sensor data displays on their insulin pumps. But, more importantly, the 530G’s “threshold suspend” feature automatically stops the pump’s basal insulin delivery when the glucose falls below a specified threshold (set by the user). The 640G’s “Smartguard” feature temporarily suspends basal delivery when hypoglycemia is approaching.
There is a significant difference in transmitters. Medtronic’s radio transmitter contains a memory chip that stores up to 40 minutes of data and submits i to the receiver in case the receiver was out of transmission range. Dexcom has no such feature. But, Dexcom has much less need for such a feature. The Medtronic signal only travels a few feet and is frequently lost even when in close proximity to the sensor. Dexcom’s signal (radio or bluetooth) tends to pick up pretty well as long as the receiver (or phone) is within 20 feet of the transmitter. Medtronic’s transmitter must be separated from the sensor and charged at least every six days. Dexcom’s transmitter never requires charging. The G4 transmitter tends to last 9-12 months and the G5 transmitter lasts 3 months.
All Dexcom CGM displays, including those in the Dexcom receiver, phone app and linked pumps, are bright/high contrast and full-color. However, the G5 Mobile app has a tendency to “lock up ” from time to time, requiring the user to shut down other programs in order to proceed with data entry. The display on the Medtronic 530G is relatively small, monochromatic (black and white), and low-contrast. The 640G screen has similar quality to Dexcom. However, many people who use an insulin pump prefer to have a receiver/display that can be carried separately for ease of access and detection of alarms. Medtronic’s cell phone app display does offer excellent size and contrast but requires one to carry a “connect” fob in order to send the data from the transmitter to the phone app.
One of the new benefits of CGM use is the ability to have caregivers and loved ones track the data in real-time on their smartphones and be alerted of potential problems. Dexcom pioneered this feature with its SHARE component (available on the G5 and later-model G4 systems). Data may be shared with several “followers” and the alerts are fully customization. Medtronic’s mobile app allows followers to be alerted in the event of a crisis, but data cannot be viewed in real-time.
Dexcom’s download software has some nice features, such as the ability to segment data by day of the week and customize analysis dates. However, Dexcom Studio software does not work on Mac systems or integrate with data from an y pumps or meters. Dexcom also took a major step backwards with its Clarity app/software (necessary for generating reports from the G5 system). Clarity eliminates many of the reporting options and customization features that were popular with Studio.
Medtronic’s Carelink software is web-based, works on virtually all operating systems, is easy to share with healthcare providers, integrates sensor data with pump and meter data, and generates reports that do a super job of revealing post-meal patterns.
As the saying goes, “If something can go wrong, it probably will”. there’s something to be said for a system that doesn’t leave a lot to chance.
Dexcom continues to be very simple to use. Other than learning the sensor insertion (which requires a bit of dexterity) and setting up the mobile app, everything about it is simple. There are very few steps, and the steps are all very logical. You put the sensor on, attach the transmitter, calibrate after 2 hours and it works.
Medtronic’s CGM has a lot of moving parts: finicky transmitters, a multitude of error messages, a lengthy/detailed sensor insertion process and a variety of issues that can impact sensor function and accuracy.
Case-in-point: I do a lot of traveling, so I decided to count up how many items I had to take with me to maintain my Dexcom G5 compared to the Medtronic Enlite (both using the respective mobile apps).
I had to tote 10 items to manage my Medtronic sensor changes, but only 3 for Dexcom (see list and photo below).
Connect device (for transmitting data from pump to phone app)
Charger for Connect device
Dexcom sensor maintenance supplies:
Sensor (with built-in inserter)
Alcohol (necessary to exfoliate skin)
** All the stuff needed to use/change/charge the Medtronic sensor w/cell-phone app (left) and Dexcom (right). The cell phone was needed for both systems.
Here is the most important piece. Most people are willing to put up with some minor inconveniences if the system generates reliable data on a consistent basis.
When reviewing this information, keep in mind that sensor accuracy is best measured in relation to LAB values, not blood glucose meter values. Fingerstick values taken with a home-use meter are intrinsically less accurate than lab values, and this contributes to a greater degree of inaccuracy by the CGM. But, given that not many of us have a fully equipped lab to tote around all day, the best we can do is evaluate the systems based on how they compared to our handy-dandy meters. In my case, I stuck with the meters that are considered to be top-of-the-line for accuracy throughout the comparisons.
Accuracy is best measured by looking at MARD – Mean Absolute RElative Difference. This represents the difference between fingerstick meter values and sensor glucose values, so a lower MARD is better. A calibration reading of 150mg/dl and a sensor value of 120mg/dl represents a MARD of 20% (a 30-point difference on a value of 150).
The bottom line is this: Medtronic still isn’t up to par with Dexcom when it comes to accuracy. But the gap is narrowing.
Note that these MARDs are higher than what is seen when using LAB values for calibration purposes. Looking at rates of accuracy, Medtronic’s sensor was closer to the fingerstick calibration 43% of the time; Dexcom was closer 57% of the time.
There were some interesting accuracy patterns related to day of Sensor use. Unlike earlier versions of both systems, accuracy on “day 1” was not all that bad (19.0% MARD for Medtronic and 14.8% for Dexcom). There was a marked difference in accuracy when sensors were used beyond their approved days of use. While Dexcom maintained its accuracy from week one to week two, Medtronic’s accuracy took a major dive when the sensor was used beyond its “approved” six days.
Dexcom MARDMedtronic MARD
Days 1-7: 13.4% Days 1-6: 16.6%
Days 8-14: 13.2% Days 7-12: 25.6%
Both systems continue to underestimate glucose levels more often than overestimating…which is good for those trying their best to avoid hypoglycemia, but not ideal for those who are striving for the tightest control possible.
Below Calibration: 62% 64%
Above Calibration: 37% 35%
Exact Match: 1% 1%
Clark Error Analysis:
Dexcom: (79% zone A – safest)
(100% zone A + B)
(0% outside zones A & B)
Medtronic: (63% zone A – safest)
(98% zone A+B)
(2% outside Zone A&B)
Take Home Messages…..
Always remember, the comparisons above are based on an “N of 1” (me only). They do not represent the findings from a multi-subject, randomized, controlled study. However, I can say that most of what I experienced reflects what I have seen among countless clients/patients who have worn the various CGM systems.
A few observations worth noting:
Dexcom continues to provide a more accurate, user-friendly system, but Medtronic is a step ahead when it comes to data analysis/reporting and integration with automated insulin delivery. If this was a boxing match, it would probably be a split decision in favor of Dexcom.
CGM accuracy continues to improve, but both systems still underestimate glucose levels overall.
Dexcom sensors appears to maintain their accuracy when used beyond 7-days. Medtronic sensors appear to fall off a cliff accuracy-wise when used beyond six days. Personally, I’m very excited about the “next steps” from each company. Envision a CGM that does not require fingerstick calibration, is simple to use, and integrates easily into daily diabetes management decisions. I guess that means people won’t need my services as much, but that’s OK. I can always pursue my dream of becoming a traffic engineer.
An award-winning Certified Diabetes Educator, Masters-level Exercise Physiologist and person with type-1 diabetes since 1985, Gary Scheiner has dedicated his professional life to improving the lives of people with insulin-dependent diabetes. He was named 2014 Diabetes Educator of the year by the American Association of Diabetes Educators.
Gary has authored six books: You Can Control Diabetes (1997), Think Like A Pancreas (2004, 2012, 2020), The Ultimate Guide to Accurate Carb Counting (2007), Get Control of Your Blood Sugar (2009), Until There’s A Cure (2012), Practical CGM (2015) and Diabetes-How To Help (2018), as well as dozens of published articles related to diabetes education for consumer and trade magazines and diabetes websites. He speaks at local, national and international conferences on a multitude of topics in diabetes care. Gary is certified to train on all models of insulin pumps, continuous glucose monitors and hybrid closed loop systems, and has personally used every system that is currently on the market.
Stay far far away from the Medtronic sensor…This sensor is a piece of CR–. I have to return every third sensor. I have talked with most of the tech support staff and each one gives me different “trick” to try and fool the sensor into working. This includes esoteric ISIG calculations to simply testing and waiting and testing and waiting until the sensor figures things out. I am ready to buy my own Dexcom regardless of what my insurance tells me.
I am ready to update or change my CMG system. Any new information on either Dexcom or Medtronic – 2018 information out there? thanks
Here’s our run down on the 2 CGM options available
Having just experienced six out of eight Medtronic sensors not performing and/or telling me to replace them prematurely, I am so disillusioned with the Medtronic system. I do like the way the integrated Medtronic system suspends basal before a low bg, but when I check with a finger prick, I am already perilously low because their SG readings read higher than the actual levels. And, despite what other consumers report, I have not had any luck getting my malfunctioning sensors replaced. So far it has cost me just short of $300.
I can’t afford to change to Dexcom, although I would like to try their system, so I will return to Libre. Feeling mightily fed up!
I am a Dexcom G5 user and wish there were more options than the two suppliers (Dexcom or Medtronic/Enlite) to choose from. The customer service at Dexcom was great at first, but declined greatly after I had come to rely on the sensors. Last week, I had sensor issues in the middle of the night and so had to remove the sensor and relplace it – the replacement failed as well (probaly due to it being the middle of the night and I desperatley needed my sleep and so may have been too quick explore trouble-shooting options). Anyway, Dexcom had replaced sensors in the past (they are SO expensive), but this time they told me, in essence , “Sorry, we don’t replace sensors if it is not our fault”. I want so badly to find a new supplier! Any advice would be appreciated.
Thanks , I was leaning towards the g-5
I can’t take anymore of dexcom mistakes. It is taking them forever to send supplies that are authorized by my insurance. The are incompetent. The g5 is terrible and keeps disconnecting for no reason and is very inaccurate. Diabetics keep your money and learn how to eat properly. In 1947 I did not even have a glucometer to test my sugar. These companies are becoming multimillionaire on sick peoples money.don’t say you can’t manage it without a cgm. It has been done for years if u stop paying for it the price will come down
UnfortunatelyI ‘broke up’ with my Dexcom G5 because I thought the Medtronic 670G would be a simpler system and would ultimately help me stay in my target range more. Wrong. I want you back Dex!
I’m on the phone with Medtronic way too often. Their sensors have malfunctioned countless times, and my transmitter malfunctioned and left me in a l lurch.
I would love an updated comparison now that Dex Has partnered with another company and has an integrated pump. I’d like to know how the upcoming iLet stacks up too.
We are updating our pump comparison page now, it should be ready any time now.
Also we ahve a tech night oevent on facebook tonight 12/6 @ 730 eastern and will eb going over new diabetes technology!
I spent the last two years using a Dexcom CGM (G4) and really liked it. My final A1C with that system and using a Minimed 630 pump manually (they don’t link) was a 6.5. I recently took advantage of a free upgrade to a Minimed 670G with the integrated Guardian CGM system. I have to say that after a month of using the new Minimed – it’s a nightmare. Here’s why: First of all, if you sleep more than six hours a night, the Minimed system WILL wake you up asking for glucose checks. Secondly, the multiple daily calibrations needed for the Minimed sensors require at least three finger sticks each – more if you enter follow up BGs as prompted and re-calibrate instead of declining a calibration (they don’t explain that in the manual). Additionally, the Minimed sensor, which only last a week (I could get three from the Dexcom sensors) are frequently WAY off – in numerous cases by more than 100 mg/dl. That never happened when I was on the Dexcom system. There’s no way this 670G integrated system and sensor will get me an A1C below 7. Bottom line: Today I’m asking for a return authorization for my unopened boxes of Minimed Guardian sensors and will return to Dexcom. Between the finger-stick hell and gross inaccuracies that are the Minimed system, I can’t take it anymore. Too bad, because I like the integration with the 670G pump.
Thank you so much this is an excellent report, I really appreciate all of the hard work.
Great review. I have been a Dexcom user for years. The G4 was great and never had issues. They released the G5 in 2015 and would say it was a little premature for the market. Have constantly been working with the poor customer service department to replace either receiver or sensors because of quality issues. Wish I had the G4 device to this day. With regard to the G5 mobile app – it gets a C+ rating at best. Poor Bluetooth connection with the iPhone and constantly miscalibrated. I have moved to using only the G5 receiver device for monitoring the past few months. As well, its important to note all the sensor issues they’ve had during 2016. The company shipped me new ones because of technical problems (per the tech) and it took two months for the devices to arrive.
In my case, I switched out a transmitter this week because of low battery errors and now the receiver has went into a OS loop were it shuts off and on constantly. Resetting device does not fix the situation and technical support said I’ve passed the 12 month window. Thus, I would need to repurchase hardware for something that is clearly a technical issue. After 3 great years and 1 really bad year, I’m going to spend money elsewhere. Dexcom’s poor performance and customer service has gotten the best of me. Again, shame on me for allowing them to talk me into the G5.
Great article – Hoping either the Abbott or Medtronic devices are better. Will keep researching.
I agree Connie and John – I also use Medtronics CGM and it has helped me catch lows and suspend the pump before lows. The main reason i went to a pump and CGM was for controlling night time lows and while it’s not 100% perfect i find it very easy and simple to use.
Also have to say that the customer support is great and they will replace any faulty sensors not questions asked.
Love love love my Medtronic 640g with Enlite sensors.
I wish I read this before getting the 630g. Contrary to what Connie says accuracy is important. When the Medtronic CGM says you are 80 and you are really 50 that means the auto suspended won’t kick in and Medtronic CGM will tell the pump to kill you with an overdose of insulin. Any and every time one computer is giving another computer instructions accuracy is important. My Medtronic has done this to me several times at work and in my sleep. Don’t let anyone ever tell you accuracy is not important. Accuracy is a matter of life and death.
The. Medtronic sensor and transmitter combo lays flatter on the body and thus is easier to wear than the Dexcom combo.
Thanks very much for such a practical evaluation. I also tried both, as I am an offshore sailor/ long distance runner/ old guy, but I decided on Medtronics, not because of the product, but how they handled their customers.
The support, and out of warranty help, form Medtronic has been exemplary.
Gary, not difficult to figure out your bias. Now mine.
First, CGMs are not devices intended for taking actions/treatments from the glucose values/readins — only blood glucose values readings are take
as an accurrate value/reading. So please remove all comparison statements regarding accuracy! We’re just not there yet. The main purpose for CGMs is to track real time data – to be alerted of potential problems ( for me lows). I do this be reviewing the rate or slope of the data. This point you missed; CGM training from the manufacturer. I believe my Medtronic training was great and they continue to give more learning tips. Not sure about Dexcom. My last correction is on the “Data Sharing”. Medtronic CareLink shares real time data to iPhones (caregivers and loved ones) for example. I’m ordering one so I’ll let you know if I like it — I may share my data with my husband.
Just my take on this matter. Connie
As a Medtronic pump user for over 9 years and CGM user for 5 years, I’d take the customizable alert alarms and
The Dexcom G5 was approved in early 2017 to be accurate enough to replace finger sticks for dosing calculations after day 1 of use. As is the Libre by Abbott, and closed loop suystems depends on this accuracy to make constant dosing adjustments.
Thanks for this! I was stressing about whether I needed to switch to Medtronic bc I mistakenly thought that Dexcom was alone in its contraindication with Tylenol. I was starting to re-evaluate my pump choice too- especially in light of the MINIMED 670G SYSTEM’s FDA approval. Oh, well… While I am disappointed to learn there is no way to get around the Tylenol problem (which is HUGE if you cannot take NSAIDs), at least I can quit researching now. Thanks!
good news is on the horizon. Dexcom has submitted their G6 sensor for approval by the FDA with projection to be on the market in mid 2018. This sensor is NOT impacted by acetaminophen! As someone who can not use NSAIDs I am very much looking forward tot his, and many other upgrades the G6 sensor will provide.
Thank you for the comparison info! I have a 12 yr old who was recently diagnosed, and is currently using the Abbott Freestyle Libre glucose monitor. They are not yet FDA approved in the US,(we have to have a friend in England order them and ship them over), but in trials right now. You should check them out! They do not pair with a pump, but they are good for 14 days and require no fingerstick calibration. They are also approved for dosing in the UK.