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by Gary Scheiner MS, CDE

As a kid, I loved reading MAD magazine.  It was sarcastic, critical, and held no prisoners.  The “Spy vs. Spy” segment was a personal favorite because of its poignant portrayal of the idiocy of war.
In the diabetes industry, the “war” has traditionally been “meter vs. meter” or “pump vs. pump.”  Today, a major side-battle is brewing between the CGM companies, namely Dexcom and Medtronic.  As insurance coverage and patient/provider acceptance of continuous glucose monitoring grows at an exponential rate, more companies are sure to enter the market.  Abbott’s Navigator system still has a small foothold in some parts of the world, but for now, Dexcom and Medtronic are the sole super CGM powers.  So how do they compare?

For the past 8 weeks, I put each company’s latest system, the Dexcom G4 and Medtronic 530G with Enlite, to a head-to-head test.  Other than wearing the sensors longer than the “approved” number of days, I pretty much followed all of the rules and recommendations that each company provides to users.  Simultaneous calibrations were performed three or 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, and the sites were rotated consistently.   Extra adhesive was applied over the sensors at the first sign of loosening.  Transmitters and receivers were charged as needed.

Here’s what I found:

Sensor Insertion & Comfort:
Both systems now use very thin introducer needles and automated insertion devices.  Enlite inserts at a 90-degree angle and is much easier to insert and more comfortable than Medtronic’s previous Sof-Sensors.
Edge:  EVEN

Sensor Longevity:
With a little ingenuity (and a lot of tape), both sensors can work well past their initial life-cycle.  Based on my personal experience and that of my clients, I have found that the Enlite sensors last an average of 8-10 days; G4 lasts an average of 12-14.

Alert Settings:
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 and allows the user to vary settings by time of day.  Medtronic’s latest system also has a “low suspend” feature that automatically stops the pump’s basal insulin delivery when the glucose falls below a specified threshold – an annoyance during most garden-variety lows, but a potential life saver for those susceptible to severe hypoglycemia without symptoms.

Alert Volume:
Both systems can beep and/or vibrate to alert the user of a potential problem and both have adjustable settings, but Dexcom’s vibrate mechanism is considerably stronger, and it can beep much louder (when desired).

Medtronic’s radio transmitter contains a memory chip that stores up to 40 minutes of data and submits it to the receiver in case the receiver was out of range of the transmission.  Which is needed:  the signal only travels a few feet (and is frequently lost even when in close proximity), and it must be removed from the sensor and charged at least every six days.  Dexcom’s transmitter never requires charging and sends a signal that reaches across a sports field or multiple rooms of a house.

For those who use a Medtronic insulin pump, the convenience of having the CGM display right on the pump itself is tremendous.  However, some people (parents, caregivers) prefer to have a receiver/display that can be carried separately or placed away from the sensor.  Dexcom’s full-color, high-contrast screen is simply superior to the Medtronic LCD screen.
Edge:  EVEN

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, it does not work on Mac systems or integrate with data from any pumps or meters (unless you happen to live in certain European countries).  Medtronic’s Carelink software is web-based, works on virtually all operating systems, is easy to use and share, integrates sensor data with pump & meter data, and generates reports that do a super job of revealing post-meal-bolus patterns.

There’s something to be said for a system doesn’t take a lot of work in order to function well.  Other than having a sensor insertion device that takes some dexterity, Dexcom is the epitome of simplicity.  The transmitter requires no charging (Medtronic’s transmitter must be charged every six days).  Calibrations may be performed at virtually any time (Medtronic’s must be done according to a schedule).  Dexcom hones its data towards calibration entries (Medtronic generates error messages when data is off).  Dexcom’s setup and sensor startup startup process is streamlined.  Out-of-range signals are much rarer with Dexcom and don’t require extra steps to re-initialize.

Here’s the biggie.  Most people are willing to put up with some minor inconveniences if the system generates reliable data on a consistent basis.  Medtronic’s original sensor, the “Sof-Sensor,” was infamous for its accuracy and dependability problems.  I saw this with my own system as well as with many of my clients who used it.  So I went into my personal trial with the 530G with Enlite without any big expectations.  And I was pleasantly surprised.  Enlite performed admirably.  There were far fewer lost signals and data gaps.  It wasn’t as “cranky” at the start-up.  And the data closely matched the fingerstick calibrations more often than not.  But it still wasn’t quite up to par with Dexcom’s accuracy.  

Accuracy is best measured by looking at MARD – Mean Absolute Relative Difference.  This represents the difference between fingerstick calibrations and sensor glucose values, so a lower MARD is better.  A calibration reading of 150 mg/dl and a sensor value of 120 mg/dl represents a MARD of 20%.  The overall MARD for each system was as follows:

Medtronic Enlite: 18.66%  MARD
Dexcom G4:  12.60%  MARD

Keep in mind that this data (particularly the Medtronic MARD) differs from data published by the companies from their own pre-market trials.  I’d like to think that I put the sensors through the rigors of a serious real-world test:  changes in sleep patterns, travel, lots of unusual foods, extreme exercise conditions, stressful situations, and so on.  In other words, the rigors of my normal life.

In terms of which system was more accurate more often, Dexcom had a definite advantage:  Dexcom G4 was more accurate than Medtronic Enlite 67% of the time; Medtronic was more accurate 27% of the time, and the two showed equal accuracy 6% of the time.

Neither system was particularly accurate during the first 24 hours after sensor insertion, with MARDs in the 20-25% range.  But things improved considerably after “day 1” and continued to improve over the life of the sensors.  Both systems also tended to underestimate glucose levels more often than overestimating (Dexcom moreso than Medtronic)… which is good for those trying their best to avoid hypoglycemia, but not ideal for those who are easily annoyed by false low alerts.

Percent of Readings
BELOW Calibration
Percent of Readings
ABOVE Calibration
Exact Matches
with Calibration







So there you have it.  Do with it what you will.  Both companies (as well as Abbott, from what I hear) are aggressively developing their next generation systems, so this may all be obsolete soon anyway.  But for now, I hope it gives you what you need to make an educated decision.  Whatever you choose, you’ll be far better off than not using CGM at all!

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