A Tale of Three Navigators

by Amy Tenderich, Dana Lewis, and Kelly Close

The FDA approved the Abbott FreeStyle Navigator® Continuous Glucose Monitoring System in March 2008. We were excited about the entry of a third option in continuous glucose monitoring (CGM) because we think more options drive further product innovation. We also know that many people love Abbott’s FreeStyle blood glucose meters and have been eager for its continuous monitor since it was submitted to the FDA for approval four years ago. Medtronic’s Paradigm REAL-Time and DexCom’s SEVEN are the other two CGM devices available. diaTribe Editor-in-Chief Kelly Close, star blogger Amy Tenderich, and diabetes advocate Dana Lewis independently put the Navigator through its paces and bring you their candid opinions in this three-for-one Test Drive.

continuous glucose monitoring: the technical background and the facts

The Abbott FreeStyle Navigator is a continuous glucose monitor that uses a sensor sitting just below your skin to measure glucose levels in the fluid that circulates around your cells – the interstitial fluid (ISF). Now, this gets kind of complicated but we’re going to give you the nitty-gritty. While changes in glucose levels in the interstitial fluid are similar to changes in blood glucose levels, the two are not absolutely equivalent. This is why we’re not supposed to use sensor numbers as a basis for insulin dosing. Note that this is not an accuracy issue - the diabetes healthcare world just does not know how to respond to ISF glucose because we have had little experience in it, so they rely on what we know and that is blood glucose values. In our view, someday we all might even find that responding to ISF glucose values is more accurate - there have been reports that glucose in the brain is closer to ISF values.

After insertion of the Navigator, the sensor reports glucose values every minute. It also depicts trends in glucose levels – one of the coolest things about CGM, with arrows pointing in one of five directions, indicating whether blood glucose is stable, moving up or down, or moving up or down very quickly. The sensor can be worn continuously for five days, and can be used anytime, including during bathing and swimming. Like other CGM devices, it is available by prescription.

Amy on the first glance: The Navigator was much-rumored and eagerly awaited, so I was quite excited to be among the first patients to be hooked up. I spent about an hour and a half going through the basics with the company’s clinical science manager, who happens to live in the next town and was kind enough to make a house call. It was an easy setup because: 1) I’ve used two other CGM systems before, and 2) I’m a long-time FreeStyle user, so I’m familiar with its test strips, lancets, etc. That probably makes me a little biased toward this CGM system. I loved the product design, but I found that I could barely see a darn thing without manually activating the screen’s backlight. You can’t change the settings to keep the backlight on, presumably because that would drain the battery too fast (estimated life of 60 days). It’s also important to remember that the new FreeStyle Lite strips don’t work with the Navigator.

Kelly on her first sensor: The first insertion was very easy even though I was still learning the system. All I did was pull out the “quick instructions” leaflet, which had the instructions to turn “on” the receiver – it was a cakewalk.

As with my DexCom SEVEN continuous monitor, I had to check to ensure the transmitter was “talking” to the receiver. I did this, got the confirmation, and prepared to put the sensor on. Although I typically wear my SEVEN sensors on my abdomen, I wanted to try the Navigator on my upper arm. As I was twisting off the yellow protective locking pin, I thought to myself, “I wish this were a little smaller and that the sound of twisting off the pin were a little less noisy.”

But this was all minor. The actual transmitter had two self-peel labels that I took off. I put it on my arm, pushed down the top, and it clicked and was ready to go! I couldn’t believe getting the transmitter into the sensor mount was this easy. I just put it on my arm and pushed two release tabs on the top and then click! It was in. A pleasantly pain-free experience. The Navigator insertion process has a more automated feel - less like an “injection” - than the SEVEN’s inserter. On the other hand, the larger Navigator transmitter – compared to the SEVEN’s - left me slightly apprehensive that it would fall off, so I covered it with one of the clear bandages that came with the Navigator. I am not sure whether this was necessary, but I didn’t want to risk losing the $60 sensor. As with my OmniPod and SEVEN disposables, I felt a twinge of guilt throwing away the large plastic insertion device after just a single use, but all in all, I didn’t think this was too significant given that the device lasts five days.

Shortly after patting myself on the back for a successful insertion, I then NEARLY proceeded to have a $60 misadventure, which was a little depressing. After the receiver beeped to confirm it had detected the new sensor, it asked me to confirm the code – I think this was sort of like a calibration. Unfortunately I pushed the RIGHT option button (for Yes) twice and inadvertently accepted the default sensor code of 105 instead of changing it to the sensor code of 106 written on the sensor package. From the furrows on my trainer’s forehead, I could tell I had pressed the wrong button. Unfortunately, she said, once the wrong code has been entered and confirmed, the sensor must be replaced or you risk recording erroneous glucose information. Oh well, I decided I’d take my chances - I didn’t want to be out $60! The accuracy was superb the entire five days, and I’m glad I was brave enough to leave it in, but “don’t try this at home” – make sure your code is right!

Amy on calibration: My very first morning with the Navigator was a workout day. I calibrated when the thing woke me up at 7 a.m. But I was a little late getting to my breakfast, so the second required calibration, at 9 a.m., was actually rejected: “Cal Failed.” Huh? It turns out the Navigator automatically rejects any attempted calibration when you are “trending,” i.e., when your blood sugar is moving rapidly up or down. I think this is a very clever and important feature, as I’m sure my many ill-timed calibrations contributed to my frustration with the DexCom SEVEN. Calibrations should always be done when your glucose levels are steady so as not to confuse the device.

On a related note, far fewer calibrations are required! You only need to conduct four calibrations in the five-day wear period — and they are super-easy because the Navigator has a FreeStyle fingerstick meter built right in. You just stick a test strip into the little port on the lower left corner and feed it some blood and the numbers are automatically read into the calibration process. This compares to a calibration required every 12 hours with the SEVEN and with the Paradigm REAL-time.

Unfortunately, the Navigator has a 10-hour warm up period. That means once you insert the transmitter, you have to wait 10 full hours until you can fire up the receiver and get monitoring. This kind of shocked me at first — until I remembered that the DexCom folks had actually suggested doing the same. It’s not officially in their product literature, but my doctor did suggest that the sensor needs time to “settle in,” so one thought is to insert it the night before and “sleep on it” before activating the session.

Kelly on the arrows: Now up and running, I was blown away by the amount of information I was able to access on-screen. The arrows? Very cool and one of my favorite features of the Navigator. The main screen defaults to a glucose number and a trend arrow. The arrows are either flat, straight up or down, or angled up or down to let you know how fast and in which direction your glucose levels are changing. The SEVEN does not have these arrows but does have a graph on the default screen from which I can get a good sense about which direction my sugars are heading. It would be great if the Navigator used some of the abundant screen space below the glucose number to display a glucose graph on the main screen. Better yet, these devices need touch screens, like the iPhone – maybe in the next generation.

Dana on the Navigator vs. the Medtronic Paradigm: I did feel like the Paradigm gave me a better idea of when I was skyrocketing and when I was plummeting with the double arrows; the Navigator’s single up or down arrows caused me to raise an eyebrow, but they often weren’t around for very long so I didn’t jump to respond. That said, on the Navigator, I found that my numbers seemed much more timely than the results I was getting from my Paradigm. It almost seems like there is a 1-3 minute delay with the Navigator instead of 15-20 minute delay typically reported! To boot, the status reports are great - I can see how much battery my transmitter has, how long the receiver will last, etc. This almost makes up for my complaints about the devices losing the (expensive!) sensor if the battery dies midway. I LOVE that the Navigator does not stop transmitting numbers or showing them on the receiver if you are slow to respond to a prompt for calibration, whereas the Paradigm stops your readings almost immediately.

Note that the Paradigm’s double up (or down) arrow indicates that your glucose has been rapidly rising (or falling) at a rate of 2 mg/dL (0.1 mmol/L) every minute. This is equivalent to the single up (or down) arrow on the Navigator. It would help to have the onscreen graphs since it might be more intuitive to see trends this way than with arrows. Kelly can speak more to this since the default screen on the SEVEN, as she previously mentioned, displays a graph. When I asked her about it, she said she loved that when she pressed a button on the SEVEN, she immediately saw a one hour trend and the number “of the moment.” Maybe both devices could learn something from each other – or they could make the options user-settable.

Kelly on navigating the Navigator: I have become used to cycling through one-, three-, and nine- hour graphs as the main on-screen functionality of my SEVEN, and I love the simplicity of the up, down, OK and C(ancel) buttons. But I was astounded to learn that with six button pushes on my Navigator, I could find out what percentage of the day I spent above, below, or within my target range – not to mention the number of lows and highs per day, AND projected lows and highs per day. That’s a lot of powerful information – made even more powerful by actually making changes to my management based on the statistics the Navigator provides. I doubt everyone wants or needs that much info but my inner ‘type A’ personality loved it. However, there’s a slight catch: each one of these excellent pieces of data comes with its own alarm. This is great if you have nothing to do with your day other than respond to alarms, but after a while, five alarms gets a bit tiresome. Before you get too worried - each one can be turned off (or switched to vibrate) independently, which is excellent thinking by the manufacturers. The more the device can be customized, the better, and Navigator scores sky-high on this front.

Amy on alarming alarms and some skin issues: I had some internal strife about setting my BG alarm. My inner perfectionist insisted that “targets” should always be very tight, like the 90-120 mg/dL (5-6.7 mmol/L) set on my pump. But the Abbott rep reminded me that CGM “targets” are really only for record-keeping, i.e., to illustrate when you are in the “zone,” that shaded stripe on the graphs in which your BG is happily within range. diaTribe advisory board members agreed you will go crazy if you set your alarms as tight as your targets. What we really need is a CGM system that allows for variable alarm settings – so you could set your low threshold for, say, 70 mg/dL (3.9 mmol/L) during the day, but a little higher at night. Likewise, you could set your high alarm threshold a little higher during the day, when you expect postprandial spikes, but a little lower overnight when you certainly don’t want to be peaking.

Finally, I must admit I had to abort my first Navigator mission a day early. That adhesive started bothering me on Day 2 and turned into a full-blown hive by Day 4. A dermatologist confirmed I was allergic to medical adhesives. According to this doctor, it’s not just the latex that irritates, but some sort of gel used in the type of adhesive that comes with the Navigator (the same type as those DexCom shower patches, which used to irritate me). When I reported this on my blog (www.diabetesmine.com), a number of other patients came forward with the same complaint. So a word to Abbott Diabetes: consider looking into alternate adhesive material, otherwise many of us may never have a chance to experience how well the Navigator really works.

final thoughts – Kelly, Amy, and Dana

Kelly: All in all, I was pretty darn impressed by the Navigator – I learned a lot while using it. I think the improvements to be made are mainly on the user interface, time for calibration, etc – things we understand they are busy working on! I would definitely recommend everyone research CGM to see what it can teach you about what your blood glucose is really doing and how much time you’re spending in your target range. It is critical to combine CGM with education – those committed to doing so can improve their diabetes management by leaps and bounds in my view.

Amy: Overall, I think it's a clever and sophisticated system. It stands up pretty well to the competition for a first-generation system – the REAL-Time Paradigm is in its third (or fourth) generation and the DexCom SEVEN is in its second generation, so they have had more time for improvement.

Dana: I love the Navigator, and time will tell if I love it enough to pass up the conveniently integrated Paradigm CGM. I have few complaints, from not seeing the graphs on the 'home' screen and having a separate device. Otherwise, it already has an A+ recommendation from me to any new CGM users! I also don't know if it will stand up to the test of Alabama humidity, but I am seriously considering giving it a try.