RE: [IP] DexCom system accuracy and Paradigm RT alerts
Thanks for providing us all with this detailed information. In response to
your message I have just two questions:
1. You mentioned if someone sweats a lot during activity the readings are
wrong. Will the readings be correct after the sweat has dried or must the
2. You mentioned for swimming to use the shower patch and ensure it is
properly sealed. If you went swimming with no shower patch would this
destroy the sensor, or could you just reconnect the transmitter afterwards
and everything would work fine?
>From: Rick Stockton <email @ redacted>
>Reply-To: email @ redacted
>To: email @ redacted
>Subject: [IP] DexCom system accuracy and Paradigm RT alerts
>Date: Fri, 23 Jun 2006 23:43:43 -0700
>Hi again Sarah, I'm responding as a Dexcom User. Good Questions!
>(1) How close are the readings to my OT-U in hi, low, and normal ranges
>of bG? How do the readings compare when bG is moving fast?
>My experience: 'Accuracy', treating the OT as 'correct', is so good that
>I've basically stopped using my Ultra-- the instructions are "confirm
>your Dexcom reading with a finger-stick test before treating your bG"
>but I've found the treatment choice indicated by the Dexcom to be 100%
>reliable over the last 6 weeks.
>But remember: I'm now treating the trend, not just the bG. If it's going
>down sharply from 130, I'm 100% confident in the "going down" part, and
>need to eat something. The "130" reading is usually 15-20 minutes behind
>my bG, and even after "extending the curve" in my mind, only accurate to
>about +/-15% (1 standard deviation, 68% of readings within this range).
>Although I can "extend the curve" in the graph to make a guess at my
>"real" bG, I don't bother anymore-- I just "eyeball" the graph and know
>what (if anything) to do. Accuracy below bG 80 is a lot worse, maybe +/-
>30 points, but that hardly matters... you already have your diagnosis,
>eat sugar tabs NOW.
>There's a special condition where the Dexcom can't be trusted: exercise
>which **MAKES YOU SWEAT A LOT, RIGHT UNDER THE SENSOR** can send it
>totally to heck. (It may read 230 while your real bG is 120.) Never
>trust it at these times, and absolutely never calibrate it under these
>conditions... you'll mess up the readings for the next 8 hours if you
>calibrate against a sweat-polluted Sensor before it dries out. (I live
>in the Nevada Desert, right now the humidity's about 5%, sweat doesn't
>last. But for most of you, this can be a problem... heavy exercise is
>exactly the time when you most want your CGMS working.)
>When I'm sleeping, relatively motionless, my ISF readings might have a
>poorer correlation with bG. Since I've never bothered to wake up merely
>to 'test against the Ultra', I don't know for sure. But I have found
>that you MUST NOT sleep with the Sensor squashed against the mattress.
>Accuracy is highest shortly after calibration, then declines until the
>next calibration re-corrects the growing divergence. Brand new Sensors
>are less accurate in the first 8-16 hours than they are in days 2-3 (or,
>as I use them, in days 2-16).
>The official Clinical study documentation provided on pages 88-102 of
>the User's Guide (wow, LOTS of graphs and tables) differ from mine in
>one key respect: The compare Dexcom versus YSI readings from blood draws
>at the exact same time, while I "extend the curve" and assume a match
>with the Dexcom reading from 15-20 minutes later (3-4 data points after
>the blood draw). Without my adjustment for ISF versus blood delay, they
>got +/-21%, which is still pretty good. But of course, the percentage
>error dramatically error increases at low bG, and they had a lot more
>data points at YSI readings above 121 than below 120. Most "regular"
>meters also de-emphasize the number of data points taken in the critical
>sub-70 bG range, IMHO to make them look better than they are. Here,
>Dexcom's study included 140 YSI-paired readings in the 40-80 range, out
>of 640 total, a FAR LARGER proportion than studies I have seen for other
>devices-- particularly including the 2005 study of Glucowatch versus
>'old' Minimed CGMS versus 'new' Minimed CGMS. (Unlike others, Dexcom has
>the guts to conduct their study in a way which shows how they do in the
>problem area, the Hypo bGs readings.)
>As I did, they saw a decline in Accuracy as time goes by after
>calibration. During hours 9-12, compared to hours 0-3, 5% more of your
>readings will fail to match One-Touch by a specified percentage 20%,
>30%, or 40%. (For example, the percentage of readings which lie within
>within 30% of a One-Touch reading taken at the same instant declines
>from 79% to 74%.)
>In indoor non-exercise conditions, you'll probably match the study
>results (+/- 21%) if you don't "extend the graph" by 15 minutes before
>making your comparison. If you "extend the graph", as I do, and use good
>site locations, then you can realistically expect +/- 15% for actual bG
>above 100, and +/- 25 points for actual bG 50-80.
>(2) What kind of alerts does it have and how loud are they (will they
>wake the dead for nighttime, and is there a vibrate feature for the day)?
>You can't modify the alarm behavior, although you can set the high and
>low bG readings at which the alarms will occur. First, it vibrates. If
>you don't respond by clicking any button (the 1-hour screen will
>display, no matter which button you press) then it will do a LOUD beep
>after 5 minutes. If you ignore it again, then it will BEEP AND VIBRATE
>after 5 more minutes. Then, there is another BEEP+BUZZ alarm after 15
>Yes, it will almost wake the dead. My poor wife faces away from me, on
>the other side of an Eastern King (the widest of all standard bed
>sizes), and she always wakens, gives me a nasty look. And I've got in in
>a shorts pocket, under the blankets! Since it isn't wired to anything,
>you could clip it to the collar of your pajamas--- for me, that would be
>EXCRUCIATING. (BTW, I'm a musician, my ears are kinda shot. And *I* find
>it plenty loud.)
>(3) How big is the sensor/transmitter portion (length, width and depth,
>measured by yourself with a ruler--not what the company
>says)? How long does the transmitter battery last (how long before you
>need to replace it with continuous use)?
>No ruler, this is eyeballing: About 4cm long at the longest part, 3cm
>wide, and 8mm tall at the tallest part. But the edges are beveled,
>clothes don't catch on it at all. Overall, it's about 50% larger in each
>direction than my Ultraflex infusion set (clear plastic portion,
>attached and running). The adhesive pad is very generous, this Sensor
>doesn't stand a chance of moving around after you put it on and properly
>press the adhesive pad down. (Assuming that you DON'T get into a heavy
>sweat or wetness situation.)
>They say the transmitter battery is good for about 6 months. The
>receiver is estimated at a year. But I think I can squeeze at least a
>month or two more out of the receiver, by recharging it before it goes
>below 80% charge (right away when the battery indicator shows anything
>less than full), and by also being sure to avoid overcharging. The
>Transmitter isn't rechargeable, when it dies, I'm stuck. If my Sensors
>average 15 days, or more, then my total costs will be dominated by
>replacing Receivers and Transmitters, not Sensors. Grrrrrr, I wish they
>had replaceable batteries, like the Minimed and unreleased Abbott devices
>(4) Do you need to cover the sensor/transmitter with a shower patch to
>(avoid) getting it wet? Can you swim with it? How irritating is the
>They sell really good "shower patches" which, if you have the sensor on
>your torso (not a muscle which gets greatly stretched during swimming)
>is definitely waterproof for swimming. I haven't done this. The receiver
>can't handle water either, so you can't have CGMS while you're in the
>water. Adhesive and the PAD itself seem identical to the new (last 2
>years) Ultraflex... absolutely no irritation for me, and the plasticized
>backing (rather than plain paper) doesn't tear apart and leave bits
>behind as you pull it off. Remember, I'm in dry Nevada, and don't sweat
>under the Sensor Pad. But also remember, I wore my last one for
>(5) Can you use other meters to calibrate it, or is it only the Ultra
>(and can you use the Ultra 2 or the UltraSmart)?
>Definitely can't use the UltraSmart. (It doesn't have the same
>down-the-wire communications protocol as the Ultra, which J&J lets
>companies have at no cost. The UltraSmart protocol is licensed with much
>tighter restrictions, for big money.) I don't know which one the Ultra-2
>might use, but I've got a bad feeling that I might need to buy some
>spare meters NOW. I have just asked Lifescan via email, but the tech
>support guys who could answer this don't work on weekends. I'll follow
>up when I get an answer, probably on Monday.
>(6) Is the sensor set metal (can you remove the sensor and reinsert it
>in another part of the body if you do need to remove it to swim or
>Yes the Sensor is metal on the outside (Titanium) but *NO*, you can't
>re-insert it after taking it out. It's very thin, I'd guess about 29
>gage, and it's SOFT, FLEXIBLE. Completely crumples if you try to do
>anything with it after taking it out. This might be, on the other hand,
>a property which helps keep the body from reacting to it. Swim using the
>"shower patch", put the patch on carefully, make sure there are no gaps
>along the on the patch adhesive portion, and leave the Sensor in.
>(7) For both, where in the body can you insert and get good BG readings
>(arms, legs, stomach, butt, hips, etc)?
>They say: at least 3 inches from current infusion sites, 1 inch from old
>scars. But *I* say, at least 3 inches from old scars too, preferably an
>area you have NEVER USED as an infusion site. I had several failures in
>trying to use sites on my belly, near long-ago infusion sites.
>I also wouldn't recommend arms and legs for active people. If you infuse
>on your belly, the putting the Sensor on your upper butt cheeks would be
>great, but you'll need dexterity to clip in the Transmitter. Or the
>places I use... straight down my sides from belt line up towards
>armpits, a couple inches apart. That's only about 4 zones, but if I get
>more than 2 weeks from each Sensor, it will be 2 months before I go near
>an old location. And, since the Dexcom Sensors don't cause the kind of
>reactions that infusion sets do, you can probably go within 1/4 inch of
>a previous Dexcom site with no problems at all.
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