This AM was a real mess up. I have been trusting the dex most days. Today I am experiencing major differences in numbers. Dex said 159 and 2 arrows up...I was 116 on meter.
I recalibrated.
Dex now says 240 and 2 arrows up and 199 on meter.
Will it go back to normal?
Please advise.
Thanks
How old is the sensor? The thing you have to remember is, while it may be spot on a majority of the time, there will be times when the meter and it will be off, especially if you’re in the middfle of fast fluctuation period. I’ve had this happen, and although frustrating, it usually does calm down and get back in line with the meter readings after a while, or, it could be start of a sensor in it’s death throes. If the sensor is less than 7 days old, and this keeps up, call Dexcom, they’ll usually send out a new one.
And the other thing to remember too is, it’s doing it’s primary job, which is to give you trend information. It said you were trending up, and you were, that’s what I try to remind myself of when dealing with these annoying time periods. .
Thanks Jackie…I needed reminder that its the trending most important. Meter is in day 6 so I will be patient with it.
It might be because you calibrated during the double arrows up. It has a harder time calibrating properly during big changes. At times like this, I just rely on my meter for the actual number and use the arrows as a guide. I will sometimes add about 5% to my correction for a single arrow up and about 10% for a double arrow (these numbers work well for me but you’ll need to play around to see what works for you), but base the initial correction on my meter. Then calibrate once your number has stabilized.
It’s frustating though, I’ll definitely agree to that, it’s such a sweet thing to just look down, and have the reassurance that you’re OK while driving, or before say starting a tennis game, it’s almost a betrayal of trust when they are more than 50 points off. BUT, I guess that’s why we still need finger sticks, BUT, they can be 20% off too, so do we ever TRULY know what are actual BG reading is at any time, it makes my head hurt if I start thinking about these things too much : )
You guys are helpful, reasurring and ease my frustrations. Thanks!!!
I will take all advice and be happy . It certainly beats the days of testing 20 plus times.
And all advice is free…we are so blessed.
please don’t ever trust just the DexCom machine, its’ only a tool, use your meter to bolus and give insulin. The dexcom will never be exact, it’s just a tool to alert you if you are going low or going high or having trouble but you still need to do fingersticks for treatment!
I agree 100% with Laura…just use the dex as a tool. I know this is heresy but when we really need to know how low or high we are, taking a finger prick is still the best way. I am just hoping the dex company will find the way to assure more accuracy, so they can hook up with Omnipod and they can build the first external artificial pancreas! We’ll just have to wait and see!
But isn’t that what the blood sugar machine is…just a tool? Most people, including myself, (incorrectly) see the Dexcom as a tool to constantly monitor blood glucose, and, in a way, replace the meter. Well, it DOES “constantly” measure sugar levels, however it is behind the actual blood sugar and this is most obvious during these fluctuations. I’ve been using the Dex for about a month and couldn’t be happier, however my idea of how it was going to work needed tweaking after I used it for a few days/weeks. The trend arrows are AWESOME! If I have a 100 blood sugar and it is trending down, does it really matter if I am 95 or 50? I’m either low now and will be soon.
In my view, the trend arrows are Dexcom’s substitute for not being able to track the sugar during periods of high fluctuations. If you have a fast changing sugar, the Dex can’t keep up, however it will tell you with the trend arrows what to EXPECT.
While I do share the frustration, the Dexcom CAN BE TRUSTED. Just beware when you have straight arrows in either direction. In those cases, double-check with the meter.
Roxy, I agree, after using dex on and off for the past 9 months, I’ve learned how much trust to give the device, and I still occasionally am surprised. But then again, every so often, my BG meter flakes out too, and will report an in range number when I KNOW I’m low, and it will take 10 minutes or so for it to show up on BG meter. These are all tools, and all machinery that can give inaccurate results. But, for the most part, they can be trusted. I’m too much of a realist to think anything is accurate 100% of the time. For me, Dex is spot on I’d say at least 90% of the time, but those are my results only, YMMV.
“20% off” isn’t real story, of course. (It’s a quick-and-dirty number to get people off the phone when they call the 800 number for “meter support”.) Here’s the whole picture:
From a single (large) blood sample, used to feed hundreds of “identical” strips into the same brand of meter, the results will vary in (roughly) a bell shaped curve which they call a “Normal Distribution”. Once in a while, a test will be off by MUCH more than 20%. Most will be closer to the middle. You describe the shape of the bell with a calculated number called “Standard Deviation”: about 68% of the readings will be less than 1x of the S.D. from the middle of the bell; about 95% will be less than 2x of the S.D. from the middle of the bell. And so on.
When they speak of a percentage, called the “Coefficient of Variance” or CV, it’s simply the S.D. divided by the mean value. (Both figures were in units of glucose per unit volume- mg/dL or mmoL/L. Divided by each other, it’s just a number. Then multiply by 100, and it’s a percentage.)
Here’s a very important thing: The CV of bG test strips, like the accuracy of Dexcom, is much lower at low levels of bG. At each particular value of bG, the bell-shaped curve changes. It’s “skinny” (showing more consistent measurements) at higher bG values, and it gets “fatter” at low bG values (i.e., less consistent readings).
For example, here’s the data which one study got by testing a large number of One-Touch Ultra strips, tested with identical control samples at 3 different levels of mg/dL:
44 mg/dL, CV of 4.4%
171 mg/dL, CV of 2.6%
366 mg/dL, CV of 2.4%
When you’re Hypo, strips and Dexcom become much less reliable. (BTW, One-Touch Ultra is one of the more reliable strips for accuracy at low bG readings. A few brands are much, much worse.) And so are the CGMS Sensors. For me, Dexcom off by more than 10% at 90 mg/dL is unusual – but Dexcom off by more than 20% at 60 mg/dL is commonplace.
OK, that pretty much covers meters versus themselves. But there’s one more “meter accuracy” factor: Reliability versus Accuracy. Even if your meter could be totally “reliable” against itself, it would still probably have “accuracy” issues in comparison with values we believe to be truly accurate (such as those generated by a properly calibrated YSI analyzer). The details are messy, but let me summarize by saying that you’d better add another 2-4% of “slop” to your figures for that issue.
Finally, we get to the most important part. (And they’re all excluding this stuff from their sloppy “20%” figures, too): Improper usage. Strip containers left open for too long; or kept at improper temperatures (for shorter periods of time); or most common of all, sugar-polluted hands. Wash 'em well!
If you do that, you can relax (a bit) about your fingerstick readings at non-Hypo bG levels. With my meter, each reading should be within 10% of true reality about 95% of the time. (That’s 7% for two standard deviations of Variance, plus 3% for Variance versus Accuracy.) Take two readings, and the Variance of their average (versus the “true” average value for the curve) goes down dramatically- they’re extremely unlikely to both be “whacked”, by large amounts, and in the same direction.
BTW, that’s why Dexcom asks for two readings at startup.
Now you and Lisa know everything! It didn’t hurt your head that much, did it? 
Go to the head of the class.
Rick, that was a great explaination. No, my head doesn’t hurt, but I’m not too sure how long I’ll retain it either : )
I mostly pay attention to average BG and SD. When I first looked at all the dexcom reports, I tried to understand all the numbers it was giving me, then I decided, after days of googling statistical terms I’ll probably never use again in my life, that average and stand dev were what I really needed to pay attention too.
Thank the LORD for people like you that not only obviously love getting into the serious “what a number means”, but can then translate the info into laymens terms for those of us that start drifting after just glancing at those inserts in the strip boxes-and are prone to the thinking too much headache : )
My expectations needed tweeking too. Now I am more accustomed to the thing, I take the numbers on the CGM and from the a finger stick with a grain of salt. If my CMG says 60, my fingerstick says 85 and I feel like crap, I take another stick to make sure the stick is close to being right. It usually happens the other way, CGM says I am 90 and I am really 60.
i am with you, Brad!
Thank you for the kudos, I love it!
When I fire up my wife’s “Windoze” computer to download Dexcom, I usually focus on only one thing: The 3-colored pie charts. (% of time between the lines, % of time below, and % of time above.) I think their new slogan, “live between the lines”, is really great: Focus on that, set your lines pretty tight, and I think that you don’t need anything else. (MANY people would disagree with this opinion, of course.)
If my pie chart for recent days is “better”, or at least equal, to the pie chart for the last several weeks, then I’m happy with my control. For me, when the pie chart stays good, everything else seems to automatically get better (A1c; Standard Deviation; physical comfort and strength; everything).
I’ve used 80 and 140 as my alarm limits for quite a while. YMMV, of course.
Rick,
You are clearly the brains of this operation. I love the pie chart thing.
But what is YMMV?
Your Mileage May Vary. Also known as YDMV, for Your Diabetes May Vary, which, as we all know here, is VERY TRUE! ; )
Brad, there are possible explanations for both of those situations.
Dexcom is always delayed, in comparison to the ISF which is being measured. It takes time for “enough” fresh ISF, with a different glucose level, to enter the Sensor wire’s perforated Titanium covering and react with the chemicals - creating a voltage change.
But big differences are usually a result of the “noise cancellation” math which Minimed, Abbott, and Dexcom all use in creating display values. In the case of MM, there’s an actual study which you can read, with the raw data presented, about the effects of their then-current formulas for converting from ISIG to dispay data. The effects were huge. MM is widely felt to be much slower than Dexcom at detecting and showing Hypos, but in the study, they found that the raw data was fairly “quick” to respond: it’s the “error-avoidance, don’t display garbage” microcode which causes it to be slower.
Data manipulation is an absolute necessity for these devices, but it seems that Dexcom has implemented better programming (in this area) than Minimed was using at that time. I don’t know if they’ve made any changes, BUT: such a change would have had a PMA in the FDA’s records, and I never saw one on this subject. (In contrast, Dexcom’s 7-plus logic and waterproofing changes did show up as a PMA.)
The other two factors are your particular site, and food/alcohol causing rapid changes in bG. These work together- when you happen to have a site with relatively “less” neighboring blood flow, there’s a bit more buffering of bG changes in the ISF at the Sensor. Most of the time though, this is extremely small. You might be surprised to hear this, because many people (including me) have frequently guessed otherwise: ISF glucose and bG usually correlate very closely, with almost no delay between one and the other. CGMS delays in display of “bG” are actually dominated by unwanted byproducts of the data-smoothing, error-rejection software.
So, what happened in your two cases? In the second, more typical situation, you were probably falling fast, and the software refused to show it for a while. The first one is harder to guess at, but you’ve given an important hint: You felt like crap.
Perhaps your liver was dumping glucose as an emergency response, you had started out much lower than 60 mg/dL. But your liver’s response was so recent that it hadn’t flowed into ISF near the Sensor, and then into the Sensor, to show the fresh glucose being dumped into your blood supply. (CGMS accuracy goes to heck at low glucose levels, you might have actually been around 40.)
Here’s what I do in such situations: If I’m suddenly feeling really bad, and ANY of my machines shows low or falling glucose (either Dex or Fingerstick), I treat it aggressively (as a Hypo bG- regardless of what the other device says). If there’s nothing specific which you can point at and say, “Well, I just hit my thumb with a hammer- of course I feel awful”, and at least one machine says that you’re getting low, then you’re probably Hypo – and your brain neurons are screaming for help.
I say “They both suck- But 2 hours at 200 is WAY better, WAY safer than 45 minutes at 45.” But your QUALIFIED medical team might disagree. Do as they say.
Even if the med team says you are wrong…I agree 100% on your last point, Rick. How many of our med teams absolutely KNOW how we feel? Quite a few, I am sure…but still not enough! and, I feel you are more than qualified!