1st day and not sure what to think

Just put my Dex on about 10:30 this a.m. Calibrated and my numbers weren't too far off. At about 4 p.m. today felt like I was low, so I checked. My blood sugars were 52, my Dex said 84. After I drank juice it started doing the low alert.(my low alert is set at 70). Well just now its saying 112 and my sugars are actually 133. So, maybe when your low or high the numbers get way off? Also I've heard you can wear them longer than 7 days. How do you do that? Thanks

The Dex is a bit delayed because it measures interstitial glucose levels and not blood glucose levels. If it went from 84 to below 70 by the time you had tested, it was actually following the trend quite well. The same seems to be valid for the second measure, it is climbing back up, just a bit delayed.

At the end of the 7 days period, when the sensor should be replaced, you simply start a new sensor on the received while leaving it in place. You recalibrate as if it was a new one and there you go.

112 to 133 is not really "way off". In fact it falls within the 20% rule of most meters and Dex. The first 24 hours of a sensor are in my experience the least accurate but as the sensor "marinates" and you calibrate it does get much closer. Dex will only rarely match your meter exactly and usually that is during times of very steady blood sugars long after you have eaten or bolused. Dex takes a very long time to recover from a low because it is not measuring your blood sugar it is measuring interstitial fluid. When you drink juice for a low the glucose in the juice goes immediately to your blood to be transported to your glucose starved brain eventually that glucose will reach your interstitial fluid. I think of it like a train going up or down a blood sugar hill. Your meter is the first car, Dex is the caboose. I use Dex for trends, not for absolutes. If a dex reading doesn't make sense, test your blood sugar.

Hi, for me, I've found the best way to be alerted when I'm going low (around 70 -80 for me) is to set the G4 low alert/alarm for 90, maybe even 95. That sort of compensates for the the difference between the interstitial fluid and the true BG. It's sort of art and science since it depends on how fast it's going down, how much IOB you have and how many Carbs are queued up waiting to enter your bloodstream. Once you have used it for a week or two, you should be able to see a trend and determine what the best "alert BG" on the G4 is right for you. Best of luck!

Hi this is because the Dexcom can work behind your blood sugar s especially if yours bg moves quickly and erratically as mine does. I tend to use the trend graph arrows to get an idea where I am heading as in high or low. That’s why you need to test and if your having a specially erratic reading day calibrate more often to try and get it more in sync hope that’s helpful. It is not the magic potion people believe but it’s a great tool. Using g4 for over two years continuously apart from mini breaks to rest the sites.

My daughter has the dex and she had it for 6 mons now. We had to go to children’s hospital for a class. It took a month for the numbers to be close to 10 points away from when you check your blood sugar and sometimes it it right on point! It’s a great thing to have and it just takes time.

First 24 hours usually can be off. Even though we calibrate twice a day I'll do it more on the first day.

The basic plan for wearing your sensor longer than 7 days is to just leave it in place. When you get near the end of the 7 days, pick a time when it is most convenient to be without your CGM readings for two hours and then press "Stop Sensor". Next, press "Start Sensor". When the two hours go by and you are prompted for the two BG calibrations enter them (using two different finger sticks 5 - 10 minutes apart) and proceed as though you inserted a brand new sensor.

Many CGM users are also using Flexi-fix tape to extend the amount of time that the sensor adheres to the body.

As others have said, because the Dex is reading your interstitial fluid and not your blood, there is a delay at both the high and low end. I find it especially so at the low end, but I have experienced the delay at the high end as well. In addition, during the first 24 hours you may experience a fairly significant "drift" between your finger-stick readings and the Dex.

I spoke with the Dex tech support two days ago because I was having wild, wild differences between my finger-sticks and the Dex readings for a new sensor. They reminded me about the 20/20 rule, which is within 20 points if your finger-stick is 80 or less, and within 20% above that. Refrain from re-calibrating if your Dex readings are within the 20/20 rule.

While my new sensor was way, way out of range it did come back after 24 hours.

Mike Ratrie,
Agree with everything you said except for the part where you say the two calibrations should be taken 5-10 minutes apart. Where did you learn that? It seems like it would defeat the purpose of a calibration if you wait that long between each one, plus the Dexcom user guide doesn't mention waiting.
Sorry to change the topic from the original post.

I thought that I may have read somewhere that the Dex may be "less" accurate at the lower range, has that been everyone's experience?

Per FDA documents, it is +/-20 points below 80 and +/-20% above that, generally speaking. Per artificial pancreas preparatory research, it is a bit better than that provided there is some effort put into the process.

The double calibration at start should be done almost simultaneously, the goal is to diminish the error introduced by the BG tester itself by averaging the two values for the starting point.

On an informal real life test done on 14 user submitted files I got this level of precision, on average, vs SMBG

Please clarify SMBG.

Thanks in advance.

standard acronym for "self monitored blood glucose" (using only the calibration points here).

In other words, this is not an evaluation in a clinical setting, but an evaluation of the average error, as a function of glycemic range, between SMBG calibrations performed by users and the Dexcom reported value at that time.

It is not a "pure" measure of the Dexcom accuracy but rather reflects the combined effect of the Dexcom Accuracy, Blood Glucometer Accuracy and user behavior.

Just like your Dexcom readings, the accuracy of BG reading from a meter may actually be + or - 20% of your actual BG. So at times when your BG is not shifting rapidly (i.e. when the directional arrow on your Dexcom receiver is horizontal), who's to say which reading is truly more accurate, the one from your meter, or the one from your Dexcom?

You said "Well just now its saying 112 and my sugars are actually 133." In reality, you don't really know that your sugars are 133; that's just the reading your meter is giving you. It is acceptable for BG meters to be 20% higher or lower than your actual blood sugar. Your meter could read 133 when your BG is actually only 110. In that case, your Dexcom reading would actually be more accurate than your meter reading.

The trick to getting accurate readings from your Dexcom is to calibrate at the right times and NOT calibrate when you shouldn't (e.g. when the directional arrow is headed straight up or straight down). I go one step further and only calibrate when my daughter's BG is not changing rapidly and the arrow is horizontal.

No, sorry about that. Most recent BG testers do better than that. It is a bit range dependent, but still...

The better precision of BG testers is the single reason why CGM users are advised to do a BG test before treating. They may not be perfect, but they are the best measure instrument available at home.

The impact of the blood tester precision was also examined (by checking the consistency of the double calibration values) - it definitely has an impact, some testers are obviously a bit better than others.

For example here is the measured performance of the AccuChek Performa Nano

Below 100 mg/dl
100% measures within 15%
98% measures within 10%
87% measures within 5%

Above 100 mg/dl
100% measures within 15%
96% measures within 10%
64% measures within 5%

More detailed range analysis are available, but they are a bit long to put here.

In a quick fall, the BG is what matters and what is "correct" because this is the sugar supply that is and will be available for your organs. IG trails be roughly 6 minutes, but is usually delayed by 15 minutes by CGM due to sampling /denoising issues. I also have results debiased by the rate of fall and projected in the future. You can't use post calibration data precisely because they have been changed by the calibration.

No, the reader that you describe wouldn't pass ISO validation. (Not to say that some readers can be bad, test strips can be bad, etc...)

From the data I have gathered, yes, there are correct moments and incorrect moments to calibrate. In some aspects, they differ markedly from some of the anecdotal advice given frequently on the net. This is btw what motivated me to look int the issue.

How do you know you are getting accurate results from your dexcom if your blood tester is inaccurate? What do you use to check the accuracy?


I agree with you.

I got that from Dex tech support when I called about my wild readings from the new sensor.

I usually do two consecutive tests; one from each hand, the tech support person said giving it more delay may help the sensor calibrate more accurately.

After reading all of the comments submitted re this post, I was reminded of a couple of items about which I emailed Terry Gregg, CEO of Dexcom (and never got a response). First, I used a Navigator for 7-8 years which provided readings every minute. The result was that it identified trends, and alerted me, much quicker than what I find with the Dexcom. My suggestion was to provide readings more often than 5 minutes (maybe every 2-3 minutes). What I find with the Dexcom is that it rarely shows upward or downward trending arrows, and most of the time, only alerts me once a warning level has been reached. I found the Navigator to alert me frequently on trends which is not the case with the Dexcom Platinum. The other item I suggested was to incorporate a "snooze" element so when corrective action has been taken, the vibration and/or beep can be silenced for a period of time.

There is a snooze element in the Dexcom. It can be found under Alerts, Advanced, High Snooze, and Low Snooze they can be set to as long as you want.
I understand the Navigator was an outstanding CGM by Abbott and I am at a loss to understand why it was pulled from the market in the US. But hopefully Abbott is again working with a pump company to integrate the CGM/Pump technology. I will not use the Medtronic system and the Vibe (Animas/Dex) is still awaiting FDA approval. So for now I'm sticking with Omnipod/Dex and am happy with both systems, I just wish they were integrated.