Inaccurate blood sugar tests and Dexcom calibration


This morning I tested my blood sugar twice in the space of a minute using 2 different fingers to calibrate a new sensor and the readings were 5.8 (104) and 8.2 (147), which I put into the receiver. I then tested again a minute later and got 5.8 (104) and 6.7 (120). This afternoon my dexcom reading was 4.2 (75) and when I tested my meter came up with 8.2 (147) and 7.4 (133) in the space of a minute!

To be honest, I’ve never used the calibration fluid stuff that came with the meter (OneTouch UltraEasy) but I wonder if the tests would be more accurate if I were to get a new meter?

Also, when you have to enter 2 start up BGs how do you test? Do you use the same finger? Do you do one straight after the other? I know you only need to enter a BG test every 12 hours but I wonder if doing it more often improves accuracy?

I’d be interested to hear if anyone else has had this problem!

Calibrating the Dexcom is a fun game isn’t it? I think we all do it differently but here’s my take on it, which I have found to be very accurate, and I’ve been using this for a little over a year now. When I start it up, I take one BG reading, and if that number seems to be accurate, than I put that in twice. I believe the only reason why Dexcom asks for two is to make sure that your BG meter is being reasonably accurate. Putting in two very different numbers like 5.8 and 8.2 will only throw it off, and might even ask for another calibration right then. It sounds like your meter had an issue if 2 readings were similar and one was much higher.

As for the 12 hours, I just ignore those. I’ve tried all sorts of things, and overall the most accurate numbers come from when I put in very few calibrations, usually about 5 a week. I only calibrate when I know my sugars are fairly level too, since putting one in when you are rising or falling just throws it off because the Dexcom will be delayed.

I know it might sound counterintuitive to put in calibrations less frequently, but it works for me much better than the first few weeks of use when I was following Dexcom’s directions. I just tested BG and checked the Dexcom which I haven’t calibrated since about 24 hours ago, and it’s just a few points off, as usual. I keep hearing that people still test ~5-10 times a day to make sure, but I find that rather pointless. Even if the Dexcom is “having a bad day” it still always gets the trends right for me. This past year has been a great new freedom to not have to test my fingers all the time, and my a1cs just keep going down - last was 5.3%.

When I enter calibrations I use the same finger prick for both (you just need to make sure you get enough blood to do two, which isn’t usually a problem for me!). If I get two start up tests that are more than about 20 points apart I usually do a third and then put the two closest in. That helps avoid mis-calibration. Also, if I put in a blood glucose and then realize that it was a bad test I’ve found it can usually be fixed by 2-4 correct calibrations at 10-15 minutes apart. You don’t want to feed it to many numbers but if it’s off you do definitely want to correct it (it doesn’t do you any good to have wrong numbers!) If it’s reading correctly there’s no real reason to do it more often though. And don’t worry to much about the calibration fluid, I think most of us miss the normal calibration recommendations! If you’re worried you can call the meter company though and order a new bottle of the stuff to see if it’s still working right.

My experience is a little different than Andy's - your mileage may vary.

I always do the double fingerstick as Dexcom suggests, for the plain reason that I don't want to calibrate my Dexcom using an inaccurate fingerstick reading. I take readings from different hands. Then, if the two readings are more than 10mg different, I take a third, and enter the two closest readings, discarding the third as an error.

In your case, I definitely would have taken a third reading, because one of your two was way off - possibly due to a bad strip, a bit of rubbing alcohol or food on one of your fingers (false high), maybe one of your hands was wet (which would provide a false low). Since your startup readings were so far apart, I might suggest restarting this sensor.

During the first 24-36 hours of a new sensor, I enter a calibration before eating, 2 hours after, and 2 hours after that. What I find is that, during the first 24-36 hours, the Dexcom may exaggerate trends. For instance, it might report that my bg has gone from 80 to 200, when in reality it's gone from 80-150. So, at that peak, I enter a correction/calibration. Then, when the postprandial high is wearing off, the Dexcom may report that my bg has fallen from 150 to 30, when in reality it's only fallen from 150 to 90, so - again - at the bottom of the trend, I enter a correction calibration.

In general, I try to only enter calibrations when the Dexcom is reporting that my bg is level; entering a correction during a rise or fall practically guarantees inaccuracies. And, keep in mind, that Dexcom recommends against entering more than one calibration in a 15-minute span after the initial startup.

I would not have put those two numbers in since they are so far apart – remember the saying “garbage in, garbage out.” I also do what Andrew does, if I get 2 readings on my meter that are too far apart, I will do a third one.

Have you ever taken your meter with you when you get blood work done to see how close it is to the lab? That is really the best way to know how accurate your meter is.

Andrew, that high and low calibration you mention is exactly why I put in fewer calibrations. It’s a fairly well known trait of Dexcoms to report at least slightly exaggerated highs, so always trying to calibrate those will only lead to having to correct again when it comes back down. I found that I could either calibrate more often to make sure the Dexcom is always spot on, or just anticipate the exaggeration and skip both calibrations. I do check this occasionally to make sure it’s still the case though, and I’ve only been off when the sensor has long expired and starts to lose the trends.

I also tend to put in to few calibrations unless the Dex is seeming way off. I probably do about 7-10 glucose tests a week and if they agree with the Dex (which they usually do) I often don’t bother to put them in.

You might want to pick up a new meter. That’s a 40% variation in readings in one minute. Your endo might have samples if you don’t want to pay, or your insurance may have a deal to get you a free one.

We use the same readings 2x, but I’m pretty confident in our meter. I think entering two numbers that vary that much is going to cause bad sensor readings. Either the 104 or 147 is wrong - so why give them to Dex?

Our dex is pretty darn accurate. My daughter often forgets to calibrate, and it is very forgiving. I think the one thing that helps with our accuracy is that I pull sensors once I get a weird reading. Usually around day 9 or so we will see a reading 30-40 pts from meter in a somewhat stable time. The sensor is still working and I certainly could leave it in, but I’d rather pull it and be able to trust Dex.

I’ve noticed a similar type of thing with my meter these past few weeks (OneTouch Ultra Mini). This may have caused some of the errors that my Dexcom has been displaying with increased frequency (Sensor Error #1), which means it’s not able to properly calibrate. I suspect a couple of things could be contributing to the issue:

-For the past year or two, every single test-strip vial that I’ve purchased (from the same Pharmacy) has been Code 25. Because code numbers are used to calibrate the meter, I’m wondering if the lack of changes to the code on my meter means it’s not calibrating itself, causing it to become less accurate the longer I keep using test strips with the same code.

-I have not used the control solution to verify the accuracy of my meter, even though I should. Still, the margin of error for control-solutions tests is usually around 35 mg/dLs, which doesn’t exactly instill confidence.

A few weeks ago, my Dexcom reported somewhere in the 100s (something like 145). However, in an attempt to calibrate the CGM, I tested with my meter and got a reading around 350. I pulled out the Dexcom sensor in absolute frustration and injected insulin to get me back within range. In a surprisingly short time (about two hours later) my meter test reported 44. This is much quicker than any other time that I’ve had to correct a high level. While it’s possible other factors could have played into this, there was obviously an issue with accuracy.

Since this incident, I’ve seen similar differences to what you describe when performing multiple tests. This makes me think I should have left the Dexcom sensor in that day, and tested with another meter.

Thanks for all the suggestions - I think I have been calibrating far too often and it makes sense to do it when my BG is stable. I might also restart the sensor this evening as in the past they have been fairly accurate but this one has been way off all day (hardly surprising!).

I have a lab test coming up so will definitely start using a new meter before then and check it for accuracy - great tip!

So far I’ve only used 3 sensors and each has lasted exactly a week, almost down to the hour. When people say they get longer out of them, is that because they just last longer or do they restart the sensor?

We restart the sensor to get past the 7-day limit.

funny, every single tub of test strips I’ve for as long as I can remember over at least the past few months (from different pharmacies in different areas) has also been Code 25…


Recent meter marketing about codeless test strips have taken over. I think OneTouch wishes to join that camp, but until they come out with a new strip style and meters to match, it was just easier to make them all Code 25. That’s my thought anyway.

I would never recommend correcting an unexpected 350 from a single meter reading. If dex said 145 and the sensor wasn’t >7 days, I’d wash my hands and test with the meter again. Single meter readings can be wacky sometimes, and correcting a significant but wrong high can be risky.

If you just restart the sensor. Your cue that it is “done” is when you start getting innacurate readings on your Dex.

Don’t rely on your One Touch to be infallible. Check this Youtube I did a few years ago. Here is the description of the video:

"My One Touch Ultra meter gave a blood glucose reading of 334 at 10:40 a.m., less than an hour after a 99 reading at 9:46 a.m. I didn’t think the 334 reading was correct so I checked my blood glucose again at 10:51 a.m. and the result was 117. "

Always wash your hands before testing with a One Touch, and if you get weird readings (on initial calibration or on daily calibrations) retest so you are entering reliable numbers.

The key is to develop a good BG test procedure. My very first double test was 96 and 136. Before then I had no idea that my BG test procedure was so bad ( being a T1 for 36 years at the time ).

For the very first calibration I test until I have 2 readings within 5 mg/dl. More than 95% of the time I achieve that with 2 tests.

During follow-on calibrations I enter the first BG result only if it is within 5 mg/dl of the dex readout. This happens about half of the time. If not within 5 mg/dl then I continue BG tests until I have 2 readings within 5 mg/dl.

This may sound like a wasteful procedure. The upside is that I can trust my dex and typically don’t prick my fingers between calibrations. I echo everybody’s sentiment that the less calibrations the better.

That’s our experience too. If we calibrate an exaggerated high, then it’s bound to give us an exaggerated low when my son comes back down into range. Some of it is just the slight lag which happens when blood sugars are changing quickly, and we have learned not to calibrate then either. So if my son treats a low, and then his finger stick is back in range, his dexcom reading takes a little longer to catch up. We used to try to make it catch up quickly by calibrating as he came up - but that just led to it being too high later.

Holy Hannah; somebody else has this issue.

So far mentality of industry is one is not cleaning fingers and using fresh lancets properly.

Utter balderdash. ( yes you need to do that but not whole answer.)

What I have found is that if not actively walking and insulin blood is not stable; on can see the differences you saw between fingers and hands. AFter hard walking, usually fingers and hands will stabalize to 1 or 2 points apart.

That said; I have found it most reliable and consistent to take a single reading from each hand and finger and average that
and enter as BG entry on Dexcom. I get excellent tracking over time doing that.

If you have big organs; it is possible to get some heart stopping readings right after munching a glucose tablet till heart has chance to pump around your body a number of times to get glucose well mixed. When my liver does liver dump; one can see some fun numbers. Through out readings and retry so that at least two fingers close and reasonable. Usually one will see one hand/finger at 250 and other at 182 or something like that.

have fun.

Andy, what you say here I found to be true for mine as well. I haven’t had the privledge of being on the 7+ as it was all self pay for me and evtl I couldn’t afford it any longer. :o( My sugars have been all over the place so I 'm thinking about getting started back up again. What are the out of pocket? Thanks