When Dexcom and fingerstick are off

What do you do when Dexcom and fingerstick are off? Right now Dexcom is showing 45, meter 70. Son is asleep. I’d really like to be too.
We really try not to overcalibrate but I’m wondering if it’s better to calibrate with the higher number simply to get some sleep? Otherwise we’ll potentially be hearing alarms every five minutes for a long time. He’s been low on Dex for more than an hour with fingersticks reading higher.


Erin informed me that the Dexcom recommendation for calibration is everytime the blood drop appears OR anytime the actual sugars (via finger prick) are >20% off from the receiver reading.

I feel you on the sleep part. It’s 0430 and I’m nursing a sudden onset 45 double arrows down atm (from 99 to a sudden 45.). He’s just turned 66 just now… Going to do another finger stick to see if I can get back to the land of Zs.

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Aside from pressure lows, I’ve heard that the interstitial fluid doesn’t behave the same way at night–causing false lows. This is likely due to the fact that you are horizontal and not vertical. I wonder if re-calibrating will get things back on track now, but be biased later when he is awake. I would be curious to know how different your next calibration is–and if it is biased high.

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I really wish there was a way to silence those alarms for a period of time, say 90 minutes. Nothing is more frustrating to get up, take action, and then listen to the alarms again and again and again…

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Nightwatch is nice because it has “Smart Alarms”. It will alarm initially, but will not alarm again if it sees the BG going in the right direction.

Do do you have as few alarms as possible set on the Iphone that you are using as the receiver. In our house, you can hear all three phones go off in sucession…

I use staggering alarms. I have an initial alarm set with Nightwatch starting at 95, 90, 85 (all set to “smart snooze”). After that, my Dexcom app will start to alarm as well as my wife’s and the up-loader iPhone in my son’s room. We also have Nightscout tied to our mounted tv running from our laptop which also will alarm (and is a convenient large display). If all else fails, we go nuclear. My SmartThings system can be integrated at this point. My home security system employs two sirens inside the house for unauthorized entry–but is now re-purposed for dangerously low BG’s (below 55). Luckily we have only had this happen once or twice–and is not pleasant to say the least.


I know! I have an iPhone though, and from what I understand I can’t get Nightwatch on my phone. My husband has it and it seems really clever.

Although I also want the low, low alarms to go off at maximum frequency for the off chance that we both sleep through them, which has happened for about 20 or 30 minutes before…

This is interesting @Bradford – I did wind up recalibrating and this morning Dex was about 20 points off. This is well within the normal error but right about the bias between his night reading and his finger stick reading.

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Official verbiage from Dexcom -

"…and whenever the sensor is inaccurate. Inaccuracy is defined as when the difference between your sensor glucose reading and blood glucose value is greater than 20% of the blood glucose reading and blood glucose value is greater than 20% of the blood glucose value for sensor readings > 80mg/dL or greater than 20 points for sensor readings < 80 mg/dL. "

Absolutely feel your pain, especially at night.


@TiaG, it is no problem at all recalibrating an extra one or two times per day if you are significantly off. It will not negatively impact your reading.

The big issue is, that if you recalibrate while you are not nicely even, you may exacerbate the problem. It’s not even enough if you have IOB to look at an even track because you don’t know how the “next 15 minutes” look like - you may still be even on the CGM but going up or down by the meter.

My three favorite spots to recalibrate are (a) when you have no IOB and you have a nice, flat, even track, and (b) if you are going down, after you have taken some glucose, when the curve has gone back up some, barely approaches horizontal but is not quite there – which hopefully means that the next 15 minutes are going to be fairly even: next point will be a bit up, next point even, next point a tiny bit down, and © same (but in the other direction) when am blunting a very low peak with insulin.

Be aware that recalibration far from being in range will not give you good numbers when in range.

When you recalibrate because you are far off, there are two scenarios:

  • your Dexcom was high. Then the post calibration number will likely go down very close to what you entered

  • your Dexcom was low. Then the post calibration number will obviously be up, but it will not go all the way to what you entered – it will probably be more or less half way between where the Dexcom was and where your new point is. So your new track will still be a bit lower, but less than before. If, however, you were to reenter the same point again within a minute or two, it will likely “force” the Dexcom to the point you want. But I do not recommend doing that, because your meter may also be off by 20% - better average the two.

Have you all noticed that, sometimes if the CGM is WAAAAAY OFFFF (Say +100 points) from the actual BG, that after you enter the correct BG into the receiver, it forces you to enter it again by appearing like it didn’t recognize your entry? Is this intentional/built in as a function to not inadvertently enter the wrong number into the receiver?

Hmm. Interesting - I don’t think I have ever seen that. It’s very possible, I’d say even likely that it is intentional.

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I have experienced this as well a couple of times, when the difference has been about 50 or 60 points off what appears to be a persistent low but isn’t. I make sure to wash my hands again before the second fingerstick. I think this is intentional and am glad when it does cue me for the second measurement since I dose off the CGM.


I see this as Dexcom’s way of saying “you’re kidding, right?” – Basically if it’s way off, Dexcom wants to make sure that it’s wrong, not the meter. A stray fleck of food, a little bit of water on fingers – and a meter could easily be way off. So I think it’s sensible. It also forces you to do the sanity check of retesting when there’s a discordancy.


Yes, that is by design. I’ve noticed this many times.

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If the number that is entered is not consistent with the calibration slope and intercept in the calibration algorithm, then the raw reading cannot be processed. That’s why it won’t return the Bg value.


It will allow it…it just takes entering it twice. I figured it was by design and glad to know it’s not just my device being wonky.

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My guess is the first number tweaks the calibration slope enough to be able to process the next wonky number.

I have had the situation that two wayward entries would give me the two blood drop symbol causing me to have to start all over again. I think that’s probably because the calibration slope is not permitted to be less that 1.00
In any event, it’s good it rights itself.

IOB = Insulin On Board, it means bolus insulin you injected that is still active.

For instance, if your Duration of Insulin Action (DIA) is 5 hours, and you inject at 8:00, at 11 am you still have IOB, but at 2:00 pm you don’t.