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Freestyle libre time lag

thewestiesmum

Well-Known Member
Messages
143
Location
North Yorkshire
Type of diabetes
Type 1
Treatment type
Insulin
Please can somebody please explain in easy term this time lag with the FreesStyle libre? For example when you scan your sensor and straight away do a blood glucose test what do the 2 different results mean? Does it mean that in so many minutes time you will be whatever the sensor reading says?

I know that they both measure different things ie interstitial fluid and blood glucose.
 
mum2westiesGill said:
For example when you scan your sensor and straight away do a blood glucose test what do the 2 different results mean? Does it mean that in so many minutes time you will be whatever the sensor reading says?
Click to expand...
It's the other way around. Your blood glucose changes first, so the sensor lags a bit behind a fingerprick. The software does compensate for this somewhat, by doing some prediction, but this won't work right after treating a hypo or such. On such moments it's important to always double check with a finger prick if the sensor shows you aren't rising yet, you might find you're already in the safe zone when Libre still thinks your hypo, causing you to overtreat.

This video has a nice visual on how the time lag works:
 


So if the blood glucose changes first and the sensor glucose is a bit behind does that mean that the sensor glucose is what your blood glucose was a few minutes ago? If this is the case then how can the libre warn you that you are going hypo or high?
 
It depends whether you are using Libre 1 or Libre 2.
The interstitial fluid reading that Libre uses is 15 minutes behind finger prick blood.
However, The algorithm Libre 2 uses to convert from interstitial fluid reading to blood glucose reading takes the delay into consideration.

So, if you are using Libre 1, take a finger prick reading, wait 15 minutes, then scan your Libre 1 and they should be close.
If you are using Libre 2, you can scan and finger prick at the same time.

The Libre 2 algorithm predicts the 15 minute delay by extending the current graph tread.
This is great if your level continues in the same level or direction. However, if you correct a high or low in the last 15 minutes, Libre will not match finger pricks until,Libre has caught up with the time when the change of direction.
This means, Libre will take longer to notice a hypo correction.
 
Also, keep in mind that there are ERROR factors involved with both the Libre system and the finger pricking method. The trends are more important than the actual numbers.
 
I'm using the libre 2.

I've lost my hypo awareness in the last year so I don't know if I'm hypo until I a finger prick test. Somebody suggested to go back to using the libre (I became very anxious of all the differences between SG readings and BGL readings so stopped using it) and use it as a low glucose alarm which I've done. I've got my low glucose alarm set to 5.6 and I've changed my target range from 5-8mmol to 6-10mmol.
 

Hi,

If it's any help for you.? I use Diabox app with the Libre 2, what I've noticed (& works for me.) with an alarm set at 3.9mmol is it can give an advanced warning of around 5 to 10 minutes? At first I thought, "no way" when the beeping kicked off but then I felt the early hypos signs.. "Hello!?" The hypos still come on if treated early but not so severe before recovery . If I'd just used "the force" & the meter in the old days..
I'm still fully hypo awair. But I can drop a little further prior to recovery after treatment sometimes.. I certainly noticed it when using just my meter too.. My Libre can reflect this as well.
How I utilise the set up I use is to take then"edge" off any lows I might get with this early warning?

Though set a target you feel comfortable with..
 

Is the Diabox on Android?
 
Last edited:
I'd suggest you use the Freestyle Libre as an early warning but treat and monitor hypos based on blood glucose readings (not the Libre). What the Libre is measuring will not follow fast changes in blood glucose and I would not rely on an algorithm guess at what is going to happen in the future.
 
I am new to all of this, but I just use the Libre as a guide (since I have found the readings can be quite a way out). I have got out of the obsession with checking every reading and comparing to a finger prick (I was finding myself getting quite stressed about it (my apologies to those who have had diabetes for years, for being so lightweight)) and like @jobathan183 I now use the trends and only double check extreme high or low readings. I do love the Libre, compared to finger pricks and found myself completely lost recently when I became sensor-less for a few days.
 
Here are a few of my SG & BG readings where I do the sensor scan first followed straight after with a BG test

05:40 SG 17.8
05:42 BG 16.5 3.0 units correction - unlike bedtime felt safe doing a correction at this time because I would be properly awake soon
08:07 SG 6.7
08:09 BG 6.4 waking. 15.0 units of tresiba
08:20 SG 5.3
08:23 BG 5.5 had 1x dextrose

07:53 SG 5.8
07:56 BG 6.8 waking. Didn't have any dextrose because BG was within target. 15.0 units tresiba
10:08 SG 6.7
10:09 BG 6.9 before breakfast
 
Those numbers are practically the same, completely within the margin even if you used a finger prick on the same drop of blood intead of comparing different methods.
 

Hi,

I normally accept within 0.6mmol of eachother. (Personal tolerance.) bit I wouldn't shrimp at those discrepancies you posted, either..

What I have come to understand is the BG meter takes a "snap shot" in time on a draw of blood from a particular appendage on the body. Which may have been 15 to 20 minutes ago..?
Some times there can be larger discrepancies regarding members enquiring about 2 meters "which one to believe?"

At the end of the day. I personally wouldn't concern to much with those differences. Just understand which direction they are traveling.
 
jape said:
The trends are more important than the actual numbers.
Click to expand...

Depends on what and why you are using it. For an insulin user the numbers are the most important as that is what you use to determine how much insulin to inject. Rightly or wrongly, given it's inaccuracy and limitations, it's all I have to calculate insulin. Of course any odd reading can be followed up with a finger prick test but since getting the libre on prescription (which I am grateful for) the test strips are heavily restricted.
 

oh crikey, I’m lucky - my surgery hasn’t thought to restrict mine yet.
 
Mungobean said:
oh crikey, I’m lucky - my surgery hasn’t thought to restrict mine yet.
Click to expand...
Yeah, it varies a lot. When I was put on the Libre, nothing changed. Then when I changed Doctor, my Doctor went "Oh you're on the Libre, I'm going to restrict how many test strips you can get." When I suggested that wasn't a good idea, considering the advice I'd been given about double-checking with Blood Tests by Abbott, he was utterly bemused and said "well, it sounds like you're going to have to order some from Amazon."

Since switching to the Pump, I've had my entitlement increased, but it took my Pump Nurse badgering them two or three times to make that happen.

Both Doctors Surgeries operate in the same health board.
 
That’s really uncalled for, especially since the official advice is to double check with a finger prick! It’s shocking, how expensive those strips are.
 
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