Interstitial fluid which is what CGMs read lags 15 minutes or so behind blood glucose.The CGM return seems to lag blood glucose by about 15-20 minutes.
Thank you, obviously my experience was a few years ago, before the algorithms were tweaked. My readings did lag.Interstitial fluid which is what CGMs read lags 15 minutes or so behind blood glucose.
However, the view that the values reported on CGMs is behind is rather old fashioned (although, sadly, it is repeated again and again on fora and, sometimes by HCP).
The algorithm used by CGMs to convert interstitial fluid readings to blood goose readings, takes this lag into consideration and “predicts“ the current reading.
Therefore, unless the current trend changes direction in the last 15 minutes (e.g. recovering from a hypo), the reported CGM value is very close to the calculated value now.
This has been mentioned many times on this forum.
Unfortunately, it looks as if it will need to be repeated many more times.
If you wish to read further on this see :Consider the individual cells in your brain. Where do they get their glucose? They do not have blood capillaries. They are bathed in interstitial fluid. How do you “Feel” your glucose level?
So often we feel a hypo before it shows in BGM results, whether from blood or CGM. Because we detect he change in interstitial.
Consider the tested levels of Lag for Libre. They used to be 7 minutes +-3 minutes but the new algorithms have cut this. They are now 2.4 minutes +-4 minutes. I’m just going from memory here so these may be just rough figure.
The point is the 2.4 minutes -4 minutes becomes a negative value. Libre is ahead of blood at certain times! Lag becomes negative!
While the average Lag may be 2.4minutes the actual reading can vary between 6.4 minutes and -2 minutes. And all of these values, BGM and Libre are only correct for 95% of the time.
So most people end up confused. I even heard a DSN telling a group that the lag for Libre is 2 minutes 4 seconds!
The issue really - Does it matter?
Precise measurement of these values is actually impossible - even in a hospital laboratory. The British Medical Journal recently published a strong warning to medical professionals that all medical measurement is imprecise and taking it as absolutely correct could lead to incorrect diagnosis. Measurements change and comparisons between two values can completely mislead.
We mistakenly think that we need ”accurate” glucose values to compute our insulin needs. But what about that insulin itself. It is Not of constant efficacy. It might be measured and tested at the factory gate but never before it is pushed into your arm. It can vary considerably depending upon the distribution and storage and indeed how we keep it. Some US testing gave truly shocking results - but it simply is not tested here.
In short, far too many people get hung up over numbers. Treatment is based upon individual estimation - guesswork.
Perhaps the best way to compare CGM ”accuracy” is to compare two sensors on the same person. Or one sensor to the best blood measurement and plot them on a chart. If perfect, they will plot onto a single 45 degree line. Differences show up as scatter to each side of that line. Here is the line for Libre. Note a few dots well off the line but most are on it or nearby. This is a chart of hundreds of measurement and not odd results from a single comparison. (Remember the 95% sample above )Consider the individual cells in your brain. Where do they get their glucose? They do not have blood capillaries. They are bathed in interstitial fluid. How do you “Feel” your glucose level?
So often we feel a hypo before it shows in BGM results, whether from blood or CGM. Because we detect he change in interstitial.
Consider the tested levels of Lag for Libre. They used to be 7 minutes +-3 minutes but the new algorithms have cut this. They are now 2.4 minutes +-4 minutes. I’m just going from memory here so these may be just rough figure.
The point is the 2.4 minutes -4 minutes becomes a negative value. Libre is ahead of blood at certain times! Lag becomes negative!
While the average Lag may be 2.4minutes the actual reading can vary between 6.4 minutes and -2 minutes. And all of these values, BGM and Libre are only correct for 95% of the time.
So most people end up confused. I even heard a DSN telling a group that the lag for Libre is 2 minutes 4 seconds!
The issue really - Does it matter?
Precise measurement of these values is actually impossible - even in a hospital laboratory. The British Medical Journal recently published a strong warning to medical professionals that all medical measurement is imprecise and taking it as absolutely correct could lead to incorrect diagnosis. Measurements change and comparisons between two values can completely mislead.
We mistakenly think that we need ”accurate” glucose values to compute our insulin needs. But what about that insulin itself. It is Not of constant efficacy. It might be measured and tested at the factory gate but never before it is pushed into your arm. It can vary considerably depending upon the distribution and storage and indeed how we keep it. Some US testing gave truly shocking results - but it simply is not tested here.
In short, far too many people get hung up over numbers. Treatment is based upon individual estimation - guesswork.
Finally Insulin concentration - look at the graphs in this medical article - USA but would ours be better? https://journals.sagepub.com/doi/10.1177/1932296817747292?icid=int.sj-full-text.similar-articles.2Consider the individual cells in your brain. Where do they get their glucose? They do not have blood capillaries. They are bathed in interstitial fluid. How do you “Feel” your glucose level?
So often we feel a hypo before it shows in BGM results, whether from blood or CGM. Because we detect he change in interstitial.
Consider the tested levels of Lag for Libre. They used to be 7 minutes +-3 minutes but the new algorithms have cut this. They are now 2.4 minutes +-4 minutes. I’m just going from memory here so these may be just rough figure.
The point is the 2.4 minutes -4 minutes becomes a negative value. Libre is ahead of blood at certain times! Lag becomes negative!
While the average Lag may be 2.4minutes the actual reading can vary between 6.4 minutes and -2 minutes. And all of these values, BGM and Libre are only correct for 95% of the time.
So most people end up confused. I even heard a DSN telling a group that the lag for Libre is 2 minutes 4 seconds!
The issue really - Does it matter?
Precise measurement of these values is actually impossible - even in a hospital laboratory. The British Medical Journal recently published a strong warning to medical professionals that all medical measurement is imprecise and taking it as absolutely correct could lead to incorrect diagnosis. Measurements change and comparisons between two values can completely mislead.
We mistakenly think that we need ”accurate” glucose values to compute our insulin needs. But what about that insulin itself. It is Not of constant efficacy. It might be measured and tested at the factory gate but never before it is pushed into your arm. It can vary considerably depending upon the distribution and storage and indeed how we keep it. Some US testing gave truly shocking results - but it simply is not tested here.
In short, far too many people get hung up over numbers. Treatment is based upon individual estimation - guesswork.
And some people can react badly to a sensor and never get a good reading from it (the reason why I had to stop using the original libre 1).bedding in time. Some bodies react to the alien object in their arm so readings can take a couple of days to settle.
Interestingly, due to a bodge on my script, despite ordering a week in advance to process at my local & the chemist remembering me order the previous Friday.And some people can react badly to a sensor and never get a good reading from it (the reason why I had to stop using the original libre 1).
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