Formula to determine BG mmol/l change per unit of insulin

[AJB_81]

Hi all, has anyone used a formula to work out BG change per unit of insulin for a T1D?
I know the effect of a unit of insulin will, potentially, be different depending on many factors (age, time of diagnosis, current state of pancreas and any remains beta cells etc...).
I have always been told that the general rule of thumb is 1 unit changes blood glucose by 3 mmol/l. However, the more I consider this the more I think this is, as described, a rule of thumb and we should calculate this, perhaps from TDD which takes into account our weight. Any thoughts on this would be appreciated!

If the ratio between insulin and change in BG is incorrect then all meal carb counting and corrections will be causing BGs to fluctuate.

One other thing I find confusing is that if we are more sensitive to insulin in the morning then why does the 1 unit:3mmol/l change apply all day?!?

 

[tim2000s]

Hi @andybraith your questions are totally valid and absolutely right.

The 1u:3mmol/l ISF value is a totally arbitrary number (based on the rule of 100) that is really a starting point. As you see on a pump, you have the ability to set different ISF and Insulin Carb Ratios for different periods of the day.

This is why it's really important to understand whether your basal levels are correct by doing a basal test, then following that up with a Correction Factor (or Insulin Sensitivity Factor) test and an Insulin Carb Ratio test for different times of the day.

So to sum up, yes, the start is really just that and you need to adjust for yourself around those numbers, and yes most people have different ratios throughout the day. Plenty to think about!

 

[Deleted Account]

I am not sure there is a general formula: everyone is different. And, as you say, they are different at different times of the day.
If you are using MDI, you will be advised to have the same ratio at all times. As it is an approximation and you are using it to make a correction, the chances are you will closely monitor the effect. Also, with a pen, you can only go down to 1 unit (or some pens have .5 units) so there is no chance to be accurate.

I was advised the same as you (1:3). However, I checked this by making an a correction and checking the impact it had 3 - 4 hours later. Don't forget to take into consideration any insulin-on-board (most "fast acting insulin", like Novorapid, is in your system for up to 4 hours).

If/when you change to a pump, you will have a lot more control: you can set different I:C ratios throughout the day; you don;t have to make the IOB calculation, etc.

 

[AJB_81]

That's great @tim2000s thanks for getting back.
Long story but my wife is the "proper" T1D in the family and has been so since 4, I've only been diagnosed 3 months and the honeymoon has kicked in very strong and require very little insulin.
We both recently got a Libre and it's this that has made us look closer at my wife's control, fluctuations all over the place that we just weren't aware of. Steadying out night time is the challenge as we know it's important to start the day well. To get the night right BG before bed needs to be good, we're chasing our tail at the minute but determined to get it right.
Stoping night hypos by trimming back her night time basal has been the first challenge, we now need to have a period of fasting to check back ground. If this is good I'd be keen to check insulin sensitivity - any tips on how to test insulin sensitivity? Is it as simple as eat exact amount of carbs have insulin for the carbs (perhaps starting at a ratio calculated from TDD) and monitor BG?

 

[tim2000s]

My ISF testing approach is, once you've got the basal steady, to eat a rough amount of carbs from a fasted state to raise your glucose level to somewhere between 8mmol/l and 10mmol/l. When it's steady at the the higher level, then administer 1u and see what the drop is, and how long it takes. It will typically take 4 to 5 hours to come down fully and will give you both a DIA and ISF.

To determine CSF, from a fasted state, eat an exact amount of glucose (basically 12g in the form of 4 dextrosols or 3 glucotabs) and see what your rise is from flat to a higher flat level.

If you then find that, say 10g carbs raises you 4mmol/l and 1u drops you 2 mmol/l, you can see that your I:C ratio is 2:10 or 1:5.

You can then go away and test that with an as accurately counted as possible meal. (Caveat is that they are never 100% accurate).