I woke up a couple of weeks ago with a Dexcom reading of 362, and my Accu Chek aviva concurred with a 361. Since I clearly had some time to wait before my blood sugar would be in a breakfast worthy state, I decided to log my blood sugar on the hour, as well as when and how much carbs I ate, after taking my breakfast shot of regular. I'd been wanting to figure out exactly how potent Regular was, by itself and in comparison to Novolog.
I constructed a formula for retroactively finding my insulin sensitivity/carb ratio for a particular shot.
For the times that I did this, I computed my retroactive insulin to carb ratio like this: I took the blood sugar reading right when I did the shot (I counted a Dexcom reading as a blood sugar reading, usually). I took the blood sugar reading exactly four hours later. I counted all of the carbs eaten in those four hours, including hypo treatments. I assumed a BCR of 5.
Then my retroactive I:C was as follows- ((Starting BG - Ending BG)/5 + carbs)/units injected.
I did not include any times if I had to give another shot within the four hour period, if I had eaten in the hour before the shot, or taken another shot in the four hours prior or if I just didn't document all the stuff above.
I ended up doing this computation for eight breakfasts with regular. The retroactive four hour I:Cs were: 6.06, 6.5, 6.63, 7.7, 8.5, 6.0, 6.375, and 6.5.
This is much more consistent than expected and the standard deviation is small enough that I should be able to compare this to Novolog fairly confidently. The consistency suggests to me that I picked the right BCR.
I only did this computation for three lunches, partly because I often don't take insulin for lunch or don't eat lunch. The retroactive I:Cs for those were 16.4, 11, and 22. Clearly all higher than breakfast, but that's about all I can say.
Dinners I almost never did this for because I tend to eat foods for dinner where I can't get a good carb count but I suspect the numbers would have been very variable. The only dinner I did do this for, I got a 14.4.
I sort of kept an eye on how this worked for corrections and my feeling is that for morning corrections, the morning ISF of 30 (which is the same as an I:C of 6 if my BCR of 5 is correct) worked well. For the rest of the day I used an ISF of 75 (which translates to a I:C of 15) and that was usually a bit stronger than I needed, but sometimes in the evenings it wasn't enough.
Yesterday the vial of Regular ran out and I'm back to Novolog. I intend to collect data on the Novolog too, to see if it works at the same strength as Regular. I suspect that the Novolog will prove to be slightly stronger. I think next time I go back to Regular (which I plan to do in October) I'm going to try to look at the 5 hour data as well, but that will be harder because spacing shots that far apart and also it introduces more of the basal variability.
P.S. After posting, I decided this was not very easy to understand. So let me give an example.
At 8 AM on Tuesday morning, my blood sugar was 138. I took 12 units of Regular, and ate 38 grams of carbohydrate. At 9 AM my blood sugar was 125. At 10 AM my blood sugar was 117 and I ate another 38 grams of carbohydrate. At 11 AM my blood sugar was 117 again and at noon my blood sugar was 103.
In a four hour period, my blood sugar fell 35 mg/dl, and I ate 76 grams of carbohydrate. To have kept my blood sugar totally stable, I would have needed to eat enough to stop the 35 mg/dl fall. I assume one gram of carbohydrate raises my blood sugar by 5 mg/dl, so I would have needed to eat 7 more grams. 76+7= 83. I would have needed to eat 83 grams of carbohydrate to have stayed stable with the 12 units of Regular I took. Pretending that basal issues don't exist, that means that to stay stable, I needed 12 units per 83 carbs, which is 1 unit for every 6.916 grams of carbohydrate.
So my retroactive I:C for that four hour period would have been 6.916.
I could also calculate that as a retroactive insulin sensitivity factor just by multiplying by 5, since I'm assuming one gram of carbohydrate raises me 5 mg/dl. Then I would assume one unit would have lowered my blood sugar by 34.583 mg/dl that morning.