Now that you know how to use an ICR (insulin:carb ratio) and how to use an ISF (insulin sensitivity factor), it is time to put them together!
Using both an ICR and ISF will give you the most flexibility and control when using rapid-acting insulin.
Doing the calculations can be tricky at first, but practice will help and it will get easier over time. Ask your diabetes educator about resources that can help you calculate your insulin dose.
Remember that the ICR tells you how many grams of carbohydrate 1 unit of rapid insulin will cover. The ISF will tell you how many mmol/l (‘blood sugar points’) 1 unit of rapid insulin will lower your blood sugar.
To put them together, we can use the following steps to calculate the meal time insulin dose:
- Test your blood sugar
- Count the carbohydrate in your meal
- Calculate the amount of rapid insulin needed to cover carbohydrate (using ICR)
- Calculate the amount of rapid insulin needed for correction if blood sugar is high (using ISF)
- Add value from step 3+4 to get the total dose of rapid insulin to take at that meal.
For example, let’s say that I test my blood sugar and it is at 12.0 mmol/l. I have counted 45g of carbohydrate for my lunch. My ICR is 1:15 and my ISF is 1:2. My goal is to get my blood sugar back to 6.0 mmol/l.
I can use the steps above to calculate my total insulin dose at the meal:
- My blood sugar is 12.0 mmol/l
- My meal has 45g of carbohydrate
- 45 (carbs)/15 (ICR) = 3 units of rapid insulin to cover my carbs
- 12.0(BG)-6.0 (target) = 6.0 mmol/l I need to come down to reach target
—>6.0 (need to come down) / 2 (ISF) = 3 extra units of rapid insulin to come back to target
- 3 (insulin for carbs) + 3 (insulin for correction) = 6 units of rapid insulin that I will need to take at this meal.
Remember that the ICR and ISF do not take into consideration whether not you have been or plan to be active or if you are sick. Your ICR and ISF may change in these situations as you may need more or less insulin during these times. Speak to your diabetes educator about adjusting your ICR and ISF in these situations.