If you like to vary your carbohydrate intake from one day to the next, you can use an insulin-to-carbohydrate ratio (ICR). This will help you get the right amount of insulin for the carbohydrates you will be eating.
What is the Insulin-to-Carbohydrate-Ratio?
The ICR means that you take 1 unit of rapid-acting insulin for a particular amount of carbs.
For example, if your ICR is 1:12 you would have to take 1 unit of rapid-acting insulin (either Apidra, Fiasp, Humalog, or Novorapid) for every 12 grams of carbohydrate eaten. If the meal contains 36 grams of carb and you have an ICR of 12 it would mean that you would need 3 units of rapid-acting insulin for this particular meal (36 : 12 = 3). Whereas a person with an ICR of 8 would need 4.5 units for the same amount of carbohydrates.
Your credentialled diabetes educator (CDE) or your endocrinologist can help you decide on what your ICR should be.
How is the ICR calculated?
You can calculate the ICR using the 500-rule.
First you need to calculate your Total Daily Dosage (TDD) by adding up both the rapid-acting (bolus or pre-meal insulin) and long-acting (basal or background) insulin doses in a 24-hour period. It will be more accurate if you calculate the TDD over a few days and take the average.
Then calculate the ICR by taking the number 500 and divide it by your current TDD.
For example: If you take 55 units of Optisulin at night and 8 units of Novorapid at breakfast, 6 units at lunch and 7 units at dinner your TDD will be:
55 + 8 + 6 + 7 = 76 units/day
500 : 76 = 6.6
This means that you will need 1 unit of rapid-acting insulin for 6.6 grams of carbohydrate. In other words, your ICR is 6.6.
Some healthcare professionals prefer to use the 450-Rule. This works the same as the examples above, except for using the number 450 instead of 500 in the calculations. Using the 450-Rule is a little more conservative. For example, if we use the case study above the calculation would be as follows:
55 + 8 + 6 + 7 = 76 units/day
450 : 76 = 5.9
This means that you will need 1 unit of rapid-acting insulin for 5.9 (or 6 to round it off nicely) grams of carbohydrate. In other words, your ICR is 6.0.
How do I use the ICR?
Work out how much carbohydrate you will have for your next meal and divide this amount by the ICR. For example: If you are going to have some pasta with a sauce and you calculate that this will contain 87 grams of carbs for the whole meal, you will need 13 units of rapid-acting insulin (if using the example above) as: 87 : 6.6 = 13.
It is easiest if you count your carbs in grams, rather than portions or exchanges. Talk to your dietitian for more information or consider attending the OzDafne program.
It is important to remember to bolus before the meal. In most cases 10-15 minutes before you start eating is ideal. Bolusing after the meal will see your glucose level rise a few hours later.
Taking insulin before eating and then not eating all of the planned carbohydrate may cause a hypo when the rapid-acting insulin peaks.
Another thing to consider is that fat and protein also have a role to play in glucose metabolism, find out more here.
Many people have different ICRs for different meals. For example: you may have an ICR of 10 for breakfast, 8 for lunch and 12 for dinner. If your snack size exceeds 30 grams of carbohydrate you may need to bolus for this snack to avoid glucose rises afterwards. Talk to your healthcare professional to see if
this applies to you.
How do I know if the ICR is correct?
To check if your ICR is correct you should check your BGL 2-3 hours after eating. If the BGL is 1-2 mmol/l higher than it was before the meal your ICR was spot on (and you estimated the carbs for that meal brilliantly).
If your after meal BGL is more than 3 mmol higher (or lower) than what it was before the meal, you need to consider making your carb ratio stronger by lowering the number (or weaker by increasing the number) or review your carb counting skills.
Rounding up or down?
In most cases you will have to round of the total amount of insulin to the next nearest full unit, unless you use an insulin pen device that can provide dosing in half unit increments. The question is: should you round up or down?
Well, this depends on your sensor glucose (SG) or blood glucose level (BGL) at the time. (If you are using continuous or flash glucose monitoring devices you should remember that for some devices a finger prick is required for insulin dosing – talk to your CDE for more details).
Generally speaking, if your BGL is high at the time it is usually recommended that you round up. But if your BGL is on the lower side it is worthwhile rounding down (and hopefully avert a hypo).
You should also consider what you will be doing during the following few hours. If you are going to be sitting around you may want to round up, but if you are planning physical activity you would do well rounding it down.
Check the basal first
Before calculating or changing the ICR it is important to check if your basal/background insulin (Toujeo, Semglee, Optisulin or Levemir) dose is correct. You can do this by comparing your pre-bed BGL with your fasting BGL. If your FBGL is often more than 4mmol higher or lower than the night before you may need to change your basal insulin dose by 1-2 units (follow the directions of your diabetes HCP).
Whenever the basal insulin dosage is changed it is advisable to give it at least 3 days to see the new pattern before making any further changes.
A few words of caution
In most cases you should avoid taking extra injections outside your pre-meal boluses, as this may lead to something called insulin stacking and can increase your risk of developing (severe) hypoglycaemia. However, as mentioned before, if your snack size exceeds 30 grams of carbohydrate you may need to bolus for this snack to avoid glucose rises afterwards. Talk to your healthcare professional to see if this applies to you.
Calculating and using ICR only works if you are on a basal-bolus insulin regimen. It does not work if you are using pre-mixed insulins such as Humalog Mix25 or Mix50, Novomix, or Ryzodeg. If you are using a pre-mixed insulin talk to your healthcare professional for advice on insulin dose adjustments.
The above information is to be used as a guide only. Always follow the recommendations of your healthcare professional.