Insulin-to-Carbohydrate Ratio (ICR)
Written by Carolien Koreneff, CDE-RN, FADEA
What is an Insulin-to-Carbohydrate Ratio (ICR)?
People with type 1 diabetes who like to vary their intake of carbohydrates from one day to the next can use an insulin-to-carbohydrate ratio (ICR). The ICR helps people get the right amount of insulin for the carbohydrates they will be eating.
The ICR means that you take 1 unit of rapid-acting insulin for a particular amount of carbs. For example, if the ICR is 1:15 the person would have to take 1 unit of Apidra, Fiasp, Humalog, or Novorapid for every 15 grams of carbohydrate eaten. If the meal contains 45 grams of carb and they have an ICR of 1:15 it would mean that they would need 3 units of rapid-acting insulin for this particular meal (45 divided by 15 = 3). Whereas a person with an ICR of 1:10 would need 4.5 units for the same amount of carbohydrates.
How to calculate the ICR
Many diabetes healthcare professionals use the 500-rule to calculate ICR. They take the number 500 and divide it by the person’s current total daily dosage of insulin.
To establish the Total Daily Dosage (TDD), add any basal/long-acting and any bolus/rapid-acting insulin used in a 24-hour period. If the insulin intake varies from one day to the next it is recommended to do this for a few days and take the average over say 3-4 days.
For example: If the person takes 26 units of Optisulin at night and they take 8 units of Novorapid at breakfast, lunch and dinner their TDD will be: 26+8+8+8=50 – 500 divided by 50 is 10. This means that their ICR is 1:10 and they will need 1 unit of rapid-acting insulin for every 10g of carbs. This can also be written as 1.5 units per 15g serve of carbs.
To check if the ICR is correct people should check their BGLs 2-3 hours after eating. If the after meal BGL is more than 3 mmol higher than what it was before the meal, you need to consider making the carb ratio stronger (by lowering the number) or review the patient’s carb counting skills.
Should you round up or down?
In most cases the total amount of insulin will need to be rounded to the next nearest full unit. This is because not many people with diabetes use insulin pen devices that can deliver insulin at half unit increments. But should the number be rounded up or down? Well, this depends on the sensor glucose (SG) or blood glucose level (BGL) at the time. (People with diabetes who use continuous or flash glucose monitoring devices should remember that for some devices a finger prick is required for insulin dosing).
Generally speaking, if the BGL is high at the time it is usually recommended that you round up. But if the BGL is on the lower side it is worthwhile rounding down (and hopefully avert a hypo). You should also consider what the person will be doing during the following few hours. If they are going to be sitting around you may want to round up, but if they are planning physical activity you would do well rounding it down.
Additional tips on the Insulin-to-Carbohydrate Ratio (ICR)
- It is important that people remember to bolus before the meal. In most cases 10-15 minutes before you start eating is ideal.
- Many people have different ICRs for different meals. For example: they may have an ICR of 1:10 for breakfast, 1:8 for lunch and 1:12 for dinner.
- For the ICR to work accurately it is important that carbohydrates are counted correctly.
- The protein and fat content of meals can also affect BGLs and may need to be considered.
- A bolus is often required for larger carb snacks.
- Taking insulin before eating and then not eating all of the planned carbohydrate may cause a hypo when the rapid-acting insulin peaks.
- Taking bolus insulin after eating will result in a high blood sugar a few hours later.
Carb counting resource for people living with diabetes
The NDSS have developed a free online education program called Carb Counting online which provides basic education about carbohydrates (carbs) and carb counting for people living with diabetes.
The program has 11 modules covering:
- common foods and drinks that contain carbohydrates and how they impact blood glucose levels
- an introduction to carb counting in 10g portions, 15g exchanges, and grams
- identifying carbohydrates from recipes and food labels
- tools and resources for carb counting