Updated: May 20
Last month’s Clinic Chat covered how to test your basal insulin dose in order to set you up with a solid foundation for accurate insulin dosing generally. This month and next, we will investigate the formulae you need to determine your rapid-acting doses, starting with the insulin-to-carbohydrate ratio or ICR, followed by your insulin sensitivity factor, or ISF. In this Clinic Chat, we discuss how to calculate and test the accuracy of your current ICR. If you use a pump or bolus calculator, your ICR will be programmed into them. Otherwise, it's something you need to know and remember.
Your ICR specifies how many grams of carbohydrate will be covered by one unit of insulin. For example, a ratio of 1:10 means that one unit of insulin will cover 10 grams of carbohydrate. For example, if a person with an ICR ratio of 1:10 eats 30 grams of carbohydrate, they will require three units of insulin to cover this meal.
Knowing your ICR helps you calculate the amount of insulin you will need to take for the carbohydrate you eat, which can help you to maintain stable blood glucose levels.
The general rule that educators use to calculate a person’s ICR is the “Rule of 500”. The calculation is as follows:
500 ÷ Total Daily Dose
Your Total Daily Dose is the total amount of basal/long-acting plus bolus/rapid-acting insulin you use in one day. For example:
Lantus dose = 20 units
Average Novorapid doses = 6 units breakfast, 6 units lunch, 5 units dinner
Average daily correction doses = 4 units
Total Daily Dose = 20 + 6 + 6 + 5 + 4 = 41 units
In this example, the ICR calculation would be as follows:
500 ÷ 41 = 12.2
Rounding this to the closest number, the ICR would be 1:12, ie 1 unit of insulin will cover every 12 grams of carbohydrate.
If this person were to eat 48 grams of carbohydrate, the calculation would be as follows:
48 ÷ 12 = 4 units of insulin to cover the 48 grams of carbohydrate
The Rule of 500 is intended as a general starting point, and it is very likely that you will need to adjust the result to fine tune your ICR.
Testing your insulin-to-carbohydrate ratio: Before you start
It’s best to test your ICR in the context of a correct basal or long-acting insulin dose; read our April clinic chat to learn more about testing and adjusting your basal/long-acting insulin so you’re set up with the right dose before you test your ICR.
In order to use an ICR properly, you must be able to accurately calculate the amount of carbohydrate you are about to eat. So before you test your ICR, make sure your carb-counting is on-point. The Family Centre’s online carbohydrate counting course, Cyber Carbs, is a comprehensive 10-module program that will get you to carb-counting ninja status in no time.
Testing your insulin-to-carbohydrate ratio
What you need
A blood glucose monitor, a continuous glucose monitor or a Freestyle Libre.
An accurately carbohydrate-counted meal. Avoid anything too high in fat or fibre, as it will delay the digestion and absorption of the meal. A ham and cheese sandwich is an option some of my patients like to use.
Your usual rapid-acting insulin.
When to test
Many factors impact blood glucose levels and can affect the results of your ICR test. Choose a time when you know you can prepare an accurately carbohydrate-counted meal and will have the opportunity to test your blood glucose over the subsequent 3-4 hours.
Avoid ICR testing at the following times:
Immediately before or after exercise
If you are unwell or highly stressed
Immediately after a low blood glucose level
At the start of your menstrual cycle if you are female
How to perform an insulin-to-carbohydrate ratio test
1. Prepare your meal and test your blood glucose level before you eat. If you are using a continuous glucose monitor or Freestyle Libre simply scan/view the reading and note it down.
2. Using the ICR you wish to test (for example, 1:10 or 1:12), count your carbs and calculate the amount of rapid-acting insulin you need for the meal. Deliver your insulin around 10-15 minutes before you eat to give the insulin time to start working.
3. If you are finger stick testing, check your blood glucose levels every 30-60 minutes for the next four hours. If you are using a continuous glucose monitor or Freestyle Libre simply review your data four hours later, at the end of the test.
Assessing your results
Type 1 diabetic Adam Brown, editor of global diabetes news site diaTribe and author of ‘Bright Spots and Landmines’, has identified that no fewer than 42 factors that impact blood glucose levels! With this in mind, it’s important to remember that there are always more variables in play than insulin and carbohydrate when you test your ICR. Finding patterns takes time, and it’s best to repeat your ICR test using the same meal at the same time of day at least 3-4 times before you adjust your ICR.
After you have run a few tests, analyse the data. Did your blood glucose rise or fall significantly in the hours after the test meal? If your ICR is correct, 3-4 hours after dosing and eating, your blood glucose level should land at around the same blood glucose that you started with before the meal – 1-2 mmol/L on either side is ok and still indicates an accurate dose.
Adjusting your insulin to carbohydrate ratio
If the data from your tests show that your glucose level is more than 2 mmol/L higher or lower than your starting glucose level 3-4 hours after the meal, then consider adjusting your ICR. It’s best to be conservative and methodical here: make small adjustments and reassess. Adjusting ratios can be confusing - here are some tips:
If your glucose level was higher than target after your meal, you need more insulin to cover it
Example: If the ICR you tested was 1:8, you need to decrease the ratio to take more insulin next time. Try a carbohydrate ratio of 1:7.
If your glucose level was lower than target after your meal, you need less insulin to cover the meal
Example: If your tested ICR was 1:8, you need to increase the ratio to get less insulin. Try a carbohydrate ratio of 1:9 next time.
Assess your new ICR for a few days before you decide to change anything further.
Insulin-to-carbohydrate ratios at different times of the day
Sensitivity to insulin naturally changes over the course of the day, and most people will need different ICRs for different times of the day. Many people find they are slightly more insulin resistant in the morning than later in the day, and that they need more insulin to cover their breakfast meal than if they ate the very same meal for lunch or dinner. For example, you might have an ICR of 1:8 at breakfast, 1:10 at lunch, and 1:12 at dinner – meaning the same amount of insulin will cover more carbohydrates as the day goes on. It is therefore important to work out individual ICRs for morning, afternoon, and evening (or breakfast, lunch and dinner).
A note on exercise around mealtimes
You may also find that on days you exercise before or within 3-4 hours of a meal that you will need much less rapid-acting insulin to cover the meal than if you hadn’t exercised. You’ll need to adjust your ratio accordingly, to take less insulin if you’ve exercised before you eat or plan to exercise after a meal time.
Get help if you need it
Doing an ICR test is easy but assessing and analysing the results can be challenging. When you test your ICR, enlist your diabetes educator’s support to review your results and help you make adjustments. Keep detailed records, including any factors that may have influenced the test, such as stress or an insulin pump infusion set change.
If you need help assessing your insulin-to-carbohydrate ratios, we're here to help. Get in touch with the Family Centre's dietitian and diabetes educator Amy Rush here: firstname.lastname@example.org or call the Family Centre on 9446 6446.