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Conv-Insulin:Carb Ratio

If you have not done so already, we recommend that you read the following pages on insulin adjustment within Conventional Insulin Programs as background for the information that follows:
Insulin Action & How it Affects BG
Pattern Adjustment


Adjusting Insulin-to-Carbohydrate Ratios (for Conventional Insulin programs)

(by Allison Husband, RN, MN, CDE, Clinical Nurse Specialist, Diabetes Clinic, Alberta Children's Hospital)



What is an Insulin-to-Carbohydrate Ratio?insulin to carb ratio for type 1 diabetes

An Insulin-to-Carbohydrate ratio (I:C) tells you how much rapid-acting insulin to give to cover the food your child eats. (Or, in other words, the I:C ratio specifies how many grams of carbohydrates are covered by one unit of insulin.) That is, you give 1 unit of rapid-acting insulin for every X amount of carbs your child eats.
For example, if your child’s I:C ratio is 1:10, that means that 1 unit of rapid-acting insulin is given for every 10 grams of carbs eaten; if the I:C ratio is 1:15, 1 unit of insulin is given for every 15 grams of carbs eaten.
As an extension of this example, if your child uses a 1:15 ratio at breakfast you will give 1 unit of rapid for 15 grams of carb, 2 for 30, 3 for 45, 4 for 60, and so on (as well as 1.5 units for 22grams of carbs, and so on for other fractions of a unit).

Your child may use an Insulin-to-Carbohydrate ratio (I:C) if she is not on a meal plan which sets constant the amount of carbs for each meal.


Do I Need to Adjust the Insulin-to-Carbohydrate Ratio?


You may need to adjust the ratio if there is a pattern of either high or low blood glucose following the use of the ratio.

For example: If you are using a ratio at breakfast you will examine the lunch blood glucose results to determine if the ratio is working, or if it needs adjusting. If the ratio is working, a target blood glucose at breakfast will be followed by a target blood glucose at lunch.


If the lunch blood glucose consistently rises above target between breakfast and lunch, then consider changing the breakfast ratio so that you will be giving more rapid-acting insulin.

For example, if the ratio is 1:15, you may lower the second number to, say, 1:12 to give more insulin. Before using the adjusted ratio to dose insulin, it’s always wise do a little “test” to make sure you have changed the ratio in the right direction: If your child was previously using a 1:15 ratio and ate 60 g carb, you would have given 4 units. If you change the ratio to 1:12, you would now give 5 units for the same 60g carb (60 ÷ 12). Your change results in giving more insulin for the meal, so was, therefore, correct.
(If your “test” shows that you will be giving less insulin with the adjusted ratio, and you know that your child should be getting more insulin to correct the pattern of high blood glucose, then you moved the second number of the ratio in the wrong direction.)

If the lunch blood glucose falls below target two days in a row between breakfast and lunch, then consider changing the breakfast ratio so that you will be giving less rapid-acting insulin.
For example, if the ratio is 1:15, you may raise the second number to, say, 1:20 to give more insulin. Before using the adjusted ratio to dose insulin, again it’s wise do a little “test” to make sure you have changed the ratio in the right direction: If your child was using a 1:15 ratio and ate 60 g carb you would have given 4 units. If you now change the ratio to 1:20 you would give 3 units for the same 60g carb (60 ÷ 20). Your change results in giving less insulin for the meal, so was, therefore, correct.


Note: Adjusting I:C ratios can be more challenging than a simple pattern adjustment with set doses of insulin. Contact your health care provider if you are uncertain about what to do or you have made some insulin adjustments and they have not resulted in more target blood glucose results.


As a next step in insulin adjustment for a Conventional Insulin program, you may want to also check out the following article:
Adjusting Correction Insulin




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The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.

This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional.


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