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Insulin: Carbohydrate Ration (ICR) Calculations So, how is the ICR determined for each individual patient? Knowing how to do this will be key to your patient’s success. Remember, the ICR (insulin-to-carbohydrate ratio) is the amount of carbohydrate (in grams) that is counteracted by 1 unit of rapid-acting insulin. Keeping in mind that there are no predetermined rates for everyone’s individual ICR and SF, the tables we will look at our nothing more than estimations—and good jumping off points for your patients to use as they accurately determine the amounts that work best for them. There are a few different ways for determining the ICR. Let’s look at the ‘500 Rule’ and Body Weight first. (6) The 500 Rule suggests dividing 500 by the total daily dose of insulin (including basal and bolus amounts) to calculate the grams of carbohydrates that may be covered by 1 unit of rapid acting insulin. Alternately, we can use body weight. This method suggests multiplying body weight in pounds by 2.8 and then dividing that number by the total daily dose. The numbers will be similar and remain to be tested by the patient.
Sensitivity Factor Calculations The next step is to estimate the sensitivity factor or SF. The sensitivity factor is the how much 1 unit of rapid-acting insulin will lower blood glucose (in milligrams per deciliter, or mg/dL). Here we can use the ‘1,700 Rule’, which suggests dividing 1,700 by the total daily dose to estimate the reduction of blood sugar by 1 unit of rapid-acting insulin. When making the first calculations regarding ICR and SF, it’s best to be conservative and careful. When initially calculating ICR and/or SF, it’s best to err on the side of caution, basing recommendations on a conservative dose of insulin and following up with the patient to make sure dosing is correct. The optimal amount of carbohydrate the patient needs is estimated from the calculations we just examined. However, it should be noted that the best mix of carbohydrate, protein, and fat will be unique for every patient—there is no one-size-fits-all approach to optimizing these macronutrients. Blood glucose levels that are routinely out-of-range can indicate that non-insulin variables like a calculation error in carbohydrate, delayed or missed boluses, incorrect bolus administration, hormonal affects, growth spurts, high fat or high protein content of meals, exercise and activity, or a change of routine may be to blame. However, once those have been ruled out, modifying the ICR and SF might be necessary.
Insulin: Carbohydrate Ratio Modifications ICR and SF shouldn’t be modified at the same time. When evaluating ICR, patients should be instructed to eat low-fat meals with a known amount of carbohydrates with which they are familiar. It should be noted that blood glucose levels should be assessed 24 – 72 hours before making any changes, and then again 3-7 days after making changes. (8) Note that if the two-hour post-meal blood glucose is within 30-60mg/dL of the pre-meal blood glucose, ICR does not need adjusting. 1. If the two-hour post-meal blood glucose has increased by more than 60 mg/dL from pre-meal blood glucose, ICR should be decreased by 10% to 20% (or 1 to 2 g/unit). 2. If the two-hour post-meal blood glucose has increased less than 30 mg/dL from pre-meal blood glucose, ICR should be increased by 10% to 20% (or 1 to 2 g/unit). Let’s take a look at an example. In this example our patient is using an ICR of 15. Take a look at the first line on this chart. The patient’s two-hour post-meal blood glucose rose quite a bit more than is desirable, assuming a target of 110 mg/dL for this example. Ruling out non-insulin variables that might affect dosing, the patient should decrease his ICR from 15 to 13 and see how that goes.
Sensitivity Factor Modifications Now, let’s look at how to modify the sensitivity factor. The sensitivity factor is correct if the two-hour post-correction blood glucose is halfway to the goal blood glucose (and at the goal by four hours). Decrease the SF by 10-20% if the two-hour post-correction blood glucose isn’t halfway to goal (or isn’t at goal by four hours). Increase SF by 10-20% if the two-hour post-correction blood glucose is more than halfway to final goal (or below the final goal by the four-hour mark). Let’s look at another example. This one involves a patient us an SF of 50, and with a blood glucose goal of 110 mg/dL. Here, if we look at the first line on the chart we see that the two-hour post-correction blood glucose is more than halfway to the goal of 110 (and below the goal at four hours—at 98). Ruling out non-insulin dosing variables, the patient should try increasing his SF a bit to 60.
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