Amount of Mealtime Insulin, calculation question

  1. Please bear with me here. This is likely a stupid question, but it's crucial for me to understand this.

    Before a couple weeks ago I had never heard of Insulin to Carb Ratio (I:C, ICR) or Insulin Sensitivity Factor (ISF). I understand how to calculate both of them. What I don't understand is exactly how to total things up. I need to be able to explain this accurately to patients, so I have a couple questions.

    To calculate both the ICR and ISF, you need to total up the daily amount of insulin. For example (just making up numbers), a patient is prescribed 20 U of Levemir daily and 10 U of novolog at each meal. Total = 50 U of insulin a day. For the ICR (500) it would be 10 and the ISF (1500) would be 30.

    Using the same example patient, their dinner totals up to be 60 CHO. Their pre-meal BG is 190 and their target is 100. To cover the CHO, they would take 6 U and for the correction it would be 3 U. So 6 + 3 = 9. Is that all they take? Or do they add the 10 U to the 9 and end up taking 19 U of novolog for that meal?

    Hopefully this makes some sense. I'm still learning and don't want to mess this up. I'm also afraid of looking like an idiot and asking a co-worker--bad thinking, I know, but at least I'm asking someone.
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    About adio

    Joined: Jul '11; Posts: 20; Likes: 7


  3. by   Justabitoff
    I'm a type 1 diabetic for 18 years, so although I'm not an RN, I think I can help answer your question.

    So ICR is the amount of carbohydrates that one 1 unit of insulin will cover. For instance, I'm 1:7 at breakfast. If I eat 50 grams of carbs at breakfast, I take approximately 7.14 units (on an insulin pump, if I'm on injections, 7 units).

    My ISR is how many mg/dls my blood sugar will drop if I take 1 unit of insulin. My ratio is 1 unit for every 40 mg/dls and my target is 100 mg/dl. If I wake up at breakfast and my blood glucose reads at 215 mg/dl, then I want to drop 115 mg/dl, and I take 2.87 (on an insulin pump, 3 if I'm on injections).

    So, if I woke up with a blood sugar of 215 mg/dl and I'm eating 50 carbs, then i would take a TOTAL of 10.01 units (or 10 on injections). Get it?

    What you wrote doesn't really make sense. If someone is prescribed to take 10 units of Novolog at each meal, then they need to eat the corresponding amount in the equation: carbohydrates / insulin ratio = 10. What's their ratio? Do they take 1 unit for 7 carbs? Then they eat 70 carbohydrates. If their ratio is 1:9, then for 10 units they need to eat 90 carbs.

    However, if what you meant to write was that their mealtime ratio is 1:10 (1 unit for every 10 carbs), then yes, if they ate 60 carbs, they'd take 6 units of insulin. Plus whatever they need for the correction.

    There are specific baselines for starting on insulin that you can calculate based on weight. There's a general guideline for units of insulin per kilogram of body weight. You start out conservatively, test blood glucose regularly, and adjust dosing from there. Similar for figuring out bolus ratios.

    If you give a patient 19 units with only 60 carbs when they have an insulin to carb ratio of 1:10, they are overdosing and risking a seizure.
  4. by   CDEWannaBe
    The Rule of 500 you're using to calcuate the ICR isn't reliable for a type 2 diabetic because they are still producing meal time insulin. If you decide to use it, be VERY conservative starting out. For example instead of a 1u=10g I might go instead with 1u=15g carbohydrate.

    Both ISF and ICR calculations are just a starting point. The math isn't quite that simple. For an adult a ICR of 1:15 and a ISF of 1:30 are pretty common. But then the patient needs to test 2 hours after eating to identify the actual rates. For the first few days of carb counting it's best to eat simple, meals that aren't high fat or high carb (frozen dinners) until the correct rate is identified. It's also best for the first few days to avoid exercise or any other variables that will affect blood sugar.

    John Walsh's book "Using Insulin" is a great resource for ICR and ISF. Get a copy if you don't already have one. ICR and ISF are part of "Intensified Insulin Therapy" which has been around about 20 years. It's allows for flexible eating and helps correct highs, but it's not perfect. It's important that the patient is willing to log insulin, food and activity and also that they're savvy enough to do it.

    In your example, then the patient would take just the 9 units of Novolog, and their regular Levemir at bedtime or whenever they take it.
  5. by   adio
    Thank you both for replying. To be honest, Justabitoff confused me even more so thanks CDEWannaBe for responding as well.

    I'm still learning these terms and how to apply them. I cleared things up this morning and I appreciate the book recommendation. After I go through the materials at work, I'm going to search out new information so that helps.

    Thanks a bunch!
  6. by   Justabitoff
    My response was just to show you a diabetic's life in action, since your "patient" was hypothetical. I thought it would be easier to see how calculations are made and added together by describing a breakfast dose of a real person.

    Your question was also a bit unclear, but you only add together the insulin to cover the carbs being consumed, and the correction dose to bring a specific BG down to target. Whatever ratios that have been devised using the "formula" are usually adjusted, as CDEWannabe explained, because each person is different. I'm not sure why you thought you needed to add an additional 10 units.
  7. by   CDEWannaBe
    Think the 10u was because the patient's normal insulin routine consisted of taking that amount at each meal. Adio needed clarification that the ICR and ISF dose would take the place of that 10u.

    So glad there are other type 1s on this board! I've had type 1 for 35 years and am currently in nursing school in the hopes of becoming a diabetes educator. There are some great diabetes specialists on here, and I'm so glad. The disease effects so many people, but many nursing and med schools still teach older information about diabetes management.