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Who is smarter; me or my pump?

Updated: Mar 19, 2020

When is overriding your bolus wizard appropriate? Although a somewhat advanced pump topic, it is incredibly important to know when NOT to trust your bolus wizard. I know…this is the opposite of what you’re frequently told, but remember…your pump, although very “smart”, is still just a machine. It is NOT SMARTER THAN YOU! Many of today’s pumps do not subtract insulin on board from carb boluses. Let me give you an example of what this means… Yesterday, Lisa had spaghetti for lunch around noon. To cover the carbs in the spaghetti she input 100g into her bolus calculator and took the recommended 10U of insulin (carbF=10). About an hour after lunch, Jerry (a coworker) brought her favorite cookie from across the street. She tested her BG at 1pm which read 126mg/dl. She entered 40 grams of carbs into her bolus wizard and took the 4U recommended dose. Two hours after that, Lisa was shaky and dizzy. She tested her BG which read 51mg/dl. What happened? She trusted her bolus wizard…so why did she have this low? It turns out, when Lisa tested at 1pm, she had >5U of insulin on board. Unfortunately, she miscounted the carbs in her spaghetti and gave way too much insulin. She could have eaten that cookie without taking any insulin and in fact, maybe even needed a little more carbs to prevent a low. Here is how to test if your pump’s bolus calculator subtracts IOB from carb boluses… Pretend the following…DO NOT DOSE THIS INSULIN Start the test when you have >1U of IOB and enter into your bolus calculator the number of carbs required for 1U of insulin (ex: carbF=10, input 10 carbs). Now, enter a BG that is 1mg/dl higher than your target (ex: target=80-120mg/dl, input 121mg/dl). If your bolus calculator recommends a 1U bolus then it DOES NOT subtract IOB from carb boluses. Now, enter a BG 1mg/dl under your target (Target=80-120mg/dl, input 79mg/dl). If your bolus calculator says “no carb bolus needed” then that means that your bolus calculator DOES subtract IOB when your BG is below target.

If your pump does not subtract IOB from carb boluses, you can do this yourself by using this calculation: (Carb bolus + Correction bolus) – IOB = new bolus For more awesome tips in advanced pumping check out John Walsh’s book “Pumping Insulin” or visit

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