Get the Most Out of Your Insulin Pump: insulin on board & bolus calculator
One of the benefits of an insulin pump (compared to injections) is that it does the math for you. It has a Bolus Calculator to figure out how much insulin to give for a given meal or to correct a high blood sugar; it also figures out the blood-glucose reducing power still left from a previous insulin dose, that is the amount of Active Insulin Remaining, otherwise known as Insulin on Board. Here we look closer at how these insulin pump features work, and offer some tips to use them safely and most effectively.
Remember that an insulin pump delivers a single type of rapid-acting insulin in two ways:
- a basal dose that is pumped continuously at an adjustable basal rate to deliver insulin needed between meals and at night.
- a bolus dose, given as a lump-sum dose either to cover the carbohydrate-containing food which we eat (called a “carb bolus” and specified by an Insulin-to-Carb (I:C) ratio) or to correct high blood glucose (called a “correction bolus” and specified by a Correction Factor, also called an Insulin Sensitivity Factor (ISF), setting on a pump).
When a bolus is needed in an injected insulin program, someone — either the parent or the child himself — needs to crunch the numbers. That is, for a carb bolus, the number of carbs needs to be plugged into an insulin-to-carb ratio to calculate the insulin dose; for a correction bolus, the current blood glucose reading needs to be plugged into a correction formula. In both cases, someone needs to do some form of calculating.
The Bolus Calculator feature on an insulin pump does this calculation for you. When your child plans to eat carb-containing food, you manually input the number of carbs. Then the Bolus Calculator*, using the I:C ratios that you previously programmed into the pump, will calculate the insulin dose (bolus) needed to cover those carbs. Similarly, if you also enter a current blood glucose reading (which may be automatically entered for you if you use a blood glucose meter that communicates with your insulin pump), the Bolus Calculator will calculate a correction dose of insulin using the ISF settings that you previously programmed into the pump.
*Different insulin pump companies use slightly different terminology to refer to the Bolus Calculator: Medtronic calls it “Bolus Wizard;” Omnipod, the “suggested Bolus Calculator”; t:slim, "bolus calculation;" and the YpsoPump App refers to the “bolus calculator.”
The Bolus Calculator will add or subtract insulin from a carb bolus for an above or below target blood glucose reading. It may also, under certain circumstances, subtract the amount of active insulin which remains from previous boluses (see below).
For further flexibility, if you anticipate needing less insulin because of planned exercise, or more insulin because of a test or other stressor, you can override the default I:C and ISF settings in the pump, changing the amount of insulin delivered in that one occurrence by bumping up (or down) the recommended dose.
In addition, you always have the choice to bypass the Bolus Calculator, do the calculations yourself and deliver a chosen number of units of insulin (without having to enter a carb amount nor a current blood glucose reading). That is, you can use the pump as a high-tech insulin pen/syringe when and if you choose to do so. (Medtronic and Omnipod refer to this function as “Normal Bolus”; t:slim refers to it as a "Bolus Using Units"; YpsoPump as “Standard Bolus" in units.)
Active Insulin Remaining also known as Insulin on Board (IOB) or Bolus on Board (BOB)
When we gave our son insulin by injection, sometimes we would give him a correction for high blood glucose, just to find two hours later that his BG was largely unchanged. So we repeated the dose, and (you guessed it) soon his BG was low. One likely explanation for what happened was that when we gave the second correction dose at 2 hours, there was still some active insulin remaining in his system from the first correction, since the blood-glucose-lowering effect of fast-acting insulin lasts about 3-5 hours. So the power remaining in the first dose would have continued to decrease our son's blood sugar, even if we hadn't delivered the second one. If this remaining insulin is not accurately taken into account, then the second correction dose amounts to an over-correction of blood glucose, resulting in a low when everything starts rolling downhill. This is known as “insulin stacking”.
In an insulin pump, the Insulin on Board, or Active Insulin Remaining, feature takes this already-delivered but yet-unused insulin into account, reducing the potential for over-treatment of high blood sugar.
Different insulin pump companies use slightly different terminology to refer to the active insulin remaining: Medtronic calls it “active insulin;” Omnipod, t:slim, and YpsoPump call it "insulin on board (IOB)."
How exactly it does so varies between different pump manufacturers:
- For high blood glucose: the amount of active insulin will be subtracted from the correction bolus for above-target blood glucose. For example, if the pump bolus calculator recommends a correction bolus of 1.0U, but IOB is 0.2U, the bolus calculator will recommend delivery of 0.8U to bring that high blood glucose down to target.
- To cover food: the amount of active insulin may also, under some conditions, be subtracted from a carb bolus. Some pumps subtract IOB from a bolus for carbs if blood glucose is below-target (and the current blood glucose reading is taken into account – i.e. you “add BG” into the calculation), while some pumps do not subtract IOB under these conditions. If you are unsure of how your child’s pump deals with active insulin under certain conditions, please contact the pump manufacturer for clarification.
Things to Note About Active Insulin:
- Active Insulin Remaining does not involve basal insulin; it only takes bolus insulin into account. For this reason, “Bolus On Board” (rather than “Insulin on Board”) might be a more accurate label for this feature.
- Insulin on Board will only be taken into account if you add your child's current BG to a Carb Bolus. That is, if you have not checked BG lately, or if you have checked but don't enter it into the calculation when giving a carb or correction bolus, the remaining active insulin will not be taken into account when the pump calculates a recommended insulin dose.
- IOB recommendations are not very useful in the 90 to 120 minutes post-meal. During this time, the effect of the food is still being worked out, so much of the current remaining insulin will likely be used to cover the still-digesting carbs (in which case, it is not “excess” insulin at all). Some pumpers choose to disregard IOB during this time, instead covering the intake of any additional carbs with a full carb bolus.
Tips from the Trenches
We have had to learn this lesson over and over and over again!
Our young son typically has a bedtime snack within 2 hours after supper. When we use the Bolus Calculator for this snack, we see the daunting amount of IOB quoted on the screen and often fear a low. But if we follow the pump recommendations and reduce his snack bolus to take this active insulin into account, quite predictably our son’s blood sugar is crazy high a few hours later. We are learning to ignore the IOB for his bedtime snack if that snack falls 2 hours or less after supper, as all that seemingly excess “insulin on board” is not excess at all – it’s needed to cover the still-digesting food from supper. Therefore, our son’s body still needs insulin to cover the carbs in the bedtime snack. ~Michelle
- Calculations of Active Insulin Remaining are influenced by other programmed rates, such as Duration of Insulin Action (DIA)and the Target BG:
The pump will base its calculations of Active Insulin Remaining on the amount of time you have told it that insulin will last. If you have programmed in 3 hours as the Duration of Insulin Action (DIA), it will calculate how much insulin is left in the portion of those 3 hours which remain; if you have programmed in 5 hours as the DIA, it will calculate a very different recommendation based on this very different time period. As a result, very short DIA settings may underestimate the amount of insulin that is still active, thereby increasing the risk of insulin stacking, and increasing the risk of low blood glucose as a result. Conversely, very long DIA settings may over-estimate the amount of insulin that is still active, thereby increasing the chance that BG will be high later.
In addition, since the pump subtracts active insulin for a below-target blood glucose reading, pump recommendations will also be affected by the target blood glucose that you have programmed in. If you have set 8.0 mmol/L as the target, then it may subtract insulin for any current BG reading below that, including, for example, a BG of 7.0 mmol/L. However, if 6.0 mmol/L is the target BG, then the pump would not subtract insulin for a BG reading of 7.0; in fact, it will not subtract insulin until BG is below 6.0. As you can see, these different BG target settings will result in different bolus recommendations, which will, in turn, result in different BG readings in the few hours after insulin delivery.
Therefore, the pump settings you program in for Duration of Insulin Action (DIA) and Target BG are critical for the Insulin on Board function to work effectively.
We suggest that you review and make any necessary changes to the DIA and BG Target settings in order to get the most out of the BOB feature of your child’s insulin pump. For more information on how to individualize these settings for your child’s needs, consult your child’s diabetes health care team and check out the following WaltzingTheDragon.ca articles:
The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.
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