Frequent boluses are easy to give on a pump, but this can also lead to insulin stacking of BOB. BOB, also known as insulin on board or active insulin, is the amount of bolus insulin remaining from recent carb and correction boluses that is still actively lowering your glucose. BOB measures only bolus insulin activity on the assumption that your basal rates are accurate. When the correct DIA setting is put in your pump, BOB measures the insulin activity that remains from any boluses given within the DIA time.
Insulin stacking is a buildup of bolus insulin when two or more boluses overlap. The goal is to track BOB accurately so that insulin stacking does not cause hypoglycemia. In the APP study, 64% of all boluses were given within 4.5 hours of a previous bolus, well within the time in which BOB remains active. BOB rises when a correction bolus is increased to lower a glucose faster and following increased physical activity.
Your pump bolus calculator can manage insulin stacking from BOB only after you enter your glucose reading. If you give a carb bolus without entering a glucose value, your pump will recommend a full carb bolus, regardless of how much BOB remains. Don’t give blind boluses – always test and enter your glucose before you bolus. Be sure to enter each reading by hand if your meter does not automatically send them to your pump. Future pumps will hopefully alert you if you start to give a bolus while significant BOB is present. This would allow you to test and enter a glucose to minimize insulin stacking.
One of the great advantages of an insulin pump is that it calculates your BOB for you in a reasonably accurate way. BOB is shown on one or more screens. Once the DIA is correctly set, know where to find your BOB on your pump screens.
How the Bolus Calculator Handles BOB for Bolus Recommendations
Although the DIA time has the greatest impact on whether BOB is measured accurately, how your BC calculates BOB can also create some hidden insulin stacking. Because some pump BCs ignore BOB when giving carb boluses, different pumps can recommend significantly different boluses for the same situation. An example of this is shown in Box 8.9 where for the same situation one pump will recommend that no bolus be given and another recommends taking 6 units.
|What Do Pumps or Meters Count as BOB and How Is BOB Applied?|
|Is Bolus Included in BOB?||Is BOB Subtracted from Bolus?|
|Best Practice 1||Yes||Yes||Yes||Yes|
|Accu-Chek Solo *||Yes||Yes||Yes||Yes|
1 Best practice is the least likely to allow insulin stacking and cause hypoglycemia
AccuChek Spirit and Combo pumps calculate bolus doses based on glucose values rather than BOB (most situations). Insulin stacking can occur with this method as well.
© 2012 Diabetes Services, Inc.
All pump BCs subtract BOB from any correction bolus given for a high glucose to help avoid insulin stacking. However, BOB can sometimes be greater than the correction bolus needed for the current glucose, or the glucose may already be low when BOB is still present. This excess BOB would ideally be deducted from any carb bolus being given, or trigger an alert showing how many grams of carb are needed to treat the impending low. No current pump does this.
Most of today’s pumps do not take excess BOB away from carb boluses when the glucose is above the target glucose or above the bottom of the target range. Some will once the glucose goes below these levels. Excess BOB can occur after increased physical activity or after taking a larger bolus than necessary to bring a high glucose down quickly. Box 8.10 covers how to find a safer bolus in these situations.
Today’s pump BCs use one of five methods to adjust a carb bolus for excess BOB. Carb boluses should be adjusted when the glucose is low or when there is excess BOB that will make it go low. Keep in mind that the DIA time setting has the greatest impact on insulin stacking, but how a pump adjusts a carb bolus for excess BOB will significantly change a bolus recommendation for an identical situation. Know how your own pump BC works, as well as how to calculate an accurate carb bolus yourself because it is often wise to adjust or override bolus recommendations when needed.
8.10 Accurately Calculate Carb Boluses
Anytime you’ve tested your glucose and want to give a carb and/or correction bolus, check your BOB first to determine whether you need a correction bolus (not enough BOB) or correction carbs (too much BOB). To find out:
- Check whether you have excess BOB – your BOB is larger than the bolus needed for the current glucose.
- When BOB is smaller than the correction need, take the recommended bolus.
- When BOB is larger than the correction bolus needed, use this:
Accurate Bolus = (Carb Bolus + Corr Bolus) - BOB
- If your answer is a positive number, that is how many units you need as a bolus.
If BOB is greater than the units needed to cover your glucose and planned carbs (the answer is a negative number), more carbs than planned may be needed.
Most pumps show the BOB on the same screen as the carb and correction bolus recommendations. Here, it is easy to add the carb and correction boluses together and then subtract the BOB to get the safest bolus dose.
Five methods that pump Bolus Calculators use to handle excess BOB for carb boluses:
- The pre-2012 Omnipod pump doesn’t count carb boluses as BOB. This assumes that all carb boluses are accurate and nothing like extra activity has caused excess BOB to be present. Carb boluses usually make up at least 80% of most pumper’s BOB, so this method can underestimate excess BOB. This method is not recommended. Omnipod’s 2012 pump should have an option
to include carb boluses into BOB calculations, similar to Method 2.
- Medtronic BCs do not subtract excess BOB from carb boluses. They will, however, lower a carb bolus enough when a glucose is below the target range to bring the glucose up to the bottom of the target range. When your glucose is above the bottom of the target range but excess BOB present, a recommended bolus may at times be more than you need. See Box 8.9. This approach carries less risk for those who have few lows.
With these two pumps, be careful when insulin stacking is present near bedtime. If you bolus for a snack and your glucose is above target, the BC will ignore BOB and recommend a full carb bolus. Before giving a bolus and going to sleep, check whether you need to eat carbs because you have excess BOB or you need to take a correction bolus because not enough BOB remains to cover the current glucose. For these BCs, it is important to double-check bolus recommendations at any time that BOB exceeds the correction bolus.
- Animas and Tandem BCs subtract BOB from carb bolus recommendations only when the glucose is below the correction target or target range. If the glucose is lower than the correction target or range, all excess BOB is subtracted from the carb bolus to bring the glucose to the middle of a range (Animas) or to 70 mg/dl (3.9 mmol/L) (Tandem). If the glucose is in or above target, these BCs act like those in #1, and ignore excess BOB.
Bolus recommendations from these pumps can change dramatically depending on whether the glucose is above or below target. For example, someone have have a correction target of 120 mg/dl (5.6 mmol/L) and have 6 units of excess BOB a couple of hours after their last bolus. If they want to eat carbs that normally require a 6 unit carb bolus, no bolus will be recommended if the glucose is 119 mg/dl, but a full 6 unit bolus is recommended if the glucose is 121 mg/dl. These pumps provide some protection against insulin stacking when the glucose is below target or below 70 mg/dl. They work for people in reasonable control who do not experience significant lows.
- The Accu-Chek Solo and the bolus calculator on the European version of the Abbott InsuLinx Freestyle meter (whole units only) subtract excess BOB from carb boluses regardless of whether the glucose is above or below target This method was first outlined on page 72 in the first edition (1989) of Pumping Insulin.
This carries less risk for unexplained lows because hidden insulin stacking is avoided. This pump is best for someone who has frequent or severe lows, who has hypoglycemia unawareness, or who wants reasonable readings with less risk of going low.
This method appropriately decrease carb boluses to account for excess BOB even when the current glucose may still be high. If you are having frequent highs after meals, the subtraction of excess BOB from carb and correction boluses is NOT causing the high readings. Check instead whether your carb boluses are too small (carb factor (CarbF) too large), your carb counting is off, or your basal rates are too low.
- The last method is called the HypoManagerTM. It was used by Smiths Medical in the Deltec Cozmo pump and is not currently available, but provided the safest bolus recommendations. When excess BOB is present, the pump would tell the wearer how many grams of carb they need either now or later to offset the excess BOB and avoid going low later. Refer to Box 10.4 to see how to do this yourself.
This method helps avoid over-treatment of lows and guards against an upcoming low glucose by warning the pump wearer that one may occur later from excess BOB even though their current glucose is normal or elevated.
8.9 Bolus Recommendations Differ from Different Pumps
Even with an accurate DIA time, different pump BCs subtract BOB differently from carb and correction boluses. As an example, Alex is a 46 year old pump wearer whose CarbF = 10 g, CorrF = 30 mg/dl, and correction target = 120 mg/dl. For four nights in a row, he ate the same dinner at the same time and took 12 u for 120 grams of carb.
Two hours after dinner, Alex wants to eat a 60 gram dessert when he still has 6 u of BOB left from a 12 unit carb bolus he gave for his dinner. He would usually take 6 u to cover this dessert. In this situation, however, he has to consider his current glucose and BOB to get an accurate bolus dose.
On the first two evenings he played racquetball for 45 minutes just after dinner. On the 3rd he did not. On the 4th, he learned his daughter had become engaged to a twice divorced man who has a $30,000 gambling debt. His glucose before dessert on these four evenings was 119, 121, 210, and 300 mg/dl (6.6, 6.7, 11.7, and 16.7 mmol/L).
Alex’s true bolus need is easy to find: the 6 u of BOB still left from his dinner will cover the 6 u he needs for his dessert, so the only bolus he needs would be a correction bolus if his glucose is high. The table below shows boluses recommended by each of the methods found in current pumps for these different glucose levels.
|If Alex's Glucose Is:||Alex Needs||Method 1 Says||Method 2 Says||Method 3 Says||Methods 4 & 5 say|
|Night 1: 119 mg/dl||0 u||6 u||6 u||0 u||0 u|
|Night 2: 121 mg/dl||0 u||6 u||6 u||6 u||0 u|
|Night 3: 210 mg/dl||3 u||9 u||6 u||6 u||3 u|
|Night 4: 300 mg/dl||6 u||12 u||6 u||6 u||6 u|
Method 1, with carb boluses not included in BOB, recommends 6 u of excess insulin for each glucose. Methods 2 and 3 start to be more accurate as Alex’s glucose approaches 300 mg/dl (16.7 mmol/L). With his glucose at 119 mg/dl (6.6 mmol/L), Method 3 calculates accurately and recommends 0 u, but if his glucose is 121 mg/dl (6.7 mmol/L), it recommends 6 u. Methods 4 and 5 give the most conservative (safest) doses.
Alex may realize he does not need an extra 6 units after playing racquetball when his glucose is normal, and that with a glucose of 300 mg/dl, he likely needs more than 6 units after learning his daughter’s wedding plans.
Pump wearers can more easily sort out the true setting adjustments that are needed for highs and lows when a BC has an appropriate DIA time setting, avoids insulin stacking by accounting for BOB, and recommends the carbs needed to offset insulin stacking.
No approach is perfect. For example, after eating a bean burrito, method 5 may recommend that too many free carbs be eaten because the glucose stays flat longer afterward. Some meals contain fats that create insulin resistance. Your glucose may be elevated hours later even though the BOB would normally cover this glucose.
Box 8.11 shows how to check whether your pump subtracts BOB from carb boluses when your glucose is above or below your correction target. In most situations, it is safer to subtract any excess BOB from carb boluses. If your glucose is above 140 or 150 mg/dl (7.8 or 8.3 mmol/L) with excess BOB, you may want to wait to eat carbs needed to cover this BOB until your glucose is lower.
If you are having low glucoses, Box 8.10 shows how to determine a safe bolus yourself. Remember that having frequent highs with few lows suggests that your TDD needs to be raised, along with better food choices if this is needed. Frequent lows suggests that your current TDD is too high.
Never take a recommended bolus that seems too large before checking this recommendation using Box 8.10. You may also want to recheck your glucose to ensure your reading is accurate. Visit www.diabetesnet.com/diabetes_technology/dia.php for more information on the different methods pumps use to calculate BOB.
8.11 Check How Your Pump Handles Bolus On Board (BOB)
Having excess BOB can happen anytime. It is more likely when your glucose is well controlled, following extra physical activity, or when your last bolus was increased to bring a high glucose down quickly. Do you use a pump BC that doesn’t subtract excess BOB from carb boluses? If so, unexplained lows can occur. See Box 8.8 to see how different pumps work.
To test whether your pump BC subtracts BOB from carb boluses, pretend the following:
- Any time you have more than 1 unit of BOB still active from an earlier bolus, enter your carb factor (CarbF) as the number of carbs you intend to eat into your bolus calculator. For example, if your CarbF is 1 unit for 12 grams, enter 12 grams of carb. The bolus for these carbs will be 1.0 unit.
- Next, enter a glucose that is 1 mg/dl (0.1 mmol/L) higher than your correction target (or target range). For example, if your correction target range is 100 to 120 mg/dl, enter 121 mg/dl as your glucose. (If 5.6 to 6.0 mmol/L, enter 6.1 mmol/L).
- If your pump recommends 1.0 unit, your pump BC DOES NOT subtract BOB from carb boluses when your glucose is above your target. (Don’t deliver the bolus!)
- If your pump BC says no bolus is needed for these carbs, your pump DOES subtract BOB from carb boluses.
- Next, check whether your BC will subtract excess BOB from carb boluses if your glucose is below your target. Enter a glucose value that is 1 mg/dl (0.1 mmol/L) below your correction target or target range. For example, if your correction target range is 100 to 120 mg/dl, enter 99 mg/dl as your glucose.
(If 5.6 to 6.0 mmol/L, enter 5.5 mmol/L).
If your BC says no carb bolus is needed, your BC DOES subtract BOB from carb boluses when your glucose is below your target range.
If your pump doesn’t subtract BOB from carb boluses, you may want to double-check your recommended bolus doses, as shown in Box 8.10.
BOB’s Blind Spot
Although carb and correction boluses are usually more accurate when you account for BOB, there are certain times when it is better to simply ignore BOB. After a meal bolus is given, a “blind spot” exists for about 90 to 120 minutes when the effect of carbs raising the glucose cannot be fully measured against the carb bolus insulin lowering it. Both you and your BC will have trouble determining an accurate bolus during this time. Any carbs eaten during this blind spot are typically covered with a full carb bolus.
To accurately measure BOB, the duration of insulin action (DIA) time must be correctly set in the pump.
Read more about bolus on board here.