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Insulin stacking

What is it and how can we avoid it?

Julie just finished cooking and then eating a delicious Thanksgiving meal. This was her first Thanksgiving as a diabetic and she was feeling accomplished. She counted all her carbs and even took additional injections for a second helping of mashed potatoes and for Uncle Dave’s infamous pumpkin pie. A couple of hours after dinner Julie decided to check her BG.

Her meter read 264mg/dl. What!? She thought she carb counted perfectly! She even measured out her mashed potatoes! Julie was so frustrated. She used her correction factor (1 unit: 50mg/dl > 100mg/dl) and bolused 3U of Novolog to correct her high.

Two hours later, Julie felt dizzy, tired, and sweaty. She tested her BG again and it read 46mg/dl️.

So, what happened🤦🏼‍?

Many of you have probably experienced a similar situation. This is just one of many possible examples of “insulin stacking”, a dangerous cause of low BGs for many unaware diabetics.

Although giving multiple injections (or multiple pump boluses) can allow for smaller and more exact dosing, it can make it difficult to determine how much bolus insulin remains active in the body. This is most important regarding correction boluses and is especially important at bedtime. A normal bedtime BG may end up low if there is residual insulin on board (IOB) and a high BG may not need any correction bolus if the IOB is enough to normalize it.

To determine how much, if any, insulin is needed for a correction, you need to first determine your IOB. Remember, that rapid acting insulin has an insulin action time of ~4hrs. It is therefore appropriate to assume that 25% of Novolog or Humalog is used each hour after it is dosed. For example; if 8 units are given as the original bolus, 2hrs later 4 units are still actively working.

If Julie gave herself a total of 10 units for her meal + pie, after 2 hrs she would have had 5 units left on board. If she had calculated this before giving a correction for her 264mg/dl BG, she would have seen that she didn’t need a correction at all (only 3 units needed for correction and 5 units on board)! In fact, she had 2 more units on board than she needed. Julie was probably headed for a low just based on her original insulin dose, but instead of trying to avoid one, she gave herself even more insulin! Julie’s low could have been prevented!

Remember this rule;

1 hr: 75% of original bolus active

2hrs: 50% of original bolus active

3hrs: 25% of original bolus active

4hrs: 0% of original bolus active

Although most insulin pump bolus calculators account for IOB, insulin stacking can still occur in pump users.

Say for instance you are at a potluck and have been snacking and blousing multiple times over the last hour. Your BG is 87mg/dl and you are going to eat another 20 grams of carbs. If you enter this into your pump it will calculate the dose to cover the 20 grams of carbs, but will not account for the 10 units of insulin you have on board from your previous boluses.

You need to remember; your pump is only a machine. It is not smarter than you so pay attention to your BG levels, IOB, and recommended doses! It is always a smart idea to calculate your own doses and compare them to your pump’s recommendation. Weigh all of the factors affecting your BG and then make an informed decision.

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