Ask The Expert
Insulin

In my answer about the 1500 rule, I referred to the unused insulin rule, in which I said: "A word of caution: each person is different, and there are many more details/rules to adjusting insulin that I haven't explained-for example, the unused insulin rule cautions you not to overdo giving yourself extra insulin, when you might still have insulin "on board" from the previous dose."

The terms unused insulin rule, or unused bolus rule (and also BOB for bolus on board) came from John Walsh, P.A., CDE, who has written extensively about intensive insulin therapy, especially pump therapy-most recently in Pumping Insulin, 4th edition.

The unused insulin rule helps you determine how much insulin you have left from an injection (or bolus, if using a pump) of short-acting or rapid-acting insulin. In other words, if you are tempted to take an extra correction dose of rapid or short-acting insulin within 4-5 hours of your previous dose, you must realize that there is still some insulin working from that previous dose - i.e. there will be an overlap of insulin effect. So if you are considering taking another does of insulin before the previous dose has run out, you will need to give yourself a reduced dose. Thus, the unused insulin rule can help you prevent unnecessary hypoglycemia.

Please Note: although this rule is commonly known in the diabetes field, especially with practitioners managing people with intensive insulin therapy, it is not "officially recognized" in the American Diabetes Association's textbook on Intensive Diabetes Management. This is mainly because the ADA does not publish guidelines/protocols unless there has been extensive research documenting the validity of the recommendation - i.e. evidence-based medicine. However, you will often find that there are a variety of regimens used to manage diabetes, based on trial-and-error, and on the fact that each person with diabetes may respond differently. In fact, awhile back, I had consulted the medical director of one of the major insulin companies about the unused insulin rule, and his response was similar to what I have just stated: that it makes sense, and it is probably reasonable, but that no studies have documented such specific tables and recommendations.

Consult your healthcare team before trying any of the recommendations from this link:

When you read about this rule in the related link below, you will learn that people who use insulin pumps have the option of setting their duration of insulin action in their pump; then the pump calculates the dose and will suggest a reduced dose if there has been insufficient time since the last dose. In addition, you will read that the duration of action of short or rapid-acting insulin can vary depending on the size of the dose, physical activity, and naturally, individual variation. So it is important that you discuss this with your healthcare team before following any of these principles.