All About Insulin Sensitivity Factors and Correction Factors

Whether you have type 1 or type 2 diabetes, if you have been prescribed fast-acting insulin (via pump, pen, or syringe), you need to know your “insulin sensitivity factor” or “correction factor.”

This factor describes how much one unit of fast-acting insulin will drop your blood sugar in the absence of other factors, such as exercise or food. Every person has their own correction factor, and it is vital to know yours so that you have the best chance of properly fixing high blood sugar. Delivering the right amount of insulin reduces both the risk of using too much insulin, which will lead to hypoglycemia, and the risk of using too little, which will not bring your blood sugar back into a healthy range.

What is an “Insulin Sensitivity/Correction Factor”?

First, to be clear: the terms “insulin sensitivity factor” and “correction factor” describe exactly the same thing, and are used interchangeably by experts. Throughout this article, we will use the term “insulin sensitivity factor” and its abbreviation, ISF.

Your ISF is how much 1 unit of fast-acting insulin will lower your blood sugar in the absence of other factors such as food and exercise. (Your factor will depend on the units of measurement you use to measure your blood sugar; in the United States, we usually use mg/dL.)

As an example, suppose you know that your ISF is 1:75, meaning that one unit of fast-acting insulin will drop your blood sugar by about 75 mg/dL. If your starting blood sugar level is 200 mg/dL and you want to take a correction dose to bring it back in range, 1 unit of insulin can be expected to bring your blood sugar level down to 125 mg/dL.

ISFs will vary significantly by person: depending on your level of physical activity, insulin sensitivity, age, diet, body weight, genetics, and other unknown factors, your ISF might be anywhere from 1:2 to 1:200!

In our experience, children are much more sensitive to insulin than adults, which makes sense, given their tiny bodies. Teenagers, however, can be especially insulin resistant — the hormonal changes of puberty tend to make diabetes management especially complicated. People with type 2 diabetes, a condition largely defined by insulin resistance, can have very low ISFs, requiring large doses of insulin to bring their blood sugar back down into a healthier range.

How to Estimate Your ISF:

Our writing here is highly indebted to the work of Gary Scheiner, an influential diabetes educator and physiologist. Scheiner wrote an excellent book on the use of insulin therapy in diabetes titled Think Like a Pancreas.

If you have no clue what your ISF might be, Scheiner provides a chart that can give you a very good starting point. In the left column, find your average total daily insulin usage (basal and bolus), and look across to the right. Americans will generally use the middle column, which lists blood sugar measurements in mg/dL; Europeans may use the righthand column, which uses mmol.

Please keep in mind that this is merely a starting point, an estimate based on an estimate of your overall insulin sensitivity. Consider it a ballpark figure that you will probably need to adjust through experimentation.

It’s also important to note that this chart is likely to be less accurate for people that choose low- or high-carbohydrate diets, who tend to use more or less rapid insulin for meals than they would with a more mainstream eating pattern.

How to Verify the Accuracy of Your Estimated ISF:

In this excerpt from Think Like a Pancreas, Gary Scheiner explains how to double-check the accuracy of your estimated ISF. Next time you have high blood sugar, with no fast-acting insulin on board and no plans to eat or exercise, try the following:

1. Test your blood sugar at least 4 hours after your most recent bolus of rapid-acting insulin.

2. If the blood sugar is elevated, calculate and give the appropriate dose of insulin. Go about your usual activities, but do not eat or exercise for the next several hours.

3. Test your blood sugar 4 hours later.

4. Calculate how much your blood sugar came down, and divide by the number of units you gave. This should come close to your sensitivity factor. If it does not, repeat the process the next day. If the results are similar to those from the first day, adjust your sensitivity factor accordingly.

This process allows you to perform a kind of natural experiment on your own body, observing how insulin changes your blood sugar level with as little influence as possible from extraneous factors.

Keep in mind that any one such experiment will not give you a definitive number. Blood sugar is just too complex for that, and try as we might, it’s not easy to select a time that is truly free of other diabetes management factors. If you can repeat the experiment — keeping a diabetes journal is a great idea — you’ll have an even better idea of your ISF.

Insulin Sensitivity is a Moving Target

Your insulin sensitivity factor is not set in stone; it is quite likely to change as your body changes with age and you experience shifts in your metabolic health. Some of these changes are predictable — a new medication or workout program might improve your insulin sensitivity, or weight gain might cause more insulin resistance — but other changes may seem inexplicable.

Some especially careful members of the Diabetes Daily community have also learned that their insulin sensitivity reliably shifts over time. You may be more insulin resistant in the morning than in the evening, for example, or you may experience consistent insulin resistance related to your menstrual cycle.

It’s very important to realize that your ISF, even if you’ve tested and verified it multiple times, is still just a starting point. In reality, you may rarely find yourself needing to use a correction dose when your blood sugar is in fact completely stable. Whether it’s insulin on board from an earlier meal, protein and fat from a meal hours ago, or illness, stress, hormones, or lack of sleep, there are many factors that can change the amount of insulin you need today. Attempting to adjust your ISF for all of these factors can be as much a matter of art as of science. An insulin pump, a smart insulin pen system, or a diabetes management app can help.

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