Insulin sensitivity or insulin correction factor - calculating and testing made easy

But let’s start again, from the top. If you’re on a course of intensive insulin therapy with multiple daily injections you’ll normally inject regular bolus insulin or rapid-acting bolus insulin before meals and additional long-acting basal insulin once or twice a day. The bolus insulin includes the meal insulin and the corrective insulin. For people who calculate carbohydrates, the carbohydrate-to-insulin ratio (carb ratio) is determined by the doctor at the start of the insulin therapy. The insulin sensitivity, also known as insulin sensitivity factor or insulin correction factor is also defined at the start of the therapy. Some patients are given a schedule that shows how many units of insulin they need to take when certain glucose values are reached, and others calculate the correction they need to make themselves using the insulin sensitivity factor.

But the following definition applies in both cases:

👆 The insulin sensitivity indicates how much one unit of insulin reduces the glucose level. So the corrective insulin is the insulin required to return elevated glucose values to within the target range.

Many people use the same insulin sensitivity factor/ insulin correction factor throughout the day. But since insulin effects fluctuate for each person depending on the time of day it can be helpful to use insulin sensitivity factors that vary depending on the time of day as well.

If you adjust the factors to your daily rhythm, the insulin sensitivity will look like this for many adults with diabetes:¹

These values may also differ from one individual to another depending on how sensitive they are to the insulin effect or what their insulin resistance levels are.
You can use a formula to get a rough initial idea of how much one unit of insulin can lower the blood glucose level in adults. But each individual needs to check this with their diabetes team. If necessary it can also be adjusted to reflect how the insulin requirement varies at different times of day based on tests and historical values.

In case you are confused by the formulas, here is a practical example for John, who wants to calculate his insulin sensitivity factor:

1. To get your total daily dose, add up all your usual meal time insulin and basal insulin. 10 units at breakfast, 6 units at lunch, 9 at dinner from a rapid acting analog insulin and 25 units of a long acting analog insulin at 10 p.m.. -> 10 + 6 + 9 + 25 = 50 Units Total Daily Dose (TDD)
2. Correction Factor (for someone calculating in mmol/L)  = 100/50 = 2

Therefore, one unit of rapid acting analog insulin would lower Johny’s glucose by 2 mmol/L over the next 2 to 4 hours.

The factors may change over time:

It’s important to remember that the insulin requirement doesn’t always stay the same. The bolus insulin requirement as well as the basal insulin requirement can change over time due to fluctuations in body weight or hormonal changes.

👆 So it is advisable to check the insulin sensitivity if glucose values do not return to the target range 2-3 hours after the corrective insulin is administered (when rapid-acting insulin analogs are used).

But before you take a closer look at the insulin sensitivity factor, review your injection sites. Can you feel hardened areas or even see bumps in the mirror? If you can, this may indicate “lipohypertrophy”, which extends the time needed for the insulin to be absorbed and take effect. The best thing to do in this situation is to change the injection site to achieve a reliable insulin effect.

If you are generally struggling with high values, always start by checking whether the basal insulin dose is sufficient to keep values stable within the target range without meals as well. The article “How To Successfully Test Your Basal Rate“ contains instructions on how to test for this. The next step is to test the carbohydrate ratios.

The insulin sensitivity test:

Before the test:

Start the test when your glucose value is significantly elevated and you can wait a few hours until your next meal. Food containing small amounts of protein such as a few nuts, a little cheese or a boiled egg can be eaten during the test.²

• Baseline blood glucose value above 200 mg/dL (11.1 mmol/L) (value should not be elevated for more than 4 h.)
• Do not administer insulin
  • for 3–4 hours before the test if you are using rapid-acting insulin analogs
  • for 4–6 hours before the test if you are using regular insulin⁵
• It should be 3 hours since your last meal.²
• No values < 70 mg/dL (4 mmol/L) in the last 6 hours
• No unusual physical activity before or during the test
• No alcohol in the last 12 hours.
• Do not conduct the test if you are under severe stress or have a febrile infection.
• People with type 1 diabetes should not have any metabolic abnormalities with elevated ketone levels.

Conducting the test:

• Using your current insulin sensitivity factor, correct the elevated glucose values to the target value 100 mg/dL (5.6 mmol/L) or to the target value defined for you.²
• No carbohydrate intake during the test.
• No unusual activity levels during the test.
• The glucose values should be checked hourly up to 5 hours after starting the meal.
• If hypoglycemia of < 70 mg/dL (3.9 mmol/L) occurs it is essential to discontinue the test and take grape sugar!
• If the glucose value rises despite the correction, the test may have to be discontinued and another corrective dose administered.

Evaluating the test:

• The insulin sensitivity factor is correct if the correction lowers the high glucose value to within the target range between 80–130 mg/dL (3.9–6.8 mmol/L) after a few hours.2 (If rapid-acting insulin analog is used, after approx. 3 hours, if regular insulin is used, after approx. 5 hours.)
• If the glucose values are still elevated or below the target range after a few hours, the insulin sensitivity factor is not correct. In this case, decide together with your diabetes team how to reset the correction factor and then repeat the test again.

It’s worth the effort!

It takes time and effort to check the insulin sensitivity if the therapy isn’t going so well. But it is worth the effort as it allows you to optimize the insulin therapy and get the abnormal glucose values back into the target range.

Even if it doesn’t immediately work with the adjusted factors, don’t throw in the towel. Test the new factors for a couple of days and check whether the target range is reached after correcting elevated glucose values.

Be patient, even if higher values persist. Don’t apply multiple corrections too quickly one after the other. Give the insulin time to take effect. Applying multiple corrections quickly one after the other means the dose effect curves will overlap and there may be a risk of hypoglycemia.

The insulin sensitivity is also an important parameter that you need to program a bolus calculator. A bolus calculator can provide useful support in your everyday life, particularly when using changing insulin sensitivity factors!

Sources:

[1] Thurm, Ulrike, Gehr, Bernhard (2020) CGM- und Insulinpumpenfiebel, (4.Auflage), Mainz
[2] Walsh, John et.al. (2017). Pumping Insulin, everything for success on a pump and CGM. (6.Auflage), San Diego
[3] Warshaw, (2008) 2nd edition, Practical Carbohydrate Counting: A How-to-Teach Guide for Health Professionals, American Diabetes Association, p. 61
https://doi.org/10.2337/dc23-S006
[4] SUBITO Schulungsprogramm (Schulungsporgamm für die Insulinpumpentherapie)