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Clinic Chat: Is your insulin sensitivity factor correct?

Last month’s Clinic Chat covered how to test and adjust your insulin-to-carbohydrate ratio. This month we look at how to work out your insulin sensitivity factor, or ISF. Like an insulin-to-carbohydrate ratio, your ISF is programmed into your pump or bolus calculator if you use these devices. If not, it is something you need to commit to memory. Your ISF can also be known as your “correction ratio” or “correction factor”. 

Insulin sensitivity factors

Your ISF is the number of mmol/L that one unit of insulin will lower your blood glucose level by.  For example, a ratio of 1:2 means that one unit of insulin will drop your blood glucose level by 2mmol/L. Therefore, if a person with an ISF of 1:2 needs to lower their blood glucose from 10 mmol/L down to 6 mmol/L (ie by a total of 4 mmol/L) they will need to take two units of insulin.

Target blood glucose levels are individual, and you may have different targets for different times of the day. For example, you may aim for a blood glucose level of 5 mmol/L during waking hours but prefer to aim for 6 mmol/L before bed. Knowing your ISF helps you calculate the amount of insulin you will need to take to bring your blood glucose back to your target level, which can help you to maintain stable blood glucose levels throughout the day.

The general rule that diabetes educators use to calculate a person’s ISF is called the “Rule of 100”. The calculation is as follows:

100 ÷ Total Daily Dose

Your Total Daily Dose is the total amount of basal/long-acting plus bolus/rapid-acting insulin you use in one day. For example:

Lantus dose = 22 units

Average Novorapid doses = 8 units breakfast, 6 units lunch, 8 units dinner

Average daily correction doses = 6 units

Total Daily Dose = 22 + 8 + 6 + 8 + 6 = 50 units

In this example, the ISF calculation would be as follows:

100 ÷ Total Daily Dose

100 ÷ 50 = 2

This person could work with an ISF of 2 (or 1:2 ie 1 unit will bring their levels down by 2 mmol/L). If they had a blood glucose level of 12 mmol/L and were aiming to bring this down to 6 mmol/L, the calculation would be as follows:

Current BG – Target BG = How much they want to lower their BG by

Therefore: 12 - 6 = 6 

They want to lower their blood glucose by 6 mmol/L

The amount they want to lower their BG by ÷ their ISF = The amount of units to take

6 ÷ 2 = 3 

Three units of insulin are needed to lower this person’s blood glucose by 6mmol/L

The Rule of 100 is intended as a general starting point, and it is likely that you will need to adjust the result to fine tune your ISF.

Testing your insulin sensitivity factor It is best to test your ISF in the context of a correct basal /long-acting insulin dose. Click here to access our April clinic chat, where we discussed basal/long-acting insulin testing and adjustment.

What you need

  • A blood glucose monitor, a continuous glucose monitor or a Freestyle Libre.

  • Your usual rapid-acting insulin.

When to test The best time to test an ISF is when you meet the following criteria:

  • You have not eaten for at least 4 hours. 

  • You have not taken a rapid-acting insulin dose for at least 4 hours.

  • You are willing to not eat for the next 4 hours.

  • Your blood glucose level is at least 3 mmol/L above your target blood glucose level (this might happen if you didn’t take quite enough insulin to cover the carbohydrates at your last meal)

Many other factors can impact blood glucose levels and can affect the results of your ISF test. Avoid testing at the following times:

  • Immediately before or after exercise

  • If you are unwell or highly stressed

  • Immediately after a low blood glucose level

  • After a very high fat meal

  • If you are female, at the start of your menstrual cycle

Before you test Make sure you consider these safety pointers before you do your test:

  • If your blood glucose level drops below 4 mmol/L at any point in the test, stop the test and treat the low blood sugar as you would normally.

  • If blood glucose levels reach 15 mmol/L stop the test, perform a ketone test, and act in accordance with your sick day management plan if you have ketones. 

How to perform an insulin sensitivity factor test

  1. Test your blood glucose level 4 hours after the last meal or correction insulin dose. If it is more than 3 mmol/L above your target blood glucose level, you can complete an ISF test. If you are using a continuous glucose monitor or Freestyle Libre simply scan/view the reading and note it down.

  2. Using the ISF you wish to test, calculate the amount of rapid-acting insulin you need to lower your blood glucose to your target level. Deliver your insulin and note down the time.

  3. Go about your usual activities, but do not eat or exercise for the next 4 hours. 

  4. If you are finger stick testing, check your blood glucose levels every 30-60 minutes for the next four hours. This will give the rapid-acting insulin time to fully work. If you are using a continuous glucose monitor or Freestyle Libre you can review the time period data at the end.

  5. Do your final blood glucose test at the 4 hour mark.

Assessing your results

If, after 4 hours, your blood glucose level is within 2mmol/L of your target blood glucose, then the ISF you tested worked well. A single test is a starting point to check out the accuracy of your ISF, but it’s important to repeat the test a few times before changing it.

If your blood glucose level is not within 2mmol/L of your target, you will need to adjust and re-test. 

Adjusting your insulin sensitivity factor

If the data from your test shows that your glucose level is more than 2mmol/L higher or lower than your target blood glucose level four hours after the correction dose, then consider adjusting your ISF as follows:

  • If your blood glucose is > 2mmol/L higher than your target blood glucose four hours after the dose, repeat the test again with a smaller ISF. Decreasing the ISF will give you more insulin. 

  • If your blood glucose is > 2 mmol/L lower than your target blood glucose four hours after the dose, repeat the test again with a larger ISF. Increasing the ISF will give you less insulin.

Insulin sensitivity factors at different times of the day In May’s clinic chat on insulin-to-carbohydrate ratios (ICR), we discussed how a person’s sensitivity to insulin naturally changes over the day. As such, you may require different ratios for different times of the day. Many people find they are more insulin resistant in the morning, so they may need more insulin to bring their blood glucose levels back to target for a morning correction than the rest of the day. For example, you might have an ISF of 1:2 in the morning and 1:3 for the remainder of the day. Changes in insulin sensitivity are why it’s best to perform ISF tests at different time points throughout the day.  

A note on using an insulin sensitivity factor around exercise 

Exercise can make you more sensitive to the action of insulin, both during and in the hours after exercise. If you wish to do a blood glucose correction before or in the hours after exercise, you may need less rapid-acting insulin to bring you back to your target level. To determine a correction dose of insulin before or in the hours after exercising you can do one of the following:

  • Use the same ISF dose to calculate your correction dose, then halve the result. For example, if your regular ISF calculates a total of 4 units for the correction after exercise, give 2 units and assess the result. 

  • Use a larger ISF to calculate your correction dose. This will offer you less insulin for the correction. For example, if your regular ISF is 1:2, trial using 1:4 prior to or after exercise and assess the result.

Note: Take extra caution correcting a high blood glucose level before exercise. Remember, rapid-acting insulin is at its peak level of action roughly 1.5-2 hours after the injection, and the combination of peaking insulin and exercise could send you low. 

A note on correcting around mealtimes 

Once you’ve tested your ISF, you can combine a correction dose with your mealtime dose if your blood glucose levels are high before a meal or snack. 

If you’d like to learn more about ISF, ICR and dosing insulin in the context of accurate carbohydrate counting, consider completing the Family Centre’s online carbohydrate counting course, Cyber Carbs. You can do this ten-module program at your own pace, and test your skills at the end! 

Get help if you need it

If you need help assessing your basal, insulin to carbohydrate ratio or insulin sensitivity factor, get in touch with the Family Centre clinic today. 

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