Insulin Sensitivity Factor explained

Wednesday, 24 November 2021

What is an Insulin Sensitivity Factor?

The Insulin Sensitivity Factor (ISF), sometimes also called the correction factor, reflects the power a unit of insulin has in your body. The ISF indicates how much your blood glucose level (BGL) may drop after taking 1 unit of rapid-acting insulin.

How is the ISF calculated?

Many Diabetes Healthcare Professionals use what is referred to as the ‘100-rule’. They take the number 100 and divide it by your current total daily dosage of insulin to calculate the ISF. The 100-rule was developed in response to scientific evidence.

To establish what your Total Daily Dosage (TDD) is add any basal/long-acting (or background insulin) and any bolus/rapid-acting insulin (pre-meal insulin doses) that you take in a 24-hour period. If your insulin intake varies from one day to the next it is recommended to do this for a few days and take the average over three to four days.

For example: If you take 65 units of Optisulin at night and 12 units of Novorapid at breakfast, lunch and dinner, your TDD will be:


To calculate your ISF take the number 100 divided by 101 (from our example above), which is 1. This means that your ISF is 1:1.0 and that you will need 1 unit of rapid-acting insulin for every 1.0 mmol/L drop in BGL required.

How do I use the ISF?

If your ISF is 1:1.0 mmol/L (as in the example above) you would have to take 1 unit of Apidra, Fiasp, Humalog, or Novorapid for every 1.0 mmol/L drop in BGL required.

The ISF is calculated as follows: Current BGL minus target BGL divided by ISF.

Let’s look at a different example: If your BGL is 17.2 mmol/L and you have an ISF of 1:2.5 and a target BGL of 4.0-8.0 mmol/L it would mean that you would need around 4 units of rapid-acting insulin to get your BGL back to target.

We would always work to get your BGL back to the higher end of your target range, (in this case 8.0), to reduce the risk of hypoglycaemia (low blood glucose levels).

Using the above example, the calculation would be as follows:

17.2 – 8.0 =9.2 divided by 2.5 = 3.68

Of course, you cannot take 3.68 units of insulin, as your insulin pen device will only deliver whole units (or in some cases half units). So round it off to the next nearest whole number, in this case 4.0, and your BGL should come down nicely to target. In fact your BGL will probably end up around 7.2 mmol/L.

Generally, the ISF is added to the insulin-to-carbohydrate ratio (ICR) at meal times and is based on the pre-meal BGL.

It is usually recommended NOT to take extra injections, outside your pre-meal boluses, as this may lead to something called insulin stacking and can significantly increase your risk of developing (severe) hypoglycaemia (very low blood glucose levels).

Insulin resistance

If you have insulin resistance, you are less sensitive to insulin and will need larger amounts of insulin to correct for high blood glucose levels. This means that you will have a lower ISF number, for example, 1:1.0 rather than 1:2.5.

Insulin resistance tends to be worse if you are stressed and when you gain weight. This means that generally, you will need more insulin during these times and your ISF may need to be adjusted.

On the other hand, physical activity will make you more sensitive to insulin and may increase your ISF during as well as after exercise.

It is important to always follow the recommendations of your healthcare professional when making or considering any insulin dose adjustments.

How do I know if the ISF is correct?

To check if your correction factor is right, you should check your BGLs two to three hours after taking a correction bolus or check your continuous or flash glucose monitoring data at those times.

If your BGL is not returning to target (ie your BGL remains above target or you experience low BGLs two to three hours after that meal) – and your carbohydrate counting and ICR are correct (this should be reviewed by your diabetes healthcare professional) – then you know the correction factor may need to be adjusted. Your endocrinologist or credentialled diabetes educator can help you with this.

A word of caution

Please note: Calculating and using ISF does not work if you are using pre-mixed insulins such as Humalog Mix25 or Mix50, Novomix, or Ryzodeg. If you are using a pre-mixed insulin, talk to your healthcare professional for advice on insulin dose adjustments.

Additional tips

  • Insulin pumps have an algorithm that can calculate the bolus insulin required based on your current BGL, your carbohydrate intake, your target BGL, your ISF and your insulin to carbohydrate ratio (ICR), as well as any insulin left on board (IOB).
  • As mentioned above, when using a BG target range (for example 5.2 to 7.8 mmol/L instead of a hard target of say 6.5 mmol/L) it is generally recommended to aim to decrease high BGLs using the ISF to the upper limit of the range (in this example: 7.8mM).
  • BGLs below the target can be corrected using the ISF also, but in this case the insulin amount would be deducted.
  • Check if your basal/background insulin (e.g. Lantus or Levemir) dose is correct by comparing your pre-bed BGL with your fasting BGL (FBGL). This is particularly important if frequent correction boluses are needed. If your FBGL is often more than 4mmol higher or lower than the night before, you may need to change your basal insulin dose by 1-2 units (follow the directions of your diabetes HP). Whenever the basal insulin dosage is changed it is advisable to give it at least three days to see the new pattern before making any further changes.

The above information is a guide only. Always follow the recommendations of your healthcare team.

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By Carolien Koreneff

Credentialled Diabetes Educator, Registered Nurse, FADEA

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