Insulin Sensitivity Factor (ISF) Explained

Written by Carolien Koreneff, CDE-RN, FADEA

What is an Insulin Sensitivity Factor (ISF)?

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

If the ISF is 1:2.0 mmol/L the person would have to take 1 unit of Apidra, Fiasp, Humalog, or Novorapid for every 2 mmol/L drop in BGL required.

For example: If a person’s BGL is 15.2 mmol/L and an ISF of 1:2.0, with a target BGL is 6.5, it would mean that they would need 4-4.5 units of rapid-acting insulin to get the BGL back to target. To calculate this you would take: current BGL – target BGL divided by ISF = 15.2-6.5=8.7 divided by 2.0 = 4.35 – round off to 4.0 or 4.5).

(Current BGL – Target BGL) / ISF

ISF Formula

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

How is the ISF calculated?

Many Diabetes Healthcare Professionals use what is called the 100-rule to calculate ISF. They take the number 100 and divide it by the person’s current total daily dosage of insulin.

To establish what the Total Daily Dosage (TDD) is add any basal/long-acting and any bolus/rapid-acting insulin that is taken in a 24-hour period. If the insulin intake varies from one day to the next it is recommended to do this for a few days and take the average over say 3-4 days.

For example: If the person takes 26 units of Optisulin at night and they take 8 units of Novorapid at breakfast, lunch and dinner their TDD will be: 26+8+8+8=50. To calculate this person’s ISF take 100 divided by 50, which is 2. This means that their ISF is 1:2.0 and they will need 1 unit of rapid-acting insulin for every 2.0 mmol/L drop in BGL required.

Insulin resistance

People with insulin resistance are less sensitive to insulin and will have lower ISFs as they need larger amounts of insulin to correct for high blood glucose levels. For example they may have an ISF of 1:0.5 mmol.

Stress can affect insulin sensitivity negatively, as can weight gain, both can make a person more insulin resistant, and hence can drive the ISF down.

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

How do I now if the ISF is correct?

To check if the correction factor is right, ask the person to check their BGLs 2-3 hours after taking a correction. Check the BGL logbook for times when a correction dose of insulin was added to the meal dose, and see what the BGL was 2-3 hours later (or check the continuous or flash glucose monitoring data at those times).

If the BGL is not coming back to target (i.e. the person’s BGL remains above target or they experience a low BGLs 2-3 hours after that meal) – and their carbohydrate counting seems to be appropriate and their ICR is correct – then you know the correction factor may need to be adjusted.

Additional tips on the ISF

• Insulin pumps have an algorithm in them that can calculate the bolus insulin required based on the person’s current BGL, their carbohydrate intake, their target BGL, their ISF and their insulin to carbohydrate (ICR), as well as any insulin left on board (IOB).
• When using a BG target range (for example 5.2-7.8 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 the basal/background insulin (e.g. Lantus or Levemir) dose is correct by comparing the pre bed BGL with the fasting BGL. This is particularly important if frequent correction boluses are required. If the FBGL is often more than 4mmol higher or lower than the night before change the dose by 1-2 units. Whenever the basal insulin dosage is changed it is advisable to give it at least 3 days to see the new pattern before making any further changes.

Refer the people you see to the online education program Living with insulin, it is also available as an app.