What is an Insulin Sensitivity Factor?

If you are taking rapid-acting insulin (sometimes called meal or bolus insulin, such as Novorapid, Humalog or Apidra), you educator may talk to you about the option of using an insulin sensitivity factor (ISF, for short) to better control your blood sugars.

An ISF tells you how many mmol/l (or ‘blood sugar points’) 1 unit of rapid insulin will lower your blood sugar by. You can use an ISF to calculate how much extra insulin you will need to take when your blood sugar is high, to bring it back into your target range. If you are eating a meal, this would be extra insulin you would need to take, on top of what you would normally take for that meal.

As an example, if your ISF is 1:3, this means that 1 unit of rapid insulin will lower your blood sugar by 3 mmol/l. If your ISF is 1:5, this means that 1 unit of rapid insulin will lower your blood sugar by 5 mmol/l.

Everyone is different and will use different ISFs and blood sugar targets. You educator can help you figure out what your ISF and target range should be.

Once you know what your ISF is, you can use the following steps to calculate how much extra insulin you will need to bring your blood sugar back into target range:

  1. Test blood sugar to get your current value
  2. Use current blood sugar – blood sugar target to find out how many mmol/l you need to come down to reach your target range
  3. Divide value from step 2 by your ISF.

To practice, lets say that my ISF is 1:2 (1 unit of rapid insulin lowers my blood sugar by 2 mmol/l), and when I test my blood sugar, it is 12.0 mmol/l. I have set my target to be 6 mmol/l. I can use the steps from above:

  1. My blood sugar is 12.0 mmol/l
  2. 12.0 -6.0 = 6.0 My blood sugar needs to come down 6.0 mmol/l to reach my target
  3. 6.0 / 2 (my ISF) = 3 I will need to take 3 units of additional insulin to bring my blood sugar back into target range.

** Remember that rapid acting insulin works for approximately 4 hours. It should take about 4 hours for your blood sugar to reach your target range after using your ISF for a correction. Do not correct again within 4 hours of giving a correction dose to avoid having low blood sugars. **

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