Insulin Sensitivity Factor (ISF)

An insulin sensitivity factor (ISF) or correction factor describes how much one unit of rapid or regular insulin will lower blood glucose. It is used to determine the amount of insulin to give to correct blood glucose readings that are above target. It is individualized and based on total dose of insulin (TDD), as well as trial and error.

Quick Reference Formula (rapid insulin):

ISF = 100 / TDD

  1. Creating an Insulin Sensitivity Factor

    Note: TDD = Total Daily Dose of insulin

    • 100/TDD = ISF if using rapid insulin  (the number of mmol that 1 unit drops glucose in 3-4 hours approx)

    • 83/TDD= ISF is using regular/Toronto insulin (the number of mmol/L that 1 unit  drops glucose in 3-5 hours approx)

    • EXAMPLE ONE:  Teddy, TDD 33, takes rapid insulin

      100/35= ISF of 3

      1 unit rapid insulin is expected to lower Teddy’s blood sugar by 3 mmol/L 

    • EXAMPLE TWO: Bear,  TDD 80, takes regular insulin

      83/80= ISF of 1

      1 unit of regular is expected to lower Bear’s sugar by 1 mmol/L.  This is aggressive; a safer start may be an ISF of 2.

  2. Using an Insulin Sensitivity Factor (ISF)

    • Current glucose– target glucose / ISF = units to give to correct the high reading

    • Example: 13 mmol/L – 7 mmol/L target / ISF of 2 = 3 units to correct high blood sugar

    • This correction insulin would be given in addition to the meal bolus (insulin to cover the food).

  3. Assessing An ISF

    • Assessing correction insulin against the ISF formula:  If blood glucose readings are elevated and the  ISF the client is using differs greatly from the formula answer, a change may be needed. E.g. Sandy has high readings. She is using an ISF of 3 and the formula suggests an ISF of 1.

    • Assessing the correction insulin alone:  If the client had no meal, no meal bulus but took only correction insulin,  assess if it worked.  E.g. 12 mmol at breakfast, gave correction but no meal eaten and no meal bolus, by lunch was 6.5 mmol/L. This correction worked. Clients are often hesitant to report times they've corrected but missed the meal.  Be sure to let them know this could be useful information if they had any to share.

    • Assessing correction insulin when meal bolus insulin given:

    • First assess if the meal insulin works when in target and no correction insulin is needed. 

    • If the meal insulin works when in target, next look for pre-meal readings that are above target. If blood glucose readings are: 

      • elevated after giving meal bolus AND correction insulin, consider decreasing ISF number by 0.5 or 1.0.    E.g. Breakfast 12.0 mmol/L, gave meal and correction insulin, lunch 9.5 mmol/L.  Change ISF from  3.0 to 2.0 as an option. 

      • low after giving meal bolus AND correction insulin, consider increasing ISF number by 0.5 or 1.0. E.g.  Breakfast 12.0 mmol/L, gave meal and correction insulin, lunch  3.3 mmol/L.  Change ISF from 2.0 to 3.0 as an option. 

      • in target after giving meal bolus AND correction insulin, the ISF is likely appropriate.‚Äč

  4. Supplemental Notes, Correction Tables

    • The ISF is usually dependent on the individual’s sensitivity to insulin. A person more sensitive to insulin is likely to have a larger number for ISF (eg. 3, 4 or occasionally 5). A person with insulin resistance is likely to have a lower number for ISF (eg. 1 or  less than 1).

    • An ISF of 1 (1 unit to drop 1 mmol/L) is usually not safe to start, although a very insulin resistant client may eventually require that.  An ISF of 2 (1 unit to drop blood glucose 2 mmol/L) is common and would be safer to try initially, than an ISF of 1. In lean people with established type 1 diabetes, an ISF of 3-4 is not unusual. An ISF of 5 may be needed in early type 1 diabetes, although high readings may often “self-correct” in those who are honeymooning and still making endogenous insulin.

    • Corrections after meals are often not suggested, to avoid “stacking” insulin. However, if required, advise to correct to a post meal target eg. of 10 mmol/L 

    • Eg. Current glucose– target glucose of 10 / ISF = units to give to correct the high reading

    • As with any insulin formula, expect that results can vary at times.

    • Using an ISF may be difficult for some. Consider a chart based on the client’s ISF.  E.g. if an ISF is 2, the left-hand column range for blood sugars will increase by approximately 2 mmol/L

      Sample correction table - same correction per meal. ISF=2
      (meal bolus insulin not included in chart) 

      If blood sugar is:Give this many extra units of rapid insulin along with your usual meal insulin.
       
      7.1-9 mmol/L

      1 unit

      9.1 – 11.0 mmol/L

      2 units

      11.1 – 13.0 mmol/L

      3 units

      13.1 – 15 mmol/L

      4 units

    • Correction doses may be the same for each meal, as per the chart above, or they may differ per meal based on trial and error. See the chart below for an example of the latter.

      Sample correction table - different correction per meal. ISF=2
      (meal bolus insulin not included in chart) 


      If blood sugar is:BreakfastLunchSupper
      7.1-9 mmol/L

      1 unit

      2 units

      2 units

      9.1 – 11.0 mmol/L

      2 units

      2 units

      2 units

      11.1 – 13.0 mmol/L

      3 units

      3 units

      4 units

      13.1 – 15 mmol/L

      4 units

      4 units

      4 units