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Pump-ISF

If you have not done so already, we recommend that you read the following WaltzingTheDragon.ca pages on insulin adjustment for pumps as background for the information that follows:
A Basal Bolus Approach
Insulin Action & How it Affects BG
Intro to Insulin Adjustment (for pumps)
Record Review
Basal Rates
Insulin:Carb Ratio

Advanced Insulin Adjustment Using a Pump:
Assessing and Adjusting Insulin Sensitivity Factor (ISF)

If you haven’t already done so, as background to the information that follows we suggest that you read the following page (onAdjusting Insulin Sensitivity Factor in type 1 diabetes this website):
Level 1/Basics/
Treating Highs

Correction insulin is an extra dose of insulin given to reduce high blood glucose. The correction factor, or Insulin Sensitivity Factor (ISF), specifies how much insulin is needed to correct blood glucose back to the target range. It is expressed as a ratio, such as 1:3, or 1:15, where the second number represents how much the blood glucose drops when 1 unit (the first number of the ratio) of rapid-acting insulin is given.
For example, an ISF of 1:3 means one unit of insulin would drop the blood glucose by 3 mmol/L; an ISF of 1:15 means one unit of insulin would drop the blood glucose by 15 mmol/L. We often talk about ISF in terms of this second number alone, as in “my child’s ISF is 3”.
The bolus calculator in an insulin pump uses a Correction Formula (much the same as you may have used if your child was previously on an injected Basal-Bolus with MDI program) to calculate how many units of insulin to deliver for an above-target blood glucose reading:


Correction Insulin = Current BG – Target    
                    ISF


It is informative, but not necessary, for you to understand how the pump uses this correction formula. What IS important is to understand the effect of changes to the ISF setting, as outlined below.


Initial ISF Settings

At the beginning of an insulin program, or after a major change in insulin program (ex. transition from injections to insulin pump therapy or from a pump to injections), your child’s ISF should be provided by your child’s doctor and diabetes health care team.

Your child’s ISF will change over time as he grows (and his Total Daily Dose of insulin increases). Periodically, you may want to test/verify how well your child’s existing ISF settings are meeting her current need for insulin. That is, does the current ISF setting correct high blood glucose down to target? Or instead does it under-correct (resulting in the blood glucose remaining high) or over-correct (resulting in lows)? Your correction/ISF should work at least 50% of the time.

You could use the results of an ISF assessment to make an adjustment to the ISF setting, or as a basis of discussion with your child’s diabetes health care provider.


Assessing Current ISF Settings (Pump)

If you often find that, after you have given a correction dose of insulin, your child’s blood glucose remains high or drops below the target range, you may need to adjust the ISF setting.

You could test your child’s current ISF setting (Correction Factor) by correcting a blood glucose over 10.0 (under the conditions outlined below), and then checking blood glucose for 4 hours following the delivery of the correction bolus, to see if blood glucose returns to the target range.

It is helpful to break the day up into: morning, afternoon, evening, and overnight, and test ISF in more than one of these parts of the day. The insulin needed to correct high blood glucose (specified by ISF) may vary at different times of the day. Young adults and teens may need more insulin to correct a high in the early morning hours to deal with the release of hormones involved in waking (known as the Dawn Phenomenon). Children may need more correction insulin in the late evening hours because of the release of growth hormones a few hours after they fall asleep. People of all ages may need less insulin overnight than they do during the day (or more insulin – everyone is different).

Conditions for Assessing Current ISF Settings:
  • Ketones are negative.
  • No bolus insulin has been delivered for 4 hours before the correction bolus is given. (For pump users, that means that insulin/bolus on board is zero.)
  • No carbohydrates have been consumed for 4 hours before the correction bolus is given.
  • No carbohydrates are consumed during the 4-hour test period.
(If your child’s basal insulin dosing is too low, for example, it may be appear that ISF is too low, because blood glucose remains higher-than-target after a correction bolus, when in fact ISF is correct. To remove this confounding variable, verify that your child’s basal insulin dose is correct before testing ISF. )
  • Your child is not currently sick.
  • Your child is experiencing normal stress and activity levels.

The ISF Testing Process:

1.  When blood glucose is over 10.0, deliver a correction bolus for high blood glucose as usual (according to your child’s usual ISF setting).

2.  Check your child’s blood glucose every hour for 4 hours after giving the correction bolus. (The critical checks for ISF testing are at 3 and 4 hours; the first 2 blood glucose checks are to ensure that a low is not occurring, and to provide additional information about how quickly or slowly blood glucose responds to a correction.) Have him avoid eating for 4 hours, unless his blood glucose goes low.

3.  Compare your child’s blood glucose at 3-4 hours with his target blood glucose. Is it above range? In range? Below range? An accurate ISF will bring high readings down to target within 3-4 hours.


Adjusting ISF Settings Based on the Test Results

  • If your child’s blood glucose stays higher than the target range for the 4 hours after giving the correction bolus, then your child may not be getting enough correction insulin: the ISF may be too weak (that is, that second number may be too high).
Example A: If the ISF is 3.5, the blood glucose target range is 6-10 mmol/L, and 4 hours after the correction the blood glucose reading is over 10.0 mmol/L, then consider adjusting the ISF to make it stronger (by decreasing the ISF number, to 3.0 for example).
  • If your child’s blood glucose drops lower than the target range within 4 hours after giving the correction bolus, then the ISF may be too strong (that is, the second number of the ratio may be too low).
Example B: If the ISF is 3.5, the blood glucose target range is 6-10 mmol/L, and 4 hours and after the correction the blood glucose reading is 3.8 mmol/L, then consider adjusting the ISF to make it weaker (by increasing the ISF number, to 4.0 for example).


By How Much Should I Adjust?

You could use increments of 0.5 as a reasonable starting point for ISF adjustments.

For example:
  • If you decide that the ISF setting needs to be stronger (blood glucose is still high after the test), as in Example A above, you could first try decreasing the ISF number, from 3.5 to 3.0, for example.
  • If you decide that the ISF setting needs to be weaker (blood glucose is below target after the test), as in Example B above, you could first try increasing the ISF number, from 3.5 to 4.0, for example.

After you have made an adjustment to the ISF setting, it is wise to repeat the test process and consider making further adjustments as needed.

If you have difficulty making adjustments to the ISF settings on your child’s pump, please contact your child’s diabetes health care team.





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The above information was significantly modified with permission from The Alberta Children’s Hospital Diabetes Clinic information handouts.

The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.

This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional.


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