About the Insulin Sensitivity Factor (ISF)
The Insulin Sensitivity Factor (ISF) calculator, also known as a Correction Factor calculator, is a clinical tool used to estimate how much one unit of rapid-acting or short-acting insulin will lower a person's blood glucose (BG) level. This is a fundamental component of insulin dosing for individuals with diabetes, particularly for calculating correction boluses to address high blood sugar.
Outputs Explained
The calculator provides two distinct ISF values based on well-established clinical rules:
- 1800 Rule (Rapid-Acting Insulin): This value estimates the drop in blood glucose (in mg/dL or mmol/L) for every 1 unit of a rapid-acting insulin analog, such as Lispro, Aspart, or Glulisine.
- 1500 Rule (Regular Insulin): This value estimates the drop in blood glucose for every 1 unit of regular (short-acting) human insulin, such as Humulin R or Novolin R.
How to Use This Information
To use the ISF for clinical dosing, follow these steps:
- Calculate the patient's Total Daily Dose (TDD) of insulin by summing all basal (long-acting) and bolus (mealtime) insulin taken over a 24-hour period.
- Input the TDD into the calculator to determine the appropriate ISF (using either the 1800 or 1500 rule based on the patient's insulin type).
- Measure the patient's current blood glucose level.
- Determine the target blood glucose level (e.g., 120 mg/dL).
- Apply the correction dose formula:
(Current BG - Target BG) / ISF = Correction Dose in units.
Dosing Overview
The ISF is primarily used to calculate a correction dose to bring an elevated blood glucose level back to the target range. It is often combined with an insulin-to-carbohydrate ratio (ICR) for mealtime bolus calculations.
Example: A patient using rapid-acting insulin has a current BG of 220 mg/dL, a target BG of 120 mg/dL, and a calculated ISF of 40 mg/dL/unit.
Correction Dose = (220 - 120) / 40 = 100 / 40 = 2.5 units of rapid-acting insulin.
Switching Insulin Types
When a patient switches between regular and rapid-acting insulin for corrections, the corresponding rule must also be switched. Rapid-acting insulins are more potent in their immediate glucose-lowering effect, which is reflected in the higher number (1800 vs. 1500) used in the rule, resulting in a higher, more sensitive ISF value.
Managing Missed Doses & High Blood Glucose
If a mealtime bolus dose is missed, hyperglycemia often follows. The ISF is the tool used to calculate the appropriate correction dose to manage this high blood glucose. It is crucial to consider insulin on board (IOB) from previous doses to avoid "stacking" insulin and causing hypoglycemia.
Safety Alerts
- Estimate Only: The 1800 and 1500 rules provide a starting point. The ISF must be validated and adjusted based on individual patient response.
- Hypoglycemia Risk: An incorrect TDD or an overly aggressive ISF can lead to severe hypoglycemia. Always use clinical judgment and start with conservative estimates.
- Variable Sensitivity: Insulin sensitivity is not static. It can be affected by illness, stress, physical activity, time of day, and certain medications (e.g., corticosteroids), requiring dose adjustments.
- Insulin Stacking: Administering correction doses too close together without accounting for IOB can lead to an overlap in insulin action and subsequent low blood sugar.
Frequently Asked Questions
What is the difference between the 1800 rule and the 1500 rule?
The 1800 rule is used for newer, rapid-acting analog insulins (e.g., Lispro, Aspart) which have a quicker and more potent peak effect. The 1500 rule is used for older, regular human insulin (e.g., Humulin R), which is slightly less potent on a per-unit basis for acute glucose lowering.
How is Total Daily Dose (TDD) calculated?
TDD is the sum of all insulin a patient uses in a typical 24-hour period. This includes long-acting (basal) insulin and all short/rapid-acting (bolus) insulin for meals and corrections.
Why does the ISF change for mg/dL vs. mmol/L units?
mg/dL and mmol/L are different units of measurement for blood glucose concentration. The conversion factor is approximately 18 (1 mmol/L ≈ 18 mg/dL). The formulas are adjusted to provide the correct ISF for the selected unit: 1800/TDD for mg/dL is equivalent to 100/TDD for mmol/L (since 1800/18 = 100).
Can this calculator be used for patients on an insulin pump?
Yes. The principles for calculating ISF are the same for patients on multiple daily injections (MDI) or continuous subcutaneous insulin infusion (CSII) via a pump. The TDD used in the calculation should be the total basal and bolus insulin delivered by the pump.
What is a typical target blood glucose level?
Target blood glucose levels are individualized. However, a common pre-meal target for many adults with diabetes is between 80-130 mg/dL (4.4-7.2 mmol/L), as recommended by the American Diabetes Association.
How often should the Insulin Sensitivity Factor be re-evaluated?
ISF should be re-evaluated periodically and whenever there are significant changes in a patient's lifestyle or health, such as weight change, new exercise regimen, prolonged illness, or initiation of new medications that affect glucose metabolism.
What if the patient's TDD is very high or very low?
For patients with very high TDD (indicating significant insulin resistance), the calculated ISF will be low. For patients with very low TDD (high insulin sensitivity), the ISF will be high. In extreme cases, these rules may be less accurate, and direct observation of patient data (e.g., testing the factor) may be necessary.
Does this calculator work for both Type 1 and Type 2 diabetes?
Yes, these formulas are applicable to any individual using a total daily dose of insulin, regardless of the type of diabetes. The key input is the TDD, which accounts for the individual's degree of insulin resistance or deficiency.
References
The following resources provide foundational information and clinical guidance on insulin therapy and dosing calculations.
- American Diabetes Association. 6. Glycemic Targets: Standards of Medical Care in Diabetes—2023. Diabetes Care. 2023;46(Suppl 1):S97-S110. doi:10.2337/dc23-S006
- Walsh J, Roberts R, Bailey T. The 1800 rule: a simple and accurate method for calculating insulin sensitivity. Diabetes Technol Ther. 2003;5(4):A115.
- Hirsch IB. Insulin Analogues. N Engl J Med. 2005;352(2):174-183. doi:10.1056/NEJMra040832
- Grunberger G, Sherr J, Allende M, et al. American Association of Clinical Endocrinology Clinical Practice Guideline: The Use of Advanced Technology in the Management of Persons With Diabetes Mellitus. Endocr Pract. 2021;27(6):505-537. doi:10.1016/j.eprac.2021.04.008
