How do you calculate the insulin sensitivity factor?

How to Calculate the Insulin Sensitivity Factor (ISF)

The insulin sensitivity factor (ISF), also called the correction factor, is calculated using the "1800 rule": divide 1800 by the total daily dose (TDD) of insulin, which estimates how much 1 unit of insulin will decrease blood glucose in mg/dL. 1, 2

Basic Calculation Method

• Use the 1800 rule formula: ISF = 1800 ÷ Total Daily Dose of insulin 3, 2
• The resulting number represents how many mg/dL one unit of insulin will lower blood glucose 1
• For example, if a patient uses 60 units of insulin per day total, their ISF would be 1800 ÷ 60 = 30, meaning 1 unit of insulin lowers glucose by approximately 30 mg/dL 1

Critical Consideration: Diurnal Variation

The ISF varies significantly throughout the day, with morning correction doses requiring more insulin (lower ISF values) than afternoon or evening doses due to elevated counter-regulatory hormones in the morning. 1, 4

• The 1800 rule calculation approximates the morning ISF but significantly underestimates the actual ISF needed for afternoon and evening corrections 4
• You must establish different ISF values for different time blocks throughout the day rather than using a single fixed value 1, 3
• This diurnal pattern occurs across all age groups and pubertal stages in patients with type 1 diabetes 4

Alternative Calculation Using ICR

A more accurate method for estimating ISF at any time of day is: ISF (mg/dL) = 5.14 × ICR for that same time period, which shows excellent correlation (R² = 0.95). 4

• This relationship between insulin-to-carbohydrate ratio (ICR) and ISF provides time-specific correction factors 4
• The ICR is the most significant factor associated with ISF values 4

Practical Application

When using ISF to calculate correction doses:

• Correction bolus = (Current blood glucose - Target blood glucose) ÷ ISF 2
• Modern insulin pumps have built-in calculators that apply the preprogrammed ISF automatically when the user enters their current blood glucose 1
• The pump also accounts for "insulin on board" from previous doses to prevent insulin stacking and hypoglycemia 1, 2

When to Adjust the ISF

Adjust the ISF if correction doses consistently fail to bring glucose into target range, reviewing these parameters every 3-6 months at specialized diabetes clinic visits. 3, 2

• ISF requires periodic reassessment as insulin requirements change with weight, exercise patterns, menstrual cycles, and other physiological factors 1, 2
• If carbohydrate counting is accurate but post-meal glucose remains out of range, adjust the ICR rather than the ISF 3
• Monitor whether correction doses are effectively bringing glucose to target before making adjustments 3

Common Pitfalls to Avoid

• Do not use a fixed ISF throughout the entire day - this ignores the significant diurnal variation in insulin sensitivity and leads to overcorrection at night and undercorrection in the morning 1, 2
• Do not ignore "insulin on board" - failing to account for active insulin from previous boluses causes insulin stacking and hypoglycemia 2
• Do not apply ISF calculations to patients on premixed insulins or complex regimens - these calculations are designed for basal-bolus or pump therapy 1