What is the insulin sensitivity factor?

What is the Insulin Sensitivity Factor?

The insulin sensitivity factor (ISF), also called the correction factor, is an individualized measure of how much 1 unit of insulin will decrease blood glucose—for example, with a ratio of 1:3, one unit of insulin would decrease blood glucose by 3 mmol/L (or approximately 54 mg/dL). 1

Clinical Purpose and Application

The ISF serves a specific therapeutic function distinct from meal-time insulin dosing:

• It is used to calculate correction boluses that bring elevated blood glucose back to a predetermined target level before meals, independent of carbohydrate intake 1
• The correction dose is calculated by dividing the difference between current blood glucose and target blood glucose by the ISF 2
• For example, if blood glucose is 250 mg/dL, target is 100 mg/dL, and ISF is 50 mg/dL per unit, the correction dose would be (250-100)/50 = 3 units 2

Calculation and Individualization

The ISF is calculated from the patient's total daily dose of insulin and reflects their overall insulin sensitivity, similar to the insulin-to-carbohydrate ratio 1

Key characteristics of ISF determination:

• It is preprogrammed into insulin pumps and used by the on-board calculator to automatically compute correction doses when the user inputs their current blood glucose 1
• The ISF value varies between individuals based on body weight, insulin resistance, and metabolic factors 2
• ISF is not fixed—it requires periodic reassessment as insulin requirements change with weight, exercise patterns, menstrual cycles, and other physiological changes 2

Integration with Insulin Delivery Systems

Modern insulin delivery incorporates ISF in sophisticated ways:

• Insulin pumps contain preset memory that calculates "insulin on board" from previous doses, preventing excessive correction doses ("stacking") that could cause hypoglycemia 1
• Advanced hybrid closed-loop systems may use machine learning to optimize correction factors on an ongoing basis 1
• Some newer automated insulin delivery (AID) systems provide automated correction boluses at varying frequencies, with the dose and timing determined by the programmed ISF 1

Common Clinical Pitfalls

Failing to account for insulin on board from previous boluses can lead to insulin stacking and dangerous hypoglycemia 2

Additional considerations to avoid errors:

• Hyperglycemia itself induces short-term insulin resistance, meaning the ISF may be less effective at very high glucose levels, requiring larger correction doses than the standard ISF would predict 3
• ISF typically varies throughout the day due to diurnal hormonal changes, though most systems use a single value or limited time blocks 1
• Changes in activity level, illness, or medications (especially corticosteroids) can temporarily alter insulin sensitivity, making the programmed ISF inaccurate 1

Relationship to Other Insulin Parameters

The ISF works in conjunction with other insulin dosing parameters:

• The total mealtime bolus combines both the meal bolus (based on carbohydrate-to-insulin ratio) and the correction bolus (based on ISF) 2
• Unlike the carbohydrate-to-insulin ratio which addresses food intake, the ISF specifically addresses hyperglycemia correction 1
• Both parameters should be reviewed every 3-6 months in pump therapy clinics and adjusted based on glucose patterns 1