The 1800 Rule for Calculating Insulin Sensitivity Factor in SI Units
The 1800 rule in SI units is calculated as ISF (mmol/L) = 100 divided by the Total Daily Insulin Dose (TDD). 1
Understanding the Insulin Sensitivity Factor (ISF)
The Insulin Sensitivity Factor (ISF) represents how much blood glucose will decrease with 1 unit of insulin:
- In SI units, ISF is expressed as mmol/L per unit of insulin
- For example, if a patient's ISF is 2 mmol/L, 1 unit of insulin would decrease blood glucose by 2 mmol/L
Calculating ISF Using the 1800 Rule in SI Units
The formula for calculating ISF in SI units is:
- ISF (mmol/L) = 100 ÷ TDD 1
Example Calculation:
- For a patient with a Total Daily Insulin Dose (TDD) of 50 units:
- ISF = 100 ÷ 50 = 2 mmol/L
- This means 1 unit of insulin would lower blood glucose by approximately 2 mmol/L
Clinical Application of ISF
ISF is essential for:
- Calculating correction insulin doses
- Programming insulin pumps for automated correction calculations
- Guiding multiple daily injection therapy
Diurnal Variation in ISF
Research shows that ISF varies throughout the day:
- Morning: Higher correction dose requirements (lower ISF)
- Afternoon/Evening: Lower correction dose requirements (higher ISF) 2
The 1800 rule calculation tends to match morning ISF values but may underestimate ISF for afternoon and evening, meaning it may recommend more insulin than needed during these times 2.
Factors Affecting ISF
Several factors influence an individual's ISF:
Relationship with Other Insulin Parameters
ISF has important relationships with other insulin parameters:
- ISF is closely correlated with Insulin-to-Carbohydrate Ratio (ICR)
- The relationship can be expressed as: 100 ÷ Total daily basal insulin dose = ICR = ISF ÷ 4.5 3
Common Pitfalls to Avoid
- Failing to adjust for time of day: ISF varies throughout the day, with morning requiring more correction insulin than evening
- Not updating ISF as insulin requirements change: As TDD changes due to weight changes, illness, or other factors, ISF should be recalculated
- Using the same formula for all age groups: Pediatric patients may require age-specific adjustments to the standard formula 4
- Overlooking the impact of physical activity: Exercise can significantly increase insulin sensitivity, requiring ISF adjustment
By understanding and correctly applying the 1800 rule in SI units (ISF = 100 ÷ TDD), clinicians can help patients achieve better glycemic control through more accurate correction insulin dosing.