Insulin Sensitivity Factor in SI Units
The insulin sensitivity factor (ISF) in SI units is expressed as mmol/L per unit of insulin, indicating how much blood glucose will decrease with 1 unit of insulin. 1
Definition and Clinical Application
The insulin sensitivity factor is an individualized measure that quantifies the expected reduction in blood glucose following the administration of 1 unit of insulin. In SI units, this is typically expressed as mmol/L per unit of insulin.
For example:
- An ISF of 3 mmol/L means that 1 unit of insulin would be expected to decrease blood glucose by 3 mmol/L 1
- This factor is crucial for calculating correction doses when blood glucose is above target
Calculation Methods
The ISF is typically calculated based on the total daily insulin dose (TDD) and can be estimated using established formulas:
- 1800 Rule (for SI units): ISF (mmol/L) = 100/TDD 1, 2
- This is the SI unit equivalent of the 1800 rule used in mg/dL
- Example: For a patient with TDD of 50 units, ISF = 100/50 = 2 mmol/L
Diurnal Variation
Research shows that ISF demonstrates significant diurnal variation:
- Morning ISF tends to be lower (requiring higher correction doses) 2
- Afternoon and evening ISF tends to be higher (requiring lower correction doses) 2
- The 1800 rule calculation appears appropriate for morning correction doses but may overestimate insulin needs in the afternoon and evening 2
Factors Affecting ISF
Several factors influence an individual's insulin sensitivity factor:
- Age and pubertal status 2
- Body mass index 2, 3
- Total daily insulin dose 2
- Insulin-to-carbohydrate ratio (ICR) 2
- Time of day 2
- Physical activity levels 1
- Stress levels 1
Clinical Implementation
When implementing ISF in insulin pump therapy or multiple daily injection regimens:
- ISF should be programmed into insulin pumps for automated correction calculations 1
- For multiple daily injections, ISF guides correction doses based on pre-meal blood glucose 1
- ISF and insulin-to-carbohydrate ratios are calculated from the total daily dose and reflect the patient's insulin sensitivity 1
Alternative Estimation Method
Research suggests that ISF can also be estimated based on the insulin-to-carbohydrate ratio:
- ISF (in mmol/L) ≈ 0.3 × ICR for the same time of day 2
- This conversion factor (0.3) represents the approximate conversion from the mg/dL relationship to mmol/L
Common Pitfalls
- Using a single ISF throughout the day may lead to overcorrection in the afternoon/evening 2
- Failing to adjust ISF for changes in insulin sensitivity due to exercise, illness, or stress 1
- Not recognizing that ISF may need adjustment as total daily insulin requirements change 1
- Using ISF calculated in mg/dL without proper conversion to mmol/L (divide by 18 to convert)
By understanding and properly implementing the insulin sensitivity factor in SI units, clinicians can help patients achieve better glycemic control with reduced risk of hypoglycemia.