🎯Insulin Sensitivity Factor (ISF) & the 1800 Rule
Your Insulin Sensitivity Factor tells you how much 1 unit of rapid-acting insulin will lower your blood glucose. This guide explains the 1800 Rule, correction bolus math, and how to adjust your ISF safely.
What Is the Insulin Sensitivity Factor?
The Insulin Sensitivity Factor (ISF) — also called the correction factor or sensitivity factor — tells you how many mg/dL (or mmol/L) your blood glucose will drop per 1 unit of rapid-acting insulin. It is the key number used to calculate a correction bolus when your blood sugar is above target.
Examples:
- ISF of 50 mg/dL/unit means 1 unit of rapid-acting insulin drops blood glucose by 50 mg/dL.
- ISF of 30 mg/dL/unit means 1 unit drops blood glucose by only 30 mg/dL (more insulin-resistant).
- ISF of 80 mg/dL/unit means 1 unit drops blood glucose by 80 mg/dL (more insulin-sensitive).
The 1800 Rule: Calculating ISF from TDD
The 1800 Rule (also called the 100 Rule or Sensitivity Rule) is the standard clinical method to estimate ISF from a patient's Total Daily Dose (TDD):
The constant 1800 was derived by clinical observation — it reflects the approximate amount of blood glucose (in mg/dL) that all insulin administered in a given day will process across the body.
Worked Example: Calculating ISF
Same patient from our ICR guide: TDD = 50 units/day
ISF = 1800 ÷ 50 = 36 mg/dL per unit
This means each 1 unit of rapid-acting insulin should lower blood glucose by approximately 36 mg/dL for this patient.
Using the ISF: Calculating a Correction Bolus
Once you know your ISF, calculating a correction bolus is straightforward:
Example:
- Current blood glucose: 240 mg/dL
- Target blood glucose: 100 mg/dL
- ISF: 36 mg/dL/unit
Correction Dose = (240 − 100) ÷ 36 = 140 ÷ 36 = ~3.9 units (round per clinical judgment)
Correction Bolus + Meal Bolus + IOB: The Full Picture
In real clinical use, a correction bolus is rarely given in isolation. It is typically combined with:
- Meal bolus (if eating at the same time) — calculated from your ICR and carb count
- Active IOB deduction — active Insulin on Board from previous doses is subtracted to prevent dangerous stacking
InsulinPoint's calculator computes all three components simultaneously and gives a net recommended dose.
Factors That Affect Insulin Sensitivity
ISF is not a fixed number — it changes based on many physiological and lifestyle factors:
| Factor | Effect on Insulin Sensitivity | ISF Change |
|---|---|---|
| Aerobic exercise | Greatly increases sensitivity for 12–48 hr | ISF goes UP (more mg/dL per unit) |
| Illness / infection | Raises cortisol → insulin resistance | ISF goes DOWN (fewer mg/dL per unit) |
| Stress (psychological) | Raises glucagon, epinephrine → resistance | ISF goes DOWN |
| Morning (dawn phenomenon) | Cortisol/GH surge reduces sensitivity | ISF goes DOWN in morning |
| Weight gain | Increased adipose tissue → resistance | ISF goes DOWN |
| High-fat meals (delayed) | Delayed gastric emptying → late glucose rise | Complex — may require split dosing |
Signs Your ISF Needs Adjustment
| Pattern After Corrections | What It Suggests | ISF Adjustment |
|---|---|---|
| BG still high after correction | ISF too high — insulin drops BG less than expected | Decrease ISF (e.g., 50 → 40 mg/dL/unit) |
| BG drops too far (hypoglycemia) | ISF too low — insulin is more potent than expected | Increase ISF (e.g., 40 → 55 mg/dL/unit) |