Insulin Sensitivity Factor Explained
The insulin sensitivity factor (ISF) — also called the correction factor — tells you how many mg/dL one unit of insulin will lower your blood glucose. It is used to calculate correction doses when blood glucose is above your target.
What Is the Insulin Sensitivity Factor?
ISF answers the question: "If I give one unit of rapid-acting insulin with no food, how much will my blood glucose drop?"
If your ISF is 50 mg/dL per unit, giving one unit of insulin (with no food on board) should lower your blood glucose by approximately 50 mg/dL. A blood glucose of 250 mg/dL with a target of 100 mg/dL needs a correction of (250 − 100) ÷ 50 = 3 units.
How ISF Is Calculated — 1800 Rule & 1500 Rule
1800 Rule (Rapid-Acting Insulin)
Used for rapid-acting insulin analogues (lispro, aspart, glulisine). Example: TDD = 36 → ISF = 1800 ÷ 36 = 50 mg/dL per unit.
1500 Rule (Regular / Short-Acting Insulin)
Used for Regular (R) insulin, which has a longer, flatter action curve. Example: TDD = 36 → ISF = 1500 ÷ 36 = 41.7 mg/dL per unit.
How to Calculate a Correction Dose
Example: BG = 230 mg/dL · Target = 100 · ISF = 50
Correction = (230 − 100) ÷ 50 = 130 ÷ 50 = 2.6 → round to 2.5 units
Always subtract IOB first. If you gave a correction 2 hours ago and have 1.5 units still active (IOB), the net correction is 2.5 − 1.5 = 1 unit. Stacking corrections without accounting for IOB is a leading cause of hypoglycemia. Use the IOB Calculator →
Signs Your ISF Needs Adjustment
| Pattern | What It Suggests | Action |
|---|---|---|
| Corrections bring BG too low (hypo) | ISF is too low — insulin drops BG more than expected | Increase ISF (e.g., 50 → 60) |
| Corrections don't bring BG to target | ISF is too high — insulin not dropping BG enough | Decrease ISF (e.g., 50 → 40) |
| Corrections work for lunch but not breakfast | Time-of-day ISF variation needed | Consider separate AM/PM ISF with care team |
ISF adjustments are typically made by reviewing 3–5 correction episodes in similar conditions (same time of day, no active food, no exercise).
ISF in mmol/L
The 100 Rule is the mmol/L equivalent of the 1800 Rule. Example: TDD = 36 → ISF = 100 ÷ 36 ≈ 2.8 mmol/L per unit.
Sources
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
- ADA. Standards of Medical Care — 2024. Section 7.
Last reviewed: June 2025