Endocrinology

Insulin Sensitivity Factor (ISF / 1800 Rule) & Carb-to-Insulin Ratio (CIR)

Calculates ISF correction factor, CIR carbohydrate ratio, and combined mealtime bolus dosing.

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Parameters

Insulin Sensitivity Factor (ISF / 1800 Rule) & Carb-to-Insulin Ratio (CIR) Result

Formula

Rapid-Acting Analogs (Lispro / Aspart / Glulisine):
• ISF (mg/dL per unit) = 1800 ÷ TDD
• CIR (grams per unit) = 500 ÷ TDD
• Pre-Meal Bolus = (Carbs ÷ CIR) + [(Current BG - Target BG) ÷ ISF]

Clinical Pearls

Empowers personalized intensive insulin therapy (MDI and smart insulin pumps), transforming brittle diabetes management into precise math.

Limitations

Calculations should be tailored during periods of intense exercise (which increases insulin sensitivity) or high stress / steroids (which cause severe insulin resistance).