How are correction dose and meal dose of insulin calculated?

Calculation of Insulin Correction and Meal Doses

Insulin doses are calculated using specific formulas where the meal dose is based on carbohydrate intake divided by the carbohydrate ratio, and the correction dose is calculated by dividing the difference between current and target glucose by the insulin sensitivity factor.

Meal Dose Calculation

The meal dose of insulin is required when carbohydrates are consumed and is calculated based on:

• Carbohydrate counting: Count total grams of carbohydrates in the meal
• Carbohydrate-to-insulin ratio (CIR): Defines how many grams of carbohydrate are covered by 1 unit of insulin 1

Formula for Meal Dose:

Meal Dose = Carbohydrates (g) ÷ Carbohydrate-to-insulin ratio

Determining Carbohydrate Ratio:

Several formulas exist to estimate the carbohydrate ratio:

• 300 ÷ TDD for breakfast 2
• 400 ÷ TDD for lunch and dinner 2
• 500 ÷ TDD (traditional formula, now considered to underestimate insulin needs) 3

Where TDD = Total Daily Dose of insulin

More recent research suggests these updated formulas:

• (217 ÷ TDD) + 3 4
• 300 ÷ TDD (revised formula) 5

Correction Dose Calculation

The correction dose is used to adjust for hyperglycemia above a predetermined target level:

Formula for Correction Dose:

Correction Dose = (Current glucose - Target glucose) ÷ Insulin Sensitivity Factor

Determining Insulin Sensitivity Factor (ISF):

Several formulas exist:

• 1500 ÷ TDD (revised formula) 5
• 1700 ÷ TDD (traditional formula) 4
• 1800 ÷ TDD (alternative formula) 6
• 1960 ÷ TDD (another alternative) 3

More recent research suggests this updated formula:

• (1076 ÷ TDD) + 12 4

Total Bolus Dose Calculation

When both meal coverage and correction are needed, the total bolus dose is:

Total Bolus = Meal Dose + Correction Dose

Or using the complete formula:

Total Bolus = (Carbohydrates ÷ CIR) + (Current glucose - Target glucose) ÷ ISF

Important Considerations

1. Insulin on Board (IOB): Modern insulin pumps calculate active insulin from previous doses to prevent insulin stacking 1

2. Time of day variations:

   • Insulin sensitivity may vary throughout the day
   • Many people require more insulin per carbohydrate in the morning due to counter-regulatory hormones 1

3. Individualization:

   • The carbohydrate ratio and insulin sensitivity factor should be adjusted based on blood glucose patterns
   • Adjust carbohydrate ratio if postprandial glucose is consistently out of target
   • Adjust insulin sensitivity factor if corrections don't bring glucose to target 6

4. Relationship between factors:

   • Research shows a relationship between CIR and ISF: ISF ≈ 4.44 × CIR 4
   • Another formula suggests: CIR = ISF ÷ 4.5 5

Practical Example

For a person with a Total Daily Dose (TDD) of 50 units:

1. Carbohydrate ratio: 300 ÷ 50 = 6 (1 unit per 6g of carbs)
2. Insulin sensitivity factor: 1500 ÷ 50 = 30 (1 unit lowers glucose by 30 mg/dL)
3. For a meal with 60g carbs and current glucose of 180 mg/dL (target 120 mg/dL):
   • Meal dose: 60g ÷ 6 = 10 units
   • Correction dose: (180 - 120) ÷ 30 = 2 units
   • Total bolus: 10 + 2 = 12 units

Common Pitfalls

1. Insulin stacking: Failing to account for insulin still active from previous doses
2. Inconsistent carb counting: Inaccurate estimation of carbohydrate content in meals
3. Fixed ratios: Not adjusting ratios based on time of day or changing insulin sensitivity
4. Outdated formulas: Using traditional formulas that may underestimate insulin needs
5. Not considering physical activity: Exercise can significantly affect insulin requirements

By understanding these calculations and regularly reviewing blood glucose patterns, insulin doses can be optimized to maintain target glucose levels and reduce the risk of both hyperglycemia and hypoglycemia.