What are insulin to carb (carbohydrate) ratio and correction factor?

Insulin to Carb Ratio and Correction Factor: Essential Tools for Diabetes Management

The insulin to carbohydrate ratio (ICR) is defined as the amount of carbohydrate (in grams) covered by 1 unit of insulin, while the correction factor (also called insulin sensitivity factor) is an individualized measure of how much blood glucose will decrease with 1 unit of insulin. 1

Understanding Insulin to Carbohydrate Ratio (ICR)

The ICR is a critical calculation used to determine mealtime insulin doses based on carbohydrate intake:

• Definition: The number of grams of carbohydrate that one unit of insulin will cover

• Calculation: Typically calculated from the total daily insulin dose (TDID)

  • Standard formula: 500 ÷ TDID 2
  • However, more accurate formulas may be:
    • 350 ÷ TDID for breakfast
    • 400 ÷ TDID for lunch and dinner 2, 3

• Example: With an ICR of 1:10, one unit of insulin covers 10g of carbohydrate 1

• Application: When a patient plans to eat 50g of carbohydrate with an ICR of 1:10, they would need 5 units of insulin

Diurnal Variations in ICR

The ICR often varies throughout the day due to physiological factors:

• Morning insulin requirements are typically higher (lower ICR) due to dawn phenomenon and counter-regulatory hormones 1

• Research in Mediterranean populations found:

  • Breakfast: 11.5 g/unit
  • Lunch: 12 g/unit
  • Dinner: 13.3 g/unit 2

• Japanese studies found similar patterns with CIR×TDID values of:

  • Breakfast: 288±73
  • Lunch: 408±92
  • Dinner: 387±83 3

Understanding Correction Factor (CF)

The correction factor, also called insulin sensitivity factor (ISF), determines how much insulin is needed to correct high blood glucose:

• Definition: How much blood glucose (in mg/dL or mmol/L) will decrease with 1 unit of insulin 1
• Calculation: Typically estimated as 1800 ÷ TDID 4
• Example: With a CF of 1:50, one unit of insulin will lower blood glucose by 50 mg/dL
• Application: If current glucose is 200 mg/dL, target is 100 mg/dL, and CF is 1:50, then 2 units of correction insulin would be needed

Practical Application of ICR and CF

The complete insulin dose calculation combines both carbohydrate coverage and glucose correction:

Total insulin dose = (Carbohydrates ÷ ICR) + (Current glucose - Target glucose) ÷ CF

For example:

• If eating 60g carbs with ICR of 1:15
• Current glucose is 180 mg/dL, target is 100 mg/dL
• CF is 1:40
• Total dose = (60 ÷ 15) + (180 - 100) ÷ 40 = 4 + 2 = 6 units

Important Considerations

1. Insulin stacking: Modern insulin pumps track "insulin on board" to prevent hypoglycemia from overlapping insulin doses 1

2. Protein and fat impact:

   • Protein has minimal effect on blood glucose with adequate insulin but can contribute to hyperglycemia with insulin deficiency 5
   • For high-protein meals, some research suggests using 1 unit of insulin for every 50g of protein 6

3. Regular adjustment:

   • ICR and CF should be reviewed regularly (every 3-6 months) 1
   • Adjustments may be needed based on weight changes, exercise patterns, menstrual cycles, or other physiological changes

4. Monitoring effectiveness:

   • If postprandial glucose is consistently out of target, adjust the ICR
   • If corrections don't bring glucose to target, adjust the CF 4

Pitfalls to Avoid

• Underestimating insulin needs: Traditional formulas (500/TDID) may underestimate insulin requirements, particularly at breakfast 2, 3
• Ignoring diurnal variations: Using the same ICR throughout the day may lead to suboptimal control
• Failing to account for insulin on board: Not considering active insulin from previous doses can lead to insulin stacking and hypoglycemia 1

By properly understanding and applying ICR and CF concepts, patients with diabetes can achieve better glycemic control, reducing morbidity and mortality associated with both hyper- and hypoglycemia.