Using the 500-rule, how is the insulin-to-carbohydrate ratio (ICR) derived?

Using the 500-rule, how is the insulin-to-carbohydrate ratio (ICR) derived?

• A. ICR = 600 ÷ total daily insulin (TDD); 1 unit covers roughly that many grams of carbohydrate.
• B. ICR = 500 ÷ total daily insulin (TDD); 1 unit covers roughly that many grams of carbohydrate; adjust by postmeal glucose.
• C. ICR = 400 ÷ TDD; 1 unit covers roughly that many grams of carbohydrate.
• D. ICR = 500 ÷ TDD; 1 unit covers roughly that many grams of carbohydrate; adjust by fasting glucose.

Answer: (B)

The main idea is to estimate how many grams of carbohydrate one unit of rapid-acting insulin will cover, using the total daily dose as the anchor. The 500-rule says ICR = 500 ÷ TDD, so one unit of insulin is expected to cover about that many grams of carbohydrate. As your total daily insulin increases, each unit covers fewer grams, so the ICR decreases accordingly. After meals, you monitor postmeal glucose to see if the bolus was appropriate and adjust the ICR based on that feedback to reach your target postprandial level. Adjusting by fasting glucose would mostly influence basal insulin or correction dosing, not the meal-time carbohydrate ratio. The 600- and 400-based rules aren’t the standard 500-rule.

The main idea is to estimate how many grams of carbohydrate one unit of rapid-acting insulin will cover, using the total daily dose as the anchor. The 500-rule says ICR = 500 ÷ TDD, so one unit of insulin is expected to cover about that many grams of carbohydrate. As your total daily insulin increases, each unit covers fewer grams, so the ICR decreases accordingly. After meals, you monitor postmeal glucose to see if the bolus was appropriate and adjust the ICR based on that feedback to reach your target postprandial level. Adjusting by fasting glucose would mostly influence basal insulin or correction dosing, not the meal-time carbohydrate ratio. The 600- and 400-based rules aren’t the standard 500-rule.