Advanced Carbohydrate Counting

Introduction

In patients on a basal-bolus insulin regimen with a history of well-controlled blood glucose, it may be feasible to maintain the same insulin doses each day, assuming consistent carbohydrate intake at every meal.

  • However, this fixed-dose approach can result in hyperglycemia or hypoglycemia when actual carbohydrate intake is higher or lower than usual, respectively.

A more individualized and flexible approach is to teach patients advanced carbohydrate counting.

  • This method allows adjustment of rapid- or short-acting insulin doses based on both the amount of carbohydrates consumed and the pre-meal blood glucose level, as measured by self-monitoring of blood glucose (SMBG).

NOTE: In diabetes meal planning, a patient can determine the carbohydrate content by referring to a nutrition facts label. For estimation purposes, one carbohydrate serving is equivalent to approximately 15 grams of carbohydrates.



Insulin-to-Carbohydrate Ratio (ICR)

The Insulin-to-Carbohydrate Ratio (ICR) indicates how many grams of carbohydrate are covered by 1 unit of rapid- or short-acting insulin.

  • It is used to calculate the meal-time insulin dose based on planned carbohydrate intake.

There are two commonly used estimation rules:

  • Rule of 500: For rapid-acting insulin (e.g., NovoRapid)
    ICR = 500 ÷ Total Daily Insulin Dose (TDD)
  • Rule of 450: For short-acting insulin (e.g., Insugen-R)
    ICR = 450 ÷ TDD

Meal-time insulin (units) = Total Carbohydrate Intake (g) / Insulin-to-Carbohydrate Ratio (ICR)

NOTE: Total Daily Insulin Dose (TDD) includes both basal (long-acting) and bolus (rapid- or short-acting) insulin.



Insulin Sensitivity Factor (ISF)

The Insulin Sensitivity Factor (ISF), also called the Insulin Correction Factor (ICF), estimates how much 1 unit of rapid- or short-acting insulin will lower blood glucose. The reduction is typically expressed in mmol/L (or mg/dL in some regions).

Estimation rules for blood glucose in mmol/L include:

  • Rule of 100: For rapid-acting insulin
    ISF = 100 ÷ TDD
  • Rule of 83: For short-acting insulin
    ISF = 83 ÷ TDD

A target blood glucose (BG) of 7-8 mmol/L is often used when calculating corrections to avoid hypoglycemia from overcorrection.

  • However, the target should always be individualized based on patient age, comorbidities and hypoglycemia risk.
Correction Insulin = [(Current BG – Target BG) ÷ ISF]

NOTE: To apply ISF correctly, a pre-meal blood glucose reading is required.



Summary

The full pre-meal insulin dose accounts for both the carbohydrate content of the meal and any necessary correction for elevated blood glucose.

Total Pre-meal Insulin = Meal-time insulin + Correction Insulin
Total Pre-meal Insulin = (Carbohydrate Intake ÷ ICR) + [(Current BG – Target BG) ÷ ISF]

NOTE: Correction Insulin shoudl be considered only if pre-meal blood glucose is elevated.

While this approach is more accurate and flexible, real-world application presents several challenges:

  • Carbohydrate estimation errors
    • Patients may underestimate or overestimate carbohydrate intake, leading to incorrect insulin dosing.
  • Omission of pre-meal BG check
    • Some patients may perform only carbohydrate-based bolus calculations without checking their BG, leaving pre-existing hyperglycemia uncorrected.
  • Insulin stacking
    • Patients may give multiple correction doses within a short time, unaware that the previous rapid-acting insulin dose is still active.
    • This increases the risk of hypoglycemia.



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