How is the total daily dosage of insulin calculated?

How Total Daily Insulin Dosage is Calculated

Total daily insulin dose (TDD) is calculated based on body weight, typically starting at 0.4-1.0 units/kg/day, with 0.5 units/kg/day being the standard starting point for metabolically stable patients with type 1 diabetes. 1

Initial Dosing Calculations

Weight-Based Approach

• Standard starting dose: 0.5 units/kg/day for metabolically stable adults with type 1 diabetes 1
• Range: 0.4-1.0 units/kg/day depending on clinical circumstances 1
• Lower doses (0.2-0.6 units/kg/day): Used in young children, those in partial remission ("honeymoon period"), or adults presenting with type 1 diabetes 1
• Higher doses: Required during puberty, pregnancy, medical illness, and in type 2 diabetes patients (often >1 unit/kg/day) 1

Type 2 Diabetes Initiation

For insulin-naïve type 2 diabetes patients inadequately controlled on oral medications:

• Start with 10 units per day OR 0.1-0.2 units/kg/day of basal insulin 1
• Titrate by increasing 2 units every 3 days to reach fasting plasma glucose goal 1

Distribution Between Basal and Prandial Insulin

Type 1 Diabetes

The evidence shows conflicting recommendations regarding basal-prandial split:

• Traditional teaching: 50% basal, 50% prandial 1
• Insulin pump data: Basal accounts for approximately 30-50% of TDD, with significant individual variation 1
• Recent research: Suggests basal may be closer to 27-30% of TDD when properly optimized 2, 3

The basal infusion rate should be individualized based on insulin sensitivity, dawn phenomenon, and daily activities, with adjustments made every 3-6 months 1

Calculating Pump Parameters from TDD

Once TDD is established, the following formulas derive pump settings:

Basal Insulin Dose

• TBD = 0.48 × TDD (approximately 48% of total daily dose) 4
• Alternative formula: TBD = 0.2 × weight (kg) 3

Carbohydrate-to-Insulin Ratio (CIR)

The evidence shows significant variation by time of day:

• Breakfast: CIR = 300/TDD 2, 3
• Lunch and dinner: CIR = 400/TDD 2
• Traditional formula (500/TDD) appears to underestimate insulin needs 2
• Alternative research suggests: CIR = 300/TDD for all meals 4

This ratio defines how many grams of carbohydrate are covered by 1 unit of insulin 1

Insulin Sensitivity Factor (Correction Factor)

• CF = 1500/TDD (more recent evidence) 3
• CF = 1960/TDD (alternative formula) 4
• Traditional formula: 1800/TDD

This determines how much 1 unit of insulin will lower blood glucose (e.g., 1:3 ratio means 1 unit drops glucose by 3 mmol/L or ~54 mg/dL) 1

Transitioning from IV to Subcutaneous Insulin

In hospitalized patients:

• Calculate TDD based on insulin infusion rate during prior 6-8 hours when stable glycemic goals were achieved 1
• Give subcutaneous basal insulin 2 hours before discontinuing IV infusion 1
• Consider adding low-dose basal analog (0.15-0.3 units/kg) during IV infusion to prevent rebound hyperglycemia 1

Key Clinical Pitfalls

• Avoid using outdated formulas: The 500 rule for CIR and 50/50 basal-prandial split may lead to underinsulinization 2, 3
• Account for diurnal variation: Morning insulin requirements are typically higher due to counter-regulatory hormones 1, 2
• Adjust for renal insufficiency: Lower doses needed due to decreased insulin clearance 1
• Reassess regularly: Pump parameters should be reviewed every 3-6 months, not fixed for life 1
• For hypoglycemia: If no clear cause, reduce corresponding dose by 10-20% 1