How to Calculate Prandial Insulin Dose
For a 70kg patient with type 2 diabetes already on basal insulin, start prandial insulin at 4 units of rapid-acting insulin before the largest meal, or alternatively use 10% of the current basal insulin dose. 1, 2
When to Add Prandial Insulin
Add prandial insulin when any of the following criteria are met:
- Basal insulin has been optimized (fasting glucose 80-130 mg/dL) but A1C remains above target after 3-6 months 1, 2
- Basal insulin dose exceeds 0.5 units/kg/day (35 units for a 70kg patient) 1, 2
- Significant postprandial hyperglycemia persists (>180 mg/dL at 2 hours post-meal) despite adequate fasting control 1, 2
- Clinical signs of "overbasalization" appear: bedtime-to-morning glucose differential ≥50 mg/dL, hypoglycemia episodes, or high glucose variability 1
Consider adding a GLP-1 receptor agonist before advancing to prandial insulin to minimize hypoglycemia and weight gain risks. 1
Initial Prandial Insulin Dosing
Two Evidence-Based Starting Approaches:
Option 1 (Preferred): Start with 4 units of rapid-acting insulin (lispro, aspart, or glulisine) before the largest meal 1, 2
Option 2: Calculate 10% of the current basal insulin dose 1, 2
- Example: If on 40 units of basal insulin, start with 4 units prandial insulin
Timing of Administration:
- Rapid-acting insulin analogues: 0-15 minutes before meals 1, 3, 4, 5
- Regular human insulin: 30-45 minutes before meals 1, 3, 4
Titration Algorithm
Increase prandial insulin by 1-2 units (or 10-15%) every 3 days based on 2-hour postprandial glucose readings until postprandial glucose is <180 mg/dL. 1, 2
Specific Titration Steps:
- Check 2-hour postprandial glucose after each meal where prandial insulin is given 2
- If postprandial glucose consistently >180 mg/dL: increase dose by 1-2 units 1, 2
- If hypoglycemia occurs without clear cause: reduce dose by 10-20% immediately 1
- Reassess every 3 days during active titration 1, 2
Expanding to Multiple Meals
Once the first meal is controlled, add prandial insulin to the next meal with the greatest postprandial excursion, using the same starting dose and titration approach. 1
Typical Progression:
- Start with largest meal or meal causing greatest glucose excursion 1
- Titrate to target over 1-2 weeks 1
- Add to second meal if needed 1
- Add to third meal if needed 1
Basal Insulin Adjustment When Adding Prandial Coverage
When significant additions to prandial insulin are made, particularly with the evening meal, consider decreasing basal insulin to prevent nocturnal hypoglycemia. 1
- If basal insulin dose approaches or exceeds 0.5-1.0 units/kg/day (35-70 units for 70kg patient), stop escalating basal insulin and focus on prandial coverage 1
Alternative: Carbohydrate Counting Method
For patients who count carbohydrates, calculate insulin-to-carbohydrate ratio (ICR):
- Formula: 450 ÷ total daily insulin dose (for rapid-acting analogues) 1
- Example: If total daily dose is 50 units: 450 ÷ 50 = 9 grams of carbohydrate per 1 unit of insulin
Critical Pitfalls to Avoid
- Never continue escalating basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia – this leads to overbasalization with increased hypoglycemia risk and suboptimal control 1, 2
- Never give rapid-acting insulin at bedtime – this significantly increases nocturnal hypoglycemia risk 1
- Never use sliding scale insulin as monotherapy – scheduled basal-bolus regimens are superior 1, 6
- Do not discontinue metformin when adding insulin unless contraindicated – the combination provides superior control with less weight gain 1, 6
Monitoring Requirements
- Daily fasting blood glucose monitoring during titration 1, 2
- Check 2-hour postprandial glucose to guide prandial insulin adjustments 1, 2
- Reassess every 3-6 months once stable 1, 2
- Monitor for signs of overbasalization at every visit 1, 2