How is insulin dosage calculated?

Insulin Dosage Calculation

Insulin dosage should be calculated based on the patient's weight, with typical doses ranging from 0.4 to 1.0 units/kg/day, with approximately 50% as basal insulin and 50% as prandial insulin. 1

Basal Insulin Calculation

Basal insulin provides background insulin coverage between meals and overnight, typically accounting for 50% of the total daily insulin requirement:

• Start with 0.2-0.5 units/kg/day for newly diagnosed patients or those who are metabolically stable 1
• Higher doses (0.5-0.7 units/kg/day) may be required during puberty, pregnancy, or medical illness 1
• Adjust basal insulin every 3 days to achieve target fasting glucose of 80-130 mg/dL 2
  • Increase dose by 2 units if fasting glucose remains above target
  • Decrease dose by 2-4 units if nocturnal or fasting hypoglycemia occurs

Prandial (Bolus) Insulin Calculation

Prandial insulin covers mealtime carbohydrate intake and corrects elevated blood glucose:

1. Carbohydrate Coverage:

   • Calculate using insulin-to-carbohydrate ratio (ICR)
   • Typical starting ICR: 1 unit per 10-15g carbohydrate 1
   • Formula: Carbohydrate grams ÷ ICR = Insulin units needed

2. Correction Dose:

   • Based on insulin sensitivity factor (ISF)
   • ISF represents how much 1 unit of insulin will lower blood glucose
   • Formula: (Current BG - Target BG) ÷ ISF = Correction insulin units
   • For insulin-naive patients starting prandial insulin, begin with 4 units or 10% of basal insulin dose at the largest meal 1

3. Total Bolus Calculation:

   • Total bolus = Carbohydrate dose + Correction dose

Example Calculation

For a patient with:

• Weight: 70 kg
• Blood glucose: 250 mg/dL
• Target blood glucose: 125 mg/dL
• Carbohydrate intake: 60g
• ICR: 1:10
• ISF: 1:25

The calculation would be:

• Carbohydrate dose: 60g ÷ 10 = 6 units
• Correction dose: (250 - 125) ÷ 25 = 5 units
• Total bolus dose: 6 + 5 = 11 units 1

Special Considerations

1. Hospital Setting:

   • For enteral/parenteral nutrition: Calculate 1 unit of insulin for every 10-15g of carbohydrate in the formula 1
   • For patients on glucocorticoids: Consider NPH insulin two or three times daily to match steroid pharmacokinetics 1

2. Insulin Pump Therapy:

   • Basal rates are programmed hourly to match individual needs
   • Bolus doses calculated using the same principles as above
   • Modern pumps include bolus calculators that factor in "insulin on board" to prevent stacking 3

3. Type 2 Diabetes:

   • Often require higher insulin doses (up to 1 unit/kg/day) compared to type 1 diabetes 1
   • May need adjustment of oral medications when adding insulin

Monitoring and Adjustment

• Adjust basal insulin based on fasting and overnight glucose patterns
• Adjust prandial insulin based on 2-hour postprandial glucose values
• For persistent hyperglycemia despite adequate basal coverage, consider adding or increasing prandial insulin
• For persistent hypoglycemia, reduce insulin doses accordingly

Common Pitfalls to Avoid

1. Insulin Stacking: Administering correction doses too frequently without accounting for insulin already active in the body
2. Fixed Dosing: Failing to adjust for varying carbohydrate intake or activity levels
3. Ignoring Patterns: Not recognizing consistent patterns in glucose levels that indicate a need for dose adjustment
4. Inadequate Monitoring: Insufficient blood glucose testing to make informed dose adjustments

Remember that insulin requirements may change with illness, stress, physical activity, and other factors, requiring ongoing assessment and adjustment of the insulin regimen.