What is the recommended insulin to carb ratio and correction dose for a patient on Lantus (insulin glargine) 35 units in the morning and 25 units at night, using a Humalog (insulin lispro) infusion pump?

Calculating Insulin-to-Carb Ratio and Correction Factor for Pump Therapy

Initial Calculation Based on Total Daily Dose

For this patient on 60 units of Lantus daily (35 AM + 25 PM), start with an insulin-to-carb ratio of 1:10 at lunch and dinner, 1:6 at breakfast, and a correction factor of 1:25 (meaning 1 unit lowers glucose by approximately 25 mg/dL). 1, 2, 3

Step 1: Determine Total Daily Dose (TDD)

• Current basal insulin: 60 units of Lantus daily 1
• When transitioning to pump therapy, approximately 40-60% of TDD should be basal, with the remainder as bolus insulin 4
• Initial pump TDD estimate: Start with current 60 units, then redistribute as approximately 50% basal (30 units/day) and 50% bolus 4

Step 2: Calculate Insulin-to-Carb Ratio (ICR)

Use the 300/400 rule rather than the outdated 500 rule:

• Breakfast ICR = 300 ÷ TDD = 300 ÷ 60 = 1:5 (round to 1:6 for safety) 5, 6
• Lunch/Dinner ICR = 400 ÷ TDD = 400 ÷ 60 = 1:6.7 (round to 1:10 for safety) 5

The breakfast ratio is significantly lower because counter-regulatory hormones (cortisol, growth hormone) increase insulin resistance in early morning hours 4, 1

Step 3: Calculate Correction Factor (Insulin Sensitivity Factor)

Use the 1500-1800 rule:

• Correction Factor = 1500 ÷ TDD = 1500 ÷ 60 = 1:25 2, 3
• This means 1 unit of Humalog will lower blood glucose by approximately 25 mg/dL 4, 1
• Some patients may need the more conservative 1800 rule (1800 ÷ 60 = 1:30), particularly if hypoglycemia-prone 3

Pump Programming Specifications

Basal Rate Programming

• Total basal: approximately 30 units/24 hours = 1.25 units/hour as starting average 4
• Program variable hourly rates to account for dawn phenomenon (typically higher rates 4 AM-10 AM) 4
• Verify basal rates through structured fasting tests before finalizing bolus calculations 4, 1

Bolus Calculator Settings

• Target glucose: 100-120 mg/dL pre-meal, 150 mg/dL post-meal 4
• Duration of insulin action (DIA): Set to 3 hours 6
• The 4-hour DIA commonly recommended is too long for most patients and increases insulin stacking risk 6

Critical Adjustments and Monitoring

When to Adjust ICR

• If post-meal glucose consistently >20% above pre-meal value at 2-4 hours, decrease the ICR denominator (e.g., change 1:10 to 1:8) 4, 1
• If post-meal glucose consistently drops >20 mg/dL below target, increase the ICR denominator (e.g., change 1:10 to 1:12) 4, 1
• Ensure carbohydrate counting accuracy before adjusting ratios 1

When to Adjust Correction Factor

• If correction bolus fails to bring glucose to target range within 3-4 hours, decrease the correction factor denominator (e.g., change 1:25 to 1:20) 4, 1
• If corrections cause hypoglycemia, increase the correction factor denominator (e.g., change 1:25 to 1:30) 4, 1

Common Pitfalls to Avoid

Insulin Stacking

• The pump bolus calculator must account for "insulin on board" from previous boluses 4
• With DIA set to 3 hours, any insulin given within the previous 3 hours reduces the correction dose automatically 6
• Manually overriding the calculator's recommendations frequently leads to hypoglycemia 4

Fixed Ratios Throughout Day

• Using the same ICR for all meals ignores physiologic insulin resistance patterns 4, 1
• Breakfast typically requires 40-50% more insulin per gram of carbohydrate than other meals 5, 6
• Consider programming 3 different ICR time blocks: breakfast (1:6), lunch (1:10), dinner (1:10) 1, 5

Inadequate Basal Testing

• Bolus calculations are only accurate if basal rates are properly set first 4, 1
• Perform structured meal-skipping tests to verify basal rates maintain glucose 70-170 mg/dL during fasting 4
• Incorrect basal rates will make ICR and correction factor adjustments ineffective 1

Underestimating Bolus Needs

• The outdated 500 rule significantly underestimates bolus insulin requirements 5, 3
• Research shows optimal control requires using 300/400 rules instead 5
• Patients in poor control are often relatively under-insulinized despite using more total daily insulin 2

Humalog-Specific Considerations

Timing and Administration

• Administer Humalog boluses 15 minutes before meals or immediately after meals 7
• Humalog has more rapid onset and shorter duration than regular insulin 7
• Change pump reservoir every 7 days maximum; change infusion set and site every 3 days maximum 7

Pump Compatibility

• Do not dilute or mix Humalog with any other insulin in the pump 7
• Humalog is compatible with MiniMed, Disetronic, and equivalent pump systems 7
• Rotate infusion sites within same region to reduce lipodystrophy risk 7

Reassessment Timeline

• Reassess all pump settings after 1-2 weeks of continuous glucose data 1
• ICR and correction factors change with weight, activity level, and insulin sensitivity 4, 1
• Women should monitor for menstrual cycle effects requiring temporary adjustments 1
• Exercise significantly impacts glucose; consider temporary basal rate reductions of 20-50% during and after activity 4, 1