How do I convert 21 units of lispro (insulin lispro) to NPH (neutral protamine Hagedorn) insulin?

Converting Insulin Lispro to NPH Insulin

When converting 21 units of insulin lispro to NPH insulin, use a conversion ratio of 80% of the lispro dose, resulting in approximately 17 units of NPH insulin.

Conversion Principles

• When converting from a rapid-acting insulin (lispro) to an intermediate-acting insulin (NPH), the total daily dose should be reduced to approximately 80% of the original dose due to differences in pharmacokinetic profiles 1
• For a dose of 21 units of lispro, the equivalent NPH dose would be approximately 17 units (21 × 0.8 = 16.8, rounded to 17) 1
• NPH insulin has a delayed onset and longer duration of action compared to lispro, requiring dose adjustments to prevent hypoglycemia 1

Administration Considerations

• Unlike lispro which is administered immediately before meals, NPH insulin should be administered 30-45 minutes before meals due to its delayed onset of action 1
• Consider splitting the NPH dose into morning and evening injections (2/3 morning, 1/3 evening) for better 24-hour coverage 1
• Morning administration of NPH is particularly effective for managing daytime hyperglycemia 2, 3

Dosing Adjustments

• Start with the calculated dose of 17 units and titrate by 2 units every 3 days until the target blood glucose is achieved without hypoglycemia 1
• Monitor blood glucose levels closely after conversion, particularly fasting and pre-meal values 1
• If hypoglycemia occurs, reduce the NPH dose by 10-20% (approximately 2-3 units) 1

Special Considerations

• NPH has a peak effect at 4-6 hours after administration, which differs significantly from lispro's rapid peak at 1 hour 1
• The pharmacodynamic profile of NPH makes it particularly suitable for covering basal insulin needs and addressing post-meal hyperglycemia that occurs several hours after eating 1, 3
• For patients requiring mealtime coverage, consider adding a small dose of rapid-acting insulin before meals if postprandial glucose remains elevated 1

Common Pitfalls to Avoid

• Avoid direct unit-for-unit conversion between lispro and NPH due to their different pharmacokinetic profiles 1
• Be aware that NPH has a higher risk of nocturnal hypoglycemia compared to rapid-acting insulins like lispro 1
• Remember that NPH insulin requires resuspension before administration (roll, don't shake the vial) to ensure proper dosing 1