What is the proper administration of basal (long-acting) insulin, such as glargine (insulin glargine) or detemir (insulin detemir), versus prandial (rapid-acting) insulin, such as aspart (insulin aspart), in a patient with diabetes requiring insulin therapy?

How to Administer Basal vs Prandial Insulin

Basal insulin (long-acting) should be administered once daily at the same time each day to provide 24-hour background glucose control, while prandial insulin (rapid-acting) must be given 0-15 minutes before each meal to cover postprandial glucose excursions. 1

Basal Insulin Administration

Dosing and Timing

• Administer basal insulin (glargine or detemir) subcutaneously once daily at the same time every day, typically in the evening, into the abdominal area, thigh, or deltoid 2
• Starting dose for insulin-naive type 2 diabetes patients: 10 units once daily OR 0.1-0.2 units/kg body weight 1, 3
• For type 1 diabetes: approximately 40-60% of total daily insulin dose should be basal insulin (0.4-1.0 units/kg/day total, with 0.5 units/kg/day typical for metabolically stable patients) 1, 3
• Never dilute or mix basal insulin with any other insulin or solution 2

Titration Protocol

• Increase basal insulin by 2 units every 3 days if fasting glucose is 140-179 mg/dL 1, 3
• Increase by 4 units every 3 days if fasting glucose ≥180 mg/dL 1, 3
• Target fasting plasma glucose: 80-130 mg/dL 1, 3
• If hypoglycemia occurs without clear cause, reduce dose by 10-20% immediately 1, 3

Critical Threshold

• When basal insulin exceeds 0.5 units/kg/day and approaches 1.0 units/kg/day, add prandial insulin rather than continuing to escalate basal insulin alone 1, 3
• Clinical signals of "overbasalization" include: basal dose >0.5 units/kg/day, bedtime-to-morning glucose differential ≥50 mg/dL, hypoglycemia, and high glucose variability 1, 3

Prandial Insulin Administration

Dosing and Timing

• Administer rapid-acting insulin (aspart, lispro) 0-15 minutes before each meal to effectively manage postprandial glucose 1
• Starting dose: 4 units before the largest meal OR 10% of the basal insulin dose 1, 3
• For type 1 diabetes: approximately 50-60% of total daily insulin dose should be prandial insulin, divided among three meals 1, 3

Titration Protocol

• Increase prandial insulin by 1-2 units or 10-15% every 3 days based on 2-hour postprandial glucose readings 1, 3
• Target postprandial glucose: <180 mg/dL 1
• If hypoglycemia occurs, reduce the corresponding dose by 10-20% 1

Carbohydrate Coverage

• Use insulin-to-carbohydrate ratio: 500 ÷ total daily dose (for regular insulin) OR 450 ÷ total daily dose (for rapid-acting analogs) 3
• Common starting ratio: 1 unit per 10-15 grams of carbohydrate 3

Key Physiologic Principles

Basal Insulin Function

• Basal insulin suppresses hepatic glucose production overnight and between meals, providing continuous background insulin coverage throughout the day 1, 4
• Pre-lunch glucose is controlled predominantly by basal insulin, not by breakfast prandial insulin 3
• Basal insulin has a relatively flat action profile with no pronounced peak and approximately 24-hour duration 5, 6

Prandial Insulin Function

• Prandial insulin blunts postprandial glucose excursions following meals 1, 4
• Rapid-acting insulin has a duration of action of only 3-5 hours, designed specifically for meal coverage 3
• Prandial insulin should ideally be administered prior to meal consumption, with optimal timing varying based on formulation pharmacokinetics and premeal glucose level 1

Common Pitfalls to Avoid

• Never give rapid-acting insulin at bedtime to avoid nocturnal hypoglycemia 1, 3
• Never continue escalating basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia, as this leads to overbasalization with increased hypoglycemia risk 1, 3
• Never use sliding scale insulin as monotherapy—it treats hyperglycemia reactively rather than preventing it 3
• Always rotate injection sites to reduce risk of lipodystrophy and localized cutaneous amyloidosis 2
• Never share insulin pens between patients even if the needle is changed, due to risk of blood-borne pathogen transmission 7

Monitoring Requirements

• Daily fasting blood glucose monitoring is essential during basal insulin titration 1, 3
• Check pre-meal and 2-hour postprandial glucose to guide prandial insulin adjustments 3
• Reassess adequacy of insulin dose at every clinical visit, looking specifically for signs of overbasalization 1, 3