If total parenteral nutrition (TPN) containing 170 g dextrose and 15 units regular insulin is infused over 24 hours, how many units of regular insulin should be added when the same TPN is infused over 18 hours?

Adjusting Insulin in TPN When Reducing Infusion Time from 24 to 18 Hours

Increase the insulin in the TPN bag from 15 units to approximately 20 units (a 33% increase) to maintain the same insulin-to-dextrose ratio when shortening the infusion from 24 to 18 hours. 1

Calculation Rationale

• The current regimen delivers 170 g dextrose with 15 units regular insulin over 24 hours, yielding a glucose infusion rate of approximately 7.1 g/hour and an insulin-to-dextrose ratio of 0.088 units per gram 1
• Compressing the same 170 g dextrose into 18 hours raises the glucose infusion rate to approximately 9.4 g/hour—a 33% increase in hourly delivery 1
• To preserve the same insulin-to-dextrose ratio and prevent hyperglycemia, the insulin dose must increase proportionally by 33%, from 15 units to approximately 20 units 1

Safety Protocols for Cyclic TPN

• Taper the infusion rate gradually: increase the rate over the first 1–2 hours at start-up and reduce to 50% during the final 30 minutes to prevent rebound hypoglycemia when the infusion stops 1
• Maximum safe glucose infusion rate: ensure the rate stays below 5–7 mg/kg/min; for a typical 70-kg adult, 170 g over 18 hours yields approximately 4.7 mg/kg/min, which remains within safe limits 1
• Emergency protocol for unexpected interruption: if TPN is stopped abruptly, immediately start 10% dextrose at 50–100 mL/hour because the insulin already mixed in the bag continues to be absorbed for several hours, creating high hypoglycemia risk 1

Monitoring Requirements

• Check capillary glucose at least every 6 hours during the initial 24–48 hours after the schedule change 1
• Target glucose range: 140–180 mg/dL for most hospitalized patients receiving TPN 1
• If glucose remains >180 mg/dL despite the adjustment, increase the TPN-bag insulin by 10–20% per day 1
• If glucose falls <70 mg/dL, reduce the TPN-bag insulin by 10–20% immediately and treat with fast-acting carbohydrate if the patient can take oral intake 1

Daily Insulin Adjustment Algorithm

• If >20 units of correctional (sliding-scale) insulin are required in any 24-hour period, add the total correctional insulin used to the next day's TPN bag 1
• Provide separate subcutaneous correctional insulin: regular insulin every 6 hours or rapid-acting insulin every 4 hours for hyperglycemia, in addition to the scheduled insulin in the TPN bag 1

Insulin-to-Dextrose Ratio Thresholds

• The proposed 20-unit dose yields an insulin-to-dextrose ratio of approximately 0.12 units per gram (20 U ÷ 170 g), which remains well below the 0.2 units/g threshold that would trigger consideration of increasing lipid calories and decreasing dextrose 1
• If supplemental insulin requirements exceed 0.2 units per gram of dextrose, adjust the TPN formulation by increasing lipid calories and decreasing dextrose calories rather than continuing to escalate insulin 1

Common Pitfalls to Avoid

• Do not rely solely on sliding-scale insulin for patients on TPN; this reactive approach is discouraged by major guidelines due to erratic glucose control 2, 1
• Avoid adding rapid-acting insulin analogs (lispro, aspart, glulisine) to TPN bags; only regular insulin is compatible with parenteral nutrition solutions 1
• Never omit basal insulin in type 1 diabetes or insulin-dependent type 2 diabetes, even when TPN supplies all calories, to prevent ketoacidosis 1
• Do not give a single bedtime correctional insulin dose, as this markedly raises nocturnal hypoglycemia risk 1

Expected Clinical Outcomes

• With the 33% insulin increase (from 15 to 20 units), glucose should reach the 140–180 mg/dL target range within 24–48 hours of the schedule change 1
• If glucose remains >180 mg/dL after 24 hours, repeat the adjustment protocol by adding any further correctional insulin used to the next TPN bag 1