If a patient is receiving regular insulin 6 units with 100 g dextrose in total parenteral nutrition and the dextrose is increased to 170 g, how many units of regular insulin should be added to the TPN?

Insulin Dosing Adjustment for Increased Dextrose in TPN

Increase the regular insulin to approximately 10 units when dextrose is raised from 100 g to 170 g in the TPN solution. 1

Calculation Method

• The standard initial insulin-to-dextrose ratio for TPN is 0.1 units per gram of dextrose (equivalent to 1 unit per 10 g of carbohydrate). 1
• With 100 g dextrose receiving 6 units of insulin, the current ratio is 0.06 units/g, which is below the standard starting ratio and may explain inadequate glycemic control. 1
• When increasing to 170 g dextrose, apply the standard 0.1 units/g ratio: 170 g × 0.1 = 17 units of regular insulin. 1
• However, since the patient is already on 6 units with 100 g, a proportional increase would be: (170 g ÷ 100 g) × 6 units = 10.2 units, which can be rounded to 10 units. 1

Practical Dosing Approach

• Start with 10 units of regular insulin mixed directly into the TPN bag containing 170 g dextrose. 1
• This maintains the patient's current insulin-to-dextrose ratio (0.06 units/g) while accounting for the increased carbohydrate load. 1
• Only regular human insulin should be added to TPN bags; rapid-acting insulin analogues (lispro, aspart, glulisine) are incompatible with parenteral nutrition solutions. 1

Titration Protocol After Initial Adjustment

• Check capillary glucose every 6 hours during the first 24–48 hours after the TPN change. 1
• Target glucose range: 140–180 mg/dL for most hospitalized patients receiving TPN. 1
• If >20 units of correctional (sliding-scale) insulin are required within any 24-hour period, add the total amount to the next day's TPN bag. 1
• When glucose remains >180 mg/dL despite adjustments, raise 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. 1

Critical Safety Considerations

• Continue the patient's usual basal insulin separately, even when TPN provides all nutrition, to prevent ketosis in insulin-dependent patients. 1
• Insulin can adsorb to ethylene-vinyl-acetate (EVA) TPN bags, with reported losses ranging from 5% to 56%, but the standard 1:10 ratio accounts for typical adsorption losses. 1, 2, 3
• If TPN is unexpectedly stopped, start a 10% dextrose infusion at 50–100 mL/h immediately to prevent hypoglycemia, because insulin already mixed in the discontinued TPN bag continues to be absorbed for several hours. 1

When to Reassess the Insulin-to-Dextrose Ratio

• If supplemental insulin requirements exceed 0.2 units per gram of dextrose (34 units for 170 g), adjust the TPN formulation by increasing lipid calories and decreasing dextrose calories rather than continuously escalating insulin doses. 1
• The current ratio of 0.06 units/g is well below this threshold, so increasing to 10 units (0.059 units/g) remains safe. 1
• Keep serum triglycerides <400 mg/dL (optimally <700–800 mg/dL maximum) when increasing lipid content. 1

Alternative Approach for Insulin-Resistant Patients

• For patients with type 2 diabetes and known insulin resistance, an initial ratio of 1 unit per 8–10 g carbohydrate may be required, which would translate to 17–21 units for 170 g dextrose. 1
• However, given the patient's current low ratio, start conservatively at 10 units and titrate upward based on glucose response over the next 24–48 hours. 1

Common Pitfalls to Avoid

• Do not rely solely on sliding-scale correctional insulin without adjusting the scheduled insulin in the TPN bag; this reactive approach is discouraged by major guidelines. 1
• Never discontinue basal insulin in type 1 diabetes or insulin-dependent type 2 diabetes, even if TPN provides all nutrition. 1
• Do not give a single bedtime correctional insulin dose, as it markedly raises nocturnal hypoglycemia risk. 1
• Avoid adding rapid-acting insulin analogues to TPN bags; only regular insulin is compatible. 1