TPN Calculations

The major calculations for TPN solutions are as follows:

Step 1: Calculate the TDE, apply appropriate activity and stress factors
Step 2: Calculate the daily protein (amino acids) required in grams
Step 3: Calculate the number of calories provided by the amino acids
Step 4: Calculate the volume of amino acid solution required
Step 5: Calculate the lipid requirement in kcal
Step 6: Calculate the volume of lipid emulsion required
Step 7: Calculate the number of calories provided by carbohydrate
Step 8: Calculate the daily carbohydrate (dextrose) required in grams
Step 9: Calculate the volume of dextrose solution required
Step 10: Calculate the daily fluid requirement

Solved Problem: A 64 year old, 95 kg , 5”9’ man has undergone bowel resection surgery (stress factor = 1.2), and is ambulatory (activity factor 1.2). The solutions to be used are 8.5% w/v amino acid solution, 20 % w/v lipid emulsion, and 70 % w/v dextrose solution. Calculate the volume of amino acid solution, lipid emulsion and dextrose solution and any additional fluid that the patient should receive per day.

Approach: Follow steps 1-10 as listed above.

Step 1: Calculate the TDE, apply appropriate activity and stress factors


The patient is 95 kg and measures 175 cm, first calculate the BEE using the Harris-Benedict equation, then calculate the total daily expenditure using the appropriate activity and stress factors.

$$ {BEE_{men}} = {66.5 \ + \ 13.75 (95) \ + \ 5(175) \ - \ 6.78 (64)} = {1816 \ kcal/day}$$

$$ TDE = {1814 \ kcal \times 1.2 \times 1.2 } = 2612 \ kcal/day $$

Step 2: Calculate the daily protein (amino acids) required in grams

The daily protein requirements in adults are generally estimated to be:

  • 0.8 g/kg/day for normal, unstressed patients
  • 1-1.2 g/kg/day for hospitalized patients
  • 1.5-2 g/kg/day for stressed patient (critical illness, infection or trauma)

    • The patient is hospitalized, there are no other factors such as infection, and the patient is above IBW, let's choose a number at the low end of the hospitalized category (1 g/kg/day).

    $$ { 1 \ g \over kg/day } \times 95 \ kg = {95 \ g/day}$$

    Step 3: Calculate the number of calories provided by the amino acids

    The protein requirement for the patient is 95 grams, each gram of protein provides 4 kcal. The protein source used in the TPN for the patient is 8.5 % amino acid solution. Calculate the number of calories provided by the amino acids.

    $$ {95 \ g \over day } \times {4 \ kcal \over g} = {380 \ kcal/day}$$

    Step 4: Calculate the volume of amino acid solution required

    The protein source used in the TPN for the patient is 8.5 % amino acid solution. The protein requirement for the patient is 95 g. Calculate the volume of 8.5 % amino acid solution that will provide 95 g of protein.

    $$ {8.5 \ g \over 100 \ ml} = {95 \ g \over x \ ml };x = 1118 \ ml$$

    Step 5: Calculate the lipid requirement in kcal

    The proportion of calories provided by lipids is restricted to 20-40% of the total daily calories. For this calculation let’s choose 30 % of total daily calories being provided by the lipid emulsion.

    $$ 2612 \ kcal/day \times 0.3 = 784 \ kcal/day $$

    Step 6: Calculate the volume of lipid emulsion required

    The lipid source used in the TPN for the patient is 20 % lipid emulsion. The 20 % lipid emulsion provides with 2 kcal/ml. Calculate the volume of lipid emulsion required to provide 784 kcal/day.

    $$ {784 \ kcal \over day} \times {1 \ ml \over 2 \ kcal} = 392 \ ml/day $$

    Step 7: Calculate the number of calories provided by carbohydrate

    The carbohydrate source used in the TPN for the patient is 70 % dextrose solution. In earlier steps, the total calorie requirements, calories provided by proteins and lipids have been calculated. Calculate the calories provided by the carbohydrate after accommodating the contribution of the proteins and lipids.

    $$ 2612 \ kcal/day(TDE) - 380 \ kcal/day(lipids) - 784 \ kcal/day(proteins) = 1448 \ kcal/day $$

    Step 8: Calculate the daily carbohydrate (dextrose) required in grams

    Each gram of dextrose provides with 3.4 kcal. Calculate the grams of dextrose that provide 1448 kcal.

    $$ {14448 \ kcal \over day } \times {1 \ g \over 3.4 \ kcal} = 452.8 \ g/day $$

    Step 9: Calculate the volume of dextrose solution required

    The carbohydrate source used in the TPN for the patient is 70 % dextrose solution. Calculate the volume of 70 % dextrose solution that will provide with 425.8 g of dextrose per day.

    $$ {425.8 \over day } \times {100 \ ml \over 70 \ g} = 608 \ ml/day $$

    Step 10: Calculate the daily fluid requirement

    Based on 30 ml/kg/day

    $$ {30 \ ml \over kg/day} \times 95 = 2850 \ ml/day $$

    Calculate the total volume provided by the nutrients and subtract it from the daily fluid requirement to find amount of additional fluids needs.

    $$ Volume \ provided \ by \ macronutrients = 1118 \ ml + 392 \ ml + 608 \ ml = 2118 \ ml$$

    $$ Additional \ fluids \ needed = 2850 - 2118 \ ml = 732 \ ml$$

    Answer: 1118 ml of 8.5 % amino acid solution, 392 ml of 20 % lipid emulsion, 608 ml of 70 % dextrose solution, and approximately 732 ml of additional fluids per day.