Module 13 of 14 · 14 min
Parenteral nutrition
PN orders and hospital-pharmacy exam cases ask you to turn percentages and volumes into grams, kilocalories and g/kg/day to check that a regimen meets — and does not overshoot — a patient's needs.
🧠 "3.4 – 4 – 2": dextrose, protein, lipid 20% per mL.
📌 Memorize these
- Dextrose (IV)
- $3.4\,\text{kcal/g}$
- Amino acids
- $4\,\text{kcal/g}$
- Lipid 10%
- $1.1\,\text{kcal/mL}$
- Lipid 20%
- $2\,\text{kcal/mL}$
- Lipid 30%
- $3\,\text{kcal/mL}$
- Protein target
- 0.8–2 g/kg/daytypical
- Glucose infusion rate
- ≤ 4–5 mg/kg/minusual limit
A % w/v is grams per 100 mL: D15 = 15 g per 100 mL, so 2000 mL of D15 contains 300 g of dextrose.
Energy values used for PN: IV dextrose (monohydrate) 3.4 kcal/g; amino acids (protein) 4 kcal/g; lipid emulsion 20% = 2 kcal/mL (10% = 1.1 kcal/mL, 30% = 3 kcal/mL — includes glycerol and phospholipid, so not simply 9 kcal/g).
Protein is judged per kg: $\text{g/kg/day}=\dfrac{\text{g amino acids per day}}{\text{weight}}$ (typical 0.8–2 g/kg/day). Dextrose can be checked as a glucose infusion rate in mg/kg/min (usually ≤ 4–5).
Mixing two dextrose solutions follows mass balance (or alligation): the grams in the final bag equal the sum of the grams from each component.
🧭 The method
- 1For each macronutrient, grams = % ÷ 100 × mL per 24 h (or mL/h × 24).
- 2Kcal: dextrose g × 3.4, protein g × 4, lipid mL × kcal/mL for its strength.
- 3Add kcal for the total; divide by weight for kcal/kg if asked.
- 4Protein: g ÷ kg = g/kg/day; dextrose: mg ÷ kg ÷ 1440 min = mg/kg/min.
- 5For mixtures, use mass balance (g from solution A + g from solution B = g in final).
Before each reveal, predict the next step — then check.
A PN bag of 2000 mL over 24 h provides dextrose 15% and amino acids 5%, plus 250 mL of lipid emulsion 20%. How many total kcal per day? (dextrose 3.4 kcal/g, protein 4 kcal/g, lipid 20% 2 kcal/mL)
- 1
Dextrose grams and kcal.
$$\frac{15\ \text{g}}{100\ \text{mL}}\times2000\ \text{mL}=300\ \text{g};\quad300\ \text{g}\times3.4\ \frac{\text{kcal}}{\text{g}}=1020\ \text{kcal}$$ Amino-acid grams and kcal.
$$\frac{5\ \text{g}}{100\ \text{mL}}\times2000\ \text{mL}=100\ \text{g};\quad100\times4=400\ \text{kcal}$$Lipid kcal from volume.
$$250\ \text{mL}\times2\ \frac{\text{kcal}}{\text{mL}}=500\ \text{kcal}$$Add them.
$$1020+400+500=1920\ \text{kcal/day}$$
Same method — the last steps are yours. Work on paper, then reveal.
A 70 kg patient receives 1500 mL/day of PN containing dextrose 20% and amino acids 4.25%. How much protein per kg per day?
- 1
Dextrose (for completeness).
$$0.20\times1500=300\ \text{g}\to1020\ \text{kcal}$$ - 2
Your turn
Now you: grams of amino acids.
Work it out on paper, then reveal.
$$\frac{4.25}{100}\times1500\ \text{mL}=63.75\ \text{g}$$ - 3
Your turn
Now you: per kg per day.
Work it out on paper, then reveal.
$$\frac{63.75\ \text{g}}{70\ \text{kg}}=0.91\ \text{g/kg/day}$$
Type your number. Your first try decides when this module comes back for review.
How many grams of dextrose are in 750 mL of dextrose 50%?
50% = 50 g per 100 mL.
= 375 g
Lipid emulsion 20%, 250 mL, is given daily. How many kcal does it provide? (lipid 20% = 2 kcal/mL)
Use the kcal per mL for the emulsion, not 9 kcal/g.
= 500 kcal
Amino acids 10% run at 60 mL/h for 24 h. How many grams of protein per day?
Volume per day first: mL/h × 24.
= 144 g
An 80 kg patient needs 1.5 g/kg/day of protein from amino acids 10%. How many mL per day?
Grams needed, then 10 g per 100 mL.
= 1200 mL
A 70 kg patient receives 1500 mL/day of dextrose 20%. What is the glucose infusion rate in mg/kg/min?
Convert grams to mg and a day to 1440 minutes.
= 2.98 mg/kg/min
You need 1000 mL of dextrose 25% made from dextrose 70% and dextrose 5%. How many mL of dextrose 70%?
Mass balance: 0.70x + 0.05(1000 − x) = 0.25 × 1000 (or alligation: parts 20 and 45).
= 307.7 mL
⚠️ Tricky
A 2000 mL PN bag has final concentrations of dextrose 15% and amino acids 5%; 250 mL of lipid 20% runs separately. Using dextrose 3.4 kcal/g, protein 4 kcal/g and lipid 20% 2 kcal/mL, what is the total daily energy (kcal)?
Grams of each macronutrient first; dextrose (monohydrate) is 3.4 kcal/g, not 4.
= 1920 kcal
⚠️ The trap: Using 4 kcal/g for dextrose gives 2100 kcal, overstating energy by 180 kcal.
⚠️ Tricky
A 1500 mL PN bag must have a final dextrose concentration of 15%. It is compounded from dextrose 70%. How many mL of dextrose 70% are needed?
Grams wanted in the final bag, then the volume of stock that contains them.
= 321.4 mL
⚠️ The trap: Stopping at 225 (grams) and measuring 225 mL of D70 gives only 157.5 g, a final concentration of 10.5%.
⚠️ Tricky
A daily PN supplies 1000 mL of amino acids 10%, 250 g of dextrose and 250 mL of lipid 20%. Using 6.25 g protein per g nitrogen, dextrose 3.4 kcal/g, protein 4 kcal/g and lipid 20% 2 kcal/mL, what is the non-protein kcal : nitrogen ratio?
Non-protein kcal = dextrose + lipid only. Nitrogen = grams of protein ÷ 6.25.
= 84.4
⚠️ The trap: Including the 400 kcal from protein in the numerator gives 109.4 : 1, a total-kcal ratio, not a non-protein one.
⚠️ Tricky
A PN order calls for 15 mmol phosphate as sodium phosphate (each mL: 3 mmol phosphate and 4 mEq sodium) and a total of 40 mEq sodium per day. The rest of the sodium comes from sodium chloride 4 mEq/mL. How many mL of sodium chloride should be added?
Count the sodium that the phosphate salt already brings in before adding sodium chloride.
= 5 mL
⚠️ The trap: Ignoring the sodium carried in with the phosphate gives 10 mL of sodium chloride and 60 mEq of sodium in total.
⚠️ Tricky
A 60 kg patient receives cycled PN: 1800 mL with final dextrose 20%, infused over 12 hours overnight. What is the glucose infusion rate (mg/kg/min) while the PN is running?
The dextrose is delivered in 12 h (720 min), not over 24 h.
= 8.33 mg/kg/min
⚠️ The trap: Dividing by 1440 minutes gives 4.17 mg/kg/min, hiding a glucose rate that is above the usual 4–5 mg/kg/min ceiling.
🕵️ Spot the mistake
How many kcal are provided by 500 mL of dextrose 50% given IV?
A student wrote
⚠️ Traps examiners set
- Using 4 kcal/g for dextrose instead of 3.4.
- Using 9 kcal/g for lipid emulsion instead of the per-mL value (20% = 2 kcal/mL).
- Forgetting to multiply an hourly rate by 24 h.
- Reading % as grams per mL instead of grams per 100 mL.
- Mixing solutions by averaging percentages without weighting by volume.
💬 Explain it in your own words
Why is the kcal value per mL of lipid emulsion higher than 9 kcal/g × 0.2 g/mL would suggest?