Enteral & parenteral nutrition calculations: practice problems with step-by-step solutions
Also called: tube feeding calculations · enteral feeding · TPN calculations · parenteral nutrition
Free Enteral & parenteral nutrition calculations practice problems with diagrams, hints and full worked solutions. New values every time, at three levels: tube feeding: rates, energy and protein, needs, ideal body weight and free water, parenteral nutrition and obesity rules.
Learn Enteral & parenteral nutrition calculations from scratch
Before this: Dosage calculations, IV fluids & fluid balance
1Feeding a patient who cannot eat
When a patient cannot eat enough, nutrition can be given through a tube into the stomach or small intestine (enteral nutrition, tube feeding) or straight into a vein (parenteral nutrition, PN).
Nurses, dietitians and pharmacists check these feeds with a few short calculations: the pump rate, the energy and protein the patient really receives, the water the patient gets, and the composition of a PN bag.
The numbers in this lesson and in the exercises are made up for practice. In real care the prescription, the product information and local protocols decide.
2Tube-feeding rates, energy and protein
A tube-feeding formula is described by its energy density in kcal/mL and its protein content, usually in g per litre. Standard formulas give about 1.0 kcal/mL; concentrated ones 1.5 or 2.0 kcal/mL.
- Daily volume for an energy goal: mL = kcal needed / (kcal/mL).
- Continuous feed: rate (mL/h) = daily volume / hours the feed runs. A feed that runs 20 h a day needs a higher rate than one that runs 24 h.
- Bolus feeds: volume per feed = daily volume / number of feeds.
- Energy delivered = mL × kcal/mL. Protein delivered = litres × g/L.
rate = (kcal ÷ kcal/mL) ÷ hours
Self-check A patient needs 1800 kcal a day from a 1.5 kcal/mL formula that runs for 20 hours. What is the pump rate?
- 135 mL/h
- 50 mL/h
- 60 mL/h
- 90 mL/h
Answer: 60 mL/h. Volume: 1800/1.5 = 1200 mL. Rate: 1200/20 = 60 mL/h. 50 mL/h divides by 24 h although the feed runs 20 h. 90 mL/h divides kcal by hours without converting to mL.
3Needs, ideal body weight and free water
Energy and protein needs are often estimated per kilogram: for example 25–30 kcal/kg/day and 1.2–2.0 g protein/kg/day, depending on the patient. Multiply by the weight to get the daily goal.
The Devine formula gives an ideal body weight (IBW) from the height. It works in inches (1 in = 2.54 cm) and adds 2.3 kg for every inch above 5 feet (60 in):
IBW (men) = 50 + 2.3 × (inches − 60) IBW (women) = 45.5 + 2.3 × (inches − 60)
A formula is only partly water: a 1.0 kcal/mL formula is roughly 84% free water and a 2.0 kcal/mL formula roughly 70%. The rest of the daily fluid goal is given as water flushes.
flush volume per day = fluid goal − formula volume × water fraction
Self-check A man is 180 cm tall. What is his ideal body weight by the Devine formula (to the nearest kg)?
- 213 kg
- 70 kg
- 75 kg
- 80 kg
Answer: 75 kg. 180/2.54 = 70.9 in. IBW = 50 + 2.3 × (70.9 − 60) = 75 kg. 70 kg uses the 45.5 kg start for women. 213 kg forgets to subtract the first 60 inches.
4Parenteral nutrition
A PN bag contains dextrose, amino acids and usually a lipid emulsion. Concentrations are % w/v: grams per 100 mL, so grams = % × mL/100.
- Dextrose in PN gives 3.4 kcal/g (it is glucose monohydrate), not 4 kcal/g.
- Amino acids give 4 kcal/g.
- Lipid emulsion: use the kcal/mL on the product, for example 2.0 kcal/mL for 20% and 1.1 kcal/mL for 10%.
Non-protein kcal (NPC) are the dextrose and lipid kcal. Nitrogen is protein/6.25, and the NPC : N ratio compares energy with protein.
The glucose infusion rate (GIR) checks how fast glucose is given:
GIR (mg/kg/min) = dextrose (mg) / (kg × minutes) = % dextrose × mL/h / (6 × kg)
Self-check A PN bag holds 1000 mL of 20% dextrose. How many kcal does the dextrose provide?
- 200 kcal
- 68 kcal
- 680 kcal
- 800 kcal
Answer: 680 kcal. 20% of 1000 mL is 200 g of dextrose; 200 × 3.4 = 680 kcal. 800 kcal uses 4 kcal/g. 200 is the number of grams, not kcal.
5A worked example
Follow every step and predict the next one before you open it. All values are made up for practice.
Worked exampleA 70 kg patient has a fluid goal of 30 mL/kg/day. A 1.2 kcal/mL formula (81% free water) runs at 60 mL/h for 20 hours a day. The rest of the water is given as flushes every 4 hours. How large is each flush?
130 mL/kg × 70 kg = 2100 mL/day
2Volume: 60 mL/h × 20 h = 1200 mL. Only 81% of it is water.
1200 × 0.81 = 972 mL
4Every 4 hours is 24/4 = 6 flushes a day.
1128 / 6 = 188 mL
Answer Each flush is about 188 mL of water.
6Common mistakes and exam questions
- Dividing by 24 hours when the feed runs for fewer hours.
- Dividing kcal by hours without converting kcal to mL first.
- Counting the whole formula volume as water.
- Using 4 kcal/g for dextrose in PN, or 9 kcal/g for lipid emulsion instead of the kcal/mL on the label.
- Using the actual weight where a rule asks for the ideal body weight.
Easy: pump rates, bolus volumes, and the energy or protein a feed delivers. Medium: ideal body weight, energy and protein needs, and free-water flushes. Hard: PN energy, NPC : N, glucose infusion rate, and BMI-based rules for obese patients.
Self-check An obese patient’s protein goal is 2.0 g per kg of ideal body weight. The patient weighs 120 kg and the IBW is 70 kg. What is the goal?
- 100 g/day
- 140 g/day
- 175 g/day
- 240 g/day
Answer: 140 g/day. 2.0 × 70 = 140 g/day. 240 g uses the actual weight. The rule names the ideal body weight, so the actual weight is not used.
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