TPN Calculations Primer (Optional)

TPN Calculations Primer (Optional)

Discussion: When reviewing TPN formulas to determine total caloric load, 3 items will be used to determine total calories: Carbohydrate (dextrose), Protein (amino acids), and Fat (lipids, lipid emulsion, Liposyn, Intralipid). These items will be presented in some part of the basic form of calories, weight, concentration (percentage) and volume. From this basic form you can then calculate additional elements of the formula as needed. Here are the primary methods used in these calculations. Don’t forget that in pharmaceutical terminology when speaking about solutions and percentages, percentage means concentration of grams per 100 ml (deciliter). Once you have this information and a dose, you may use ratio and proportion to calculate the 4th variable.

Calories

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Weight of Ingredients

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Concentration

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Volume

1.   Dextrose: Dextrose will be presented in one of three ways, (a) total dextrose concentration of the entire solution (15% dextrose), (b) amount of concentrated dextrose solution added to the formulation sometimes called a stock solution (500ml of dextrose 70%), or (c) total dextrose calories.

a.   If you are given the final dextrose concentration of the total solution, then determine the 24 hour volume of the TPN and use ratio and proportion (R/P) to calculate the total grams of dextrose Example: TPN solution is 25% dextrose infusing at 100 ml/hour. Total 24 hr volume is 2400ml (24hr x 100ml/hr) then R/P is 25/100 = X/2400. 2400 times 25 equals 60,000 divided by 100ml equals 600 gms dextrose. Then multiply by 3.4 kcals/gm to determine dextrose kcals. 600 x 3.4 = 2040 dextrose kcals.

b.   If you are given the amount of concentrated dextrose solution (stock solution) used to make 24 hour formulation, the calculation is the same except you use only the volume of the concentrated dextrose solution. Example: 600ml of dextrose 70% is added to TPN bag. R/P is 70/100 = X/600. 600ml times 70 equals 42000 divided by 100ml equals 420 grams dextrose multiplied by 3.4 kcals/gram equals 1428 kcals.

c.   If you are given the amount of concentrated dextrose solution (stock solution) used in each liter of the formulation, the calculation is the same except that after you calculate the number of grams per liter, you must then multiply that by the number of liters per 24 hours to get the total grams per day. Example: Dextrose 50% 500 ml used to formulate each liter of TPN, infuse at 85 ml/hr. (85 x 24 = 2040ml/day or 2.04 liters). R/P is 50/100 = X/500. 500 ml times 50 equals 25000 divided by 100 equals 250 gms/liter times 2.04 liters equals 510 gms dextrose times 3.4 kcals/gm equals 1734 kcals/day.

d.   If you are given total dextrose calories, the calculations will work in reverse order of (a), (b), and (c).

2.   Protein may be presented in these ways: (a) Concentration of the total or final solution (5% protein or aminoacids), (b) amount of concentrated protein solution (stock solution) used in the formulation (500ml of 15% protein or aminoacid), (c) amount of protein per kilogram body weight (1.5gm/kg/day), (d) total amount of protein/day, or some form of conversion of the protein to kilocalories. Protein can be converted to kcals by multiplying by 4kcals/gm.

a.   If you are given the protein concentration of the total solution, then determine the 24 hour volume of the TPN and multiply it by the percentage protein then divide by 100ml. Example: TPN is 5% protein infusing at 80 ml/hr. Total 24hr volume is 80 x 24 = 1920 ml. R/P is 5/100 = X/1920. 5 times 1920 = 9600 divided by 100ml = 96 grams of protein x 4 kcals/gram = 384 kcals.

b.

i.  If you are given amount of concentrated protein solution used in the formulation, the calculation is the same except you use only the volume of the concentrated protein solution (stock solution). Example: TPN is formulated with 600 ml of 15% amino acid solution. R/P is 15/100 = X/ 600. 600ml x 15gm = 9000 divided by 100ml = 90 grams protein x 4 kcals/gram = 360 kcals.

ii. If you are given the amount of concentrated aminoacid solution used in each liter of the formulation, the calculation is the same except that after you calculate the number of grams per liter, you must then multiply that by the number of liters per 24 hours to get the total grams per day. Example: Aminoacid 15% 500 ml used to formulate each liter of TPN, infusing at 85 ml/hr. (85 x 24 = 2040ml/day or 2.04 liters). R/P is 15/100 = X/500. 500 ml times 15gm equals 7500 divided by 100ml equals 75 gms in each liter times 2.04 liters equals 153 gms protein times 4 kcals/gm equals 612 kcals/day.

c.  If you are given the protein dose in gms/kg, then volume does not enter into the calorie calculation. Example: 78 kg patient requires 1.6 gm/kg/day protein. 78 x 1.6 = 124.8 (rounded to 125) grams protein x 4 kcals/gm = 500 kcals. However, if you are required to calculate a volume of stock solution for manufacture of a TPN formula, see part d.

d.  If you are given total weight amount of protein, divide by the percentage of the aminoacid solution (either final concentration or stock solution) to calculate the volume. Example: patient needs 125gms protein. Your institution uses 15% stock solution. R/P is 15/100 = 125/X. Multiply 100 ml by 125gm equals 12500 divided by 15 gm equals 833.3 ml. Example 2: patient needs 125gm protein/day. Standard TPN (final concentration) solution is 5% aminoacids. R/P is 5/100 = 125/X. Multiply 100 ml by 125 gm equals 12500 divided by 5 gm equals 2500 ml of standard TPN to provide protein dose.

3.   Fat or lipid calculations are a little different because although you may calculate grams of fat for a dose, you do not use the 9 kcals/gm for fat in your calculations of parenteral (IV) fat emulsion. The reason is that in the manufacture of IV fat emulsions the addition of glycerin is required to create an isotonic product and glycerin adds kcals to the final product. Here is a list of the caloric density of the IV fat or lipid emulsions.

10% lipid emulsion……………1.1 kcal/ml

20% lipid emulsion…………….2.0 kcal/ml

30% lipid emulsion…………….3.0 kcal/ml (This one is only used for TPN manufacture. It cannot be infused peripherally.)

You will see fat doses presented in several different ways as well: a) Volume of emulsion infused on specific days or at specific times, b) percentage of total kcals (this may be calculated and you are given the kcals of fat emulsion), c) volume of fat emulsion used to manufacture a TPN, d) grams of fat per kilogram of patient.

a.   Patient to receive 500ml of 20% fat emulsion 3 times weekly. 3 x 500ml = 1500ml / week divided by 7 days /week = average of 214 ml daily x 2 kcals/ml (caloric density of 20% fat emulsion) = average of 428 kcals from fat emulsion daily.

b.   Patient weighs 75 kg and will receive 35 kcals/kg/day with 20% of total kcals from 20% fat emulsion. 75kg x 35 kcal/kg = 2625 kcals total x 20% = 52500 kcals / 100% = 525 kcals from fat emulsion / 2 kcals/ml = 262.5 ml of 20% fat emulsion.

c.   Patient’s TPN has 175 ml of 30% fat emulsion as part of its formulation. 175ml x 3 kcals/ml (30% fat emulsion) = 525 kcals from fat emulsion.

d.   TPN is ordered to contain 2.5 gms/kg of fat. Patient weighs 10 kg. 2.5gms/kg x 10 kg = 25 gm. Using 20% fat emulsion (20gms/100ml…remember?) and R/P is 25/X = 20/100. X gives you the volume required to supply 25 gms = 125ml. Then 125 ml x 2.0 kcals/ml equals 250 kcals supplied by the 20% fat emulsion.

e.   If lipid is referred to as total calories of lipid, simply divide by calories/ml of appropriate emulsion to calculate volume of emulsion required(see part c)

4.   Tidbits:

a.   Normal saline = 154 mEq of sodium and 154 mEq of chloride

b.   Remember that all salts forms contribute to total concentration

c.   20 - 30 mEq/Liter of potassium will replace normal losses

d.   Change in serum potassium of 1 mEq equals 200-300mEq deficit

e.   1 gm of Magnesium sulfate = 8.12 mEq of magnesium

f.   1 gram of Ca gluconate = 4.65 mEq of Calcium

g.   Don’t forget about refeeding syndrome (phos)

h.   Calculate Ideal Body Weight in kg: Males = 50 + (2.3 x inches above 5 ft)

i.   Calculate Ideal Body Weight in kg: Females = 45.5 + (2.3 x inches above 5 ft)

j.   Fast maintenance fluid calc for adults: 1100 + ( 20 x (wt in kg)) or 1cc/kcal

TPN Calculations practice

Patient is S/P MVA with left femur fracture, right tib/fib fracture, and blunt abdominal trauma and subsequent exploratory laparotomy. He has developed post operative ileus and will be started on TPN. His height is 72 in. and his weight is 80kg. His basic assessment is 30 kcals/kg and 1.8 gms/kg protein.

A. Calculate the macro nutrients for this patient giving 25% of the total calories as fat. Use 70% dextrose, 15% aminosyn, and a 20% fat emulsion.

B.   OK now Backwards (in case you are given the formula and need to calculate what the patient has received): TPN formula for this patient is Dextrose 70% - 514.3ml, aminoacid 15% - 960 ml, fat emulsion 20% - 300ml. Calculate total calories for this patient.

C.   At your institution, TPN’s have a standard formulation of 25% dextrose and 5% aminoacid with fat emulsion 20% 500 ml given 3 times weekly. Your patient is receiving this formula at 90 ml/hr. Calculate total average daily kcal.

D.   At your institution TPN is manufactured per liter with ingredients of dextrose 70% 500ml and aminoacids 10% 500ml and fat emulsion 20% 200 ml infused separately daily. Your patient receives this formulation at 110 ml/hr plus fat emulsion. Calculate daily kcals. (Note here a calculation similar to R/P but slightly different where the fraction is first reduced. This step can speed up the calculation if you are comfortable with it)

E. Your patient weighs 10kg and will receive TPN containing 15% dextrose, 2.5gm/kg/day protein, and 3 gm/kg/day fat from 20% fat emulsion with a TPN rate of 40 ml/hour. Calculate daily kcals.