Calculations for Parenteral Compounding

How to calculate a proportions

1. Cross-multiply on both sides of the equal sign

2. Place the product of each multiplication on either
   side of the equal sign

3. Solve for X.

(examples on slide 6)

If two volumes and one concentration of a product are known or vice versa, you can use…

C₁V₁ = C₂V₂

(Examples on slide 8)

Types of % strength

• weight in weight

  • When all products involved are in solid form

  • Represents number of grams in a 100 g

• weight in volume

  • Percent strength of solids in liquids

  • Represents the number of grams in 100 mL of solution

• volume in volume

  • Percent strength of a solution in a solution

  • Represents the number of milliliters in 100 mL

How to calculate % strengths?

1. Cross-multiply on both sides of the equal sign

2. Place the product of each multiplication on either side of the equal sign

3. Solve for X

(example on slide 12)

Aspects of ratio strengths

• Used when solutions are very dilute

• Alternate method for expressing concentration of solutions

• Concentrations can be expressed in ratio strengths or percentage strengths

• Set up equations with percent strength expressed as “out of 100.”

• Set up ratio strength with a numerator of 1, which is the same as 1:X

(example on slide 14)

How to calculate alligations

1. Set up a grid using known concentrations. These numbers should be placed on the left of the grid, with the more concentrated values on the top and the less concentrated values on the bottom. Place the desired concentration in the middle.

2. Subtract diagonally. Subtract in a direction that will result in a positive integer (ie find the difference)

3. Add, vertically, the differences from Step 2. The result is the total number of parts.

4. For each product, set up the following proportion: (Parts Needed/Total Parts)

5. Multiply the desired quantity by the proportion for each product to obtain the quantity of each product to be used.

(example on slide 18)

How to calculate displacement volume (powder volume)

1. Calculate the powder displacement volume.

2. Calculate the volume of diluent needed for the new concentration.

3. Subtract the displacement volume (from Step 1) from the volume needed (from Step 2) to determine the final amount that should be added

(example on slide 21)

How to calculate doses based on weight

1. Convert weight to kilograms (kg)

2. Set up a proportion equation.

   • Use the prescribed dose of medication on one side of the equal sign and the unknown on the other side of the equal sign.

   • Be sure units are balanced on either side of the equal sign.

3. Solve for X

(example on slide 24)

How to calculate flow rates

1. Calculated based on drop factors for administration set tubing for gravity flow administration. (gtt/min)

2. When electronic infusion devices are utilized, rate is programmed into device. (mL/min)

   • Set up dimensional analysis, ensuring that all units in the numerator and the denominator cancel so that the final units are in mL/hr or gtt/min or the desired rate units.

   • Multiply all numerators

3. Multiply all denominators

4. Divide the answer from Step 2 by the answer from Step 3, ensuring that all units in the numerator and the denominator have cancelled so that the final units are in mL/hr or gtt/min

(examples on slide 29)

Methods on calculating body weight

• Ideal Body Weight (IBW)

  • Males: 50kg + 2.3kg (their height in inches - 60 inches)

  • Females: 45.5kg + 2.3kg (their height in inches - 60 inches)

• Adjusted Body Weight

  • IBW + 0.4(actual-ideal)

  • used when patient is >20% above IBW

• Body Mass Index (BMI)

  • 703 x weight in lbs/(height in inches)²

(example on slide 31)

What are the 2 types of BSA formulas?

• BSA (m2) = √ [height (inches) • weight (lb)]/3131

• BSA (m2) = √ [height (cm) • weight (kg)]/3600

(example on slide 34)

What is the Cockcroft Gault formula (Creatinine Clearance)?

• CrCl_male= [(140-age) • IBW or AdjustedBW] / (Scr • 72)

• CrCl_female= 0.85 • [(140-age) • IBW or AdjustedBW] / (Scr • 72)

  • Use AdjustedBW for obese patients (BMI greater than or equal to 25)

• Many Others (Jelliffe, MDRD, CKD-EPI)

(example on slide 36)

What is the AUC dosing formula?

• total dose (in mg) = Target AUC • (GFR+25)

  • where GFR=CrCL

(examples on slide 38)

What is the corrected calcium formula?

Corrected Calcium = Serum Calcium + 0.8(4-Serum Albumin)

(example on slide 40)

What is the corrected Phenytoin formula?

Corrected phenytoin = Serum Phenytoin Level/[(0.2 • Albumin) + 0.1]

(examples on slide 42)