NAPLEX Question of the Week: Core Calculations

An important calculation is the subject of our question of the week. There are many steps to the final answer so it is important to stay focused until the end!
NAPLEX Question of the Week: Core Calculations
AW is a 67-year-old female who presents to your hospital for a workup for colon cancer. After discussion with the oncology team, the decision was made to start FOLFOX. The order for Eloxatin comes to the inpatient pharmacy that you work at as you pull up the patient’s chart. AW has a PMH of T2DM, CHF, COPD, and depression. Her home medications include Metformin 1000 MG PO BID, Farxiga 10 MG PO QD, Entresto 97/103 mg PO BID, Metoprolol Succinate ER 50 mg PO QD, Breztri 160/9/4.8 MCG 2 puffs PO BID, Albuterol 90 MCG 1-2 puffs PO Q6H PRN, and Lexapro 20 MG PO QD. Her latest Chem-7 revealed normal renal function and she is not neutropenic. 
Pertinent patient information:
Height: 5’6’’
Weight: 220 lbs
Hospital protocol uses ideal body weight (IBW) for BSA dosing unless:
1) IBW > ABW (Actual body weight), then use ABW
2) IBW*1.2 < ABW, then use 40% adjusted body weight formula:
Adjusted body weight= IBW + 0.4*(ABW-IBW)
The BSA equation is: BSA = √((height x weight)/3600)
Eloxatin on formulary comes in vials of 50, 100, and 200 mg strengths in an intravenous aqueous solution with a concentration of 5 mg/mL.
Eloxatin is to be dosed based on body surface area (BSA), with 85 mg/m2 being the standard dose in the hospital. Calculate the appropriate dose of Eloxatin in milliliters of concentrated infusion solution (whole number) and choose the appropriate vial size to be used to add to the dextrose diluent for infusion. Round each individual calculation step to the nearest tenth except BSA which should be to the hundredth. 
Answer with rationale:
Performing calculations is common practice for pharmacists, especially in hospital pharmacy. Many chemotherapy drugs, such as Eloxatin, most often called oxaliplatin, uses BSA for dosing rather weight-based dosing. Calculations may seem daunting at first glance, but one can systematically break the problem into smaller steps to streamline the process.
The first step is to calculate the patient’s IBW to see if we are using IBW, ABW, or the adjusted body weight. IBW is calculated with the equation below:
IBW=45.5 kg+(2.3 x inches above 60'')
IBW = 45.5 kg + (2.3 x 6 in)
IBW = 45.5 kg + 13.8
IBW = 59.3 kg
The patient’s actual body weight can be converted from pounds to kilograms by dividing the weight in pounds by 2.2.
ABW = (220 lbs)/(2.2 lbs/kg) = 100 kg
We can see that 1.2*IBW, which is 59.3 kg, is 71.2kg. This is less than the patient's actual body weight of 100 kg, thus per hospital protocol we must use the 40% adjusted body weight equation.
40% adjusted body weight is calculated as follows:
40% adjusted BW = 0.4(ABW - IBW) + IBW
40% adjusted BW = 0.4(100 kg-59.3 kg) + 59.3 kg
40% adjusted BW = 0.4(40.7 kg) + 59.3 kg
40% adjusted BW = 16.3 kg + 59.3 kg
40% adjusted BW = 75.6 kg
The second step is to calculate the BSA using the Mosteller’s formula, although the formula requires converting the height into centimeters first. Centimeters is converted to inches by multiplying the inches by 2.54, shown below:
height (in cm) = 2.54 x height (in inches)
height (in cm )= 2.54 x 66 inches
height (in cm )= 167.6 cm 
Now plugging the height (in cm) and weight (using 40% adjusted body weight) into the Mosteller’s formula:
BSA = √((height x weight)/3600)
BSA = √((167.6 cm x 75.6 kg)/3600)
BSA = √3.52
BSA = 1.88 m2
Now, to calculate the dose we need to multiple the BSA to the standard dose 85 mg/m2:
Dose Eloxatin (mg) = 85 mg/m2 *1.88 m2
Dose Eloxatin (mg) = 159.8 mg
This is where it is important to read the question...many would put this answer in the blank or answer a similar answer in multiple choice format. But this would be incorrect.  The question is asking for mL, not mg!
The last step is to convert the dose of Eloxatin to milliliters using a ratio. We are given the concentration of 5 mg/mL and we will set the amount needed in milliliters as “x.” A simple cross-multiply will do the trick!
(159.8 mg)/x = 5 mg/ml
With rearrangement:
x = 159.8/5 mL = 32 mLs
Since the strengths of the vials come in 50, 100, and 200 mg with a concentration of 5mg/mL, the vial sizes are 10 mL, 20 mL, and 40 mL vials. The best choice is to select the least amount of vials, thus choosing the 200 mg/40 mL vial is appropriate. There may be waste of drug but using smaller vial sizes to get to the 160 mg dose is impractical. In hospital pharmacies where there are a number of patients receiving oxaliplatin therapy in the same day, they may choose to batch the orders together to efficiently compound those days orders, but there is no evidence of this within the question so the 40ml vial will do. This will then be added to the appropriate IV dextrose diluent and infused into the patient over generally 2 hours. Oxaliplatin is NOT compatible with normal saline. 
Final Answer: 32 mLs of Eloxatin (oxaliplatin) taken from a 200 mg (40 mL) vial
Note: Always note which rounding method is requested within a particular question. 
NAPLEX Competency Statements Covered:
2.2 – Commercial availability; prescription or non-prescription status; brand, generic, or biosimilar names; physical descriptions; or how supplied
3.4 – Drug dosing or dosing adjustments; duration of therapy
4.2 – Quantities of drugs to be dispensed or administered
4.5 – Drug concentrations, ratio strengths, osmolarity, osmolality, or extent of ionization
Good luck to all who are finishing up studying before the exam or starting their preparation journey for the NAPLEX!
Dr. B

Create a Free MyAccess Profile

AccessMedicine Network is the place to keep up on new releases for the Access products, get short form didactic content, read up on practice impacting highlights, and watch video featuring authors of your favorite books in medicine. Create a MyAccess profile and follow our contributors to stay informed via email updates.