NAPLEX Question of the Week: IV Admixture Calculations

This common task among hospital pharmacists is the subject of our question of the week!
NAPLEX Question of the Week: IV Admixture Calculations

CF is a 42-year-old female who is currently admitted to the hospital with a severe herpes simplex virus (HSV) mucocutaneous infection. She is going to be treated with IV Zovirax at a dose of 5 mg/kg/dose given Q8H. The dosing for this drug is based on ideal body weight. Zovirax comes in 10 mL vials with a concentration of 50 mg/mL. Zovirax is an IV irritant so to avoid extravasation the final infusion solution must be diluted in normal saline to a concentration of 7 mg/ml.  

Labs: Height: 5’ 7”, Weight: 180 lbs  

What is the total daily dose for this patient? How much of the 50 mg/ml solution is needed to get each Q8H dose? Finally, what is the minimum final volume of the infusion solution required to prepare each dose and administer Q8H to the patient?  

a)     992 mg, 6.6 mL, 47.2 mL 

b)    308 mg, 6.2 mL, 44 mL 

c)    924 mg, 18.5 mL, 132 mL 

d)    1227 mg, 8.18 mL, 58.4 mL 

e)     924 mg, 6.2 mL, 44 mL 

Answer: E 

Brand/Generic: Zovirax (acyclovir) 


To start this calculation problem, you first want to calculate the patient's ideal body weight.  

IBW equations:  

Male: IBW= 50 kg + (2.3 x # inches over 5 ft in height)  

Female: IBW= 45.5 kg + (2.3 x # inches over 5 ft in height) 

This patient is a 5’ 7” female, so using the correct equation you should get an answer of 61.6 kg. You would then use this weight to calculate the total daily dose. The drug is being given every 8 hours so this would be a total of 3 doses/day.  

61.6 kg x 5 mg/kg/dose x 3 doses/day= 924 mg/day 

The next question is asking how much of the 50 mg/ml stock solution is needed for each dose. To solve for this volume, you can set a proportion as detailed below.  

61.6 kg x 5 mg/kg/dose= 308 mg/dose  

Finally, because IV acyclovir is a known irritant and can lead to extravasation (especially when administered peripherally), the final infusion concentration should be no more than 7 mg/mL. So, you would need to take the mg of each dose and solve for the volume needed that would give you a concentration of 7 mg/ml. 

In clinical practice, the dose would often be rounded down to 300mg IV q8h for simplicity and each dose administered within a 50mL bag. When given for HSV encephalitis, acyclovir is often given at a dose of 10mg/kg IV q8h and thus would require a larger volume dilution and for this patient would represent at a minimum a 100mL bag. 

Answer Choice A: These are the numbers you would get if you mistakenly used the male IBW equation instead of the female one.  

Answer Choice B: This answer is incorrect because the first answer listed is not the total daily dose but just a single Q8H dose. 

Answer Choice C: This answer choice is what you would get if you used the total daily dose to calculate the next two parts instead of the individual dose.  

Answer Choice D: This answer is what you would get if you used the actual body weight of 81.8 kg (180 lb/ 2.2 kg/lb= 81.8 kg) instead of the ideal body weight to solve the problems. 

Calculations are a critical part of clinical practice and the NAPLEX. Definitely work these through until the end with each problem so that you can train your mind well for these questions. 

Exam Competencies: Area 4 – Perform Calculations (4.1 – Patient parameters or laboratory measures, 4.2 – Quantities of drugs to be dispensed or administered, 4.5 – Drug concentrations, ratio strengths) 

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