NAPLEX Question of the Week: Antibiotic Suspension Calculation

Some form of calculation accounts for 14% of the NAPLEX. Are you ready for this week's question?
NAPLEX Question of the Week: Antibiotic Suspension Calculation

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SB is a 4-year-old female presenting to your pharmacy with her mom who states she has an ear infection. The doctor has prescribed amoxicillin 90 mg/kg/day in divided doses every 12 hours for 5 days. Your pharmacy is stocked with a 400mg/5mL Amoxicillin suspension. 

Temperature: 100.4 F  Height: 39.5 in  Weight: 34 lbs

Allergy: Sulfa (rash)

What size bottle of Amoxicillin should you dispense to SB? Multiple choice.

A. 75 mL

B. 100 mL

C. 50 mL

D. Do not fill

Answer with rationale:

Brands/Generics: Amoxil/Amoxicillin

Otitis media, also known as an ear infection, is an infection of the middle portion of the year. This infection is often secondary to a recent upper respiratory tract infection. Amoxicillin is an antibiotic commonly used to treat otitis media in infants and children 3 months and older.

In order to perform this calculation, the first step is to convert the patient’s weight from pounds to kilograms. Always determine which is given in a question and make the necessary conversion if needed.

34 lbs / 2.2 = 15.45kg.

The second step is to calculate how much amoxicillin the patient needs to complete the duration of therapy.

90mg x 15.45kg = 1390.5mg / day

1390.5mg/day x 5 days = 6952.5mg

The third step is to figure out how many mLs of amoxicillin you will need to dispense to complete the course of therapy. The easiest way to do this is set up a ratio and solve for x.

(6952.5mg / x) x (400mg / 5mL) = 86.9mL

To fill this prescription, you would need to dispense at least 86.9mL to your patient. For rounding purposes and ease, the patient would likely receive either 8.5mL or 9mL BID for the 5 days.  Therefore, the stock bottle that would best be filled is Answer B which is the 100mL bottle. The other two bottles would not provide enough amoxicillin for the 5-day course (answers A and C). Answer D is not correct as this is an appropriate dose for otitis media and the patient's allergy is to sulfa, not penicillin or amoxicillin. 

The patient's mom should be counseled to refrigerate the solution when not using and discard after the 5 days of antibiotic are administered even though some will be left over in bottle. 

NAPLEX Competencies Covered: 

Area 2: Identify Drug Characteristics 

2.1 Pharmacology, mechanism of action, or therapeutic class

2.2 Commercial availability; prescription or non-prescription status; brand, generic, or biosimilar names; physical descriptions; or how supplied 

Area 3: Develop or Manage Treatment Plans 

3.4 – Drug dosing or dosing adjustments; duration of therapy

3.5 – Drug route of administration, dosage forms, or delivery systems

Area 4: Perform Calculations

4.2 – Quantities of drugs to be dispensed or administered

4.5 – Drug concentrations, ratio strengths, osmolarity, osmolality, or extent of ionization


Amoxicillin Package Insert -,050761s11,050754s12,050542s25lbl.pdf

Congratulations to all who have passed their NAPLEX. For those who have the exam coming up, keep up the studying! 

Dr. B

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