While working at a retail pharmacy, you get a call-in prescription from a prescriber for Ofloxacin 0.3% eye drops. The instructions state to instill 1-2 drops OU four times per day for 7 days. The quantity is given as “one bottle QS.” The pharmacy carries 2.5mL, 5mL, 7.5mL, and 10mL, bottles.

You would like to dispense the lowest possible amount of ofloxacin while providing the patient enough drops. What size bottle should you dispense, assuming there are 20 drops per mL of solution?

A. 2.5mL

B. 5mL

C. 7.5mL

D. 10mL

E. There is not enough information, and the prescriber must be contacted for clarification

Answer with rationale:

Prescriptions where the amount is given as “quantity sufficient (or QS)” are common in pharmacy practice. It is super important that pharmacists are comfortable making fast and accurate calculations for these prescriptions, as incorrect calculations can lead to under or overdosing a medication, often resulting in undesirable patient health outcomes.

In this case, you need to calculate the amount of a solution to be dispensed based on prescriber instructions. You should start by calculating the total amount of drops needed to accurately complete the course of therapy. When given a range to use such as “1-2 drops”, always assume the patient will use the most possible drops in your calculations to ensure you dispense enough medication. So 1-2 drops becomes 2 drops for our calculation.

2 drops x 2 eyes x 4 times per day x 7 days = 112 total drops

112 total drops / 20 drops per mL = 5.6mL

To give enough solution, you would need to dispense at least the 7.5mL bottle.

Answer A is Incorrect: 2.5mL is not enough solution for this prescription.

Answer B is Incorrect: 5mL is not enough. If we had used the lower end of the range of drops per dose, we would’ve ended up with 56 total drops and 2.8mL of solution. The 5mL bottle would have seemed appropriate, but if the patient used 2 drops, they would run out of medication before the full course could be completed.

Answer C is Correct:  7.5mL is more than the needed 5.6mL, and is also the smallest bottle size that provides enough solution. 

Answer D is Incorrect: While 10mL would be enough to fulfill the full prescription, there would be an increased amount of medication left over and wasted, so it would not be the best choice for this prescription.

Answer E is Incorrect:  The amount needed can be calculated and dispensed based on the provider’s instructions, so there is no need to call for clarification.

Brand/generics Covered:

Ofloxacin (Ocuflox)

Naplex Content Domains Covered

1.C.2: Calculations of quantities of drugs to be administered

2.A.2: Prescription interpretation of dosing regimens