Dispensing a Pediatric Syrup Against a Stock Bottle
A clinic stocks a children's antibiotic syrup labelled 250 mg per 5 mL in a 100 mL bottle. A prescription reads: 375 mg three times daily for 7 days. The nurse measures doses with a 10 mL oral syringe graduated in 0.5 mL steps. A colleague claims one bottle is more than enough because 'the bottle holds 100 doses at 1 mL each.' Before dispensing, the nurse must confirm the volume per dose, the total volume needed, and whether a second bottle is required. She also notices the label states the syrup must be discarded 14 days after opening, and the first dose will be given today.
1. Calculate the volume of a single 375 mg dose in mL, and state which graduation mark on the syringe would be used.
Read the worked answer
Set up a proportion from the label: 250 mg is contained in 5 mL, so 1 mg corresponds to 5/250 = 0.02 mL.
Volume per dose = 375 mg x 0.02 mL/mg = 7.5 mL.
Check by ratio: 375/250 = 1.5, and 1.5 x 5 mL = 7.5 mL.
The syringe is graduated in 0.5 mL steps, and 7.5 mL falls exactly on a mark, so the nurse draws to the 7.5 mL line with no rounding needed. Because the value lands on a graduation, no measurement error is introduced at this step.
2. Determine the total volume required for the full 7-day course and decide whether one 100 mL bottle suffices.
Read the worked answer
Doses per day = 3, so volume per day = 3 x 7.5 mL = 22.5 mL.
Total for 7 days = 22.5 mL x 7 = 157.5 mL.
One bottle holds 100 mL, which covers 100/22.5 = 4.44 days, i.e. four full days (90 mL) with 10 mL left over, enough for one more 7.5 mL dose plus 2.5 mL spare.
So one bottle is not enough. At least two bottles are needed: 157.5 mL requires ceiling(157.5/100) = 2 bottles.
The colleague's reasoning is wrong because it assumes a 1 mL dose. At 7.5 mL per dose, a 100 mL bottle holds only about 13 doses, not 100.
3. The bottle is opened on day 1 and must be discarded 14 days after opening. Does the 7-day course fit within the stability window, and what practical constraint does this place on the second bottle?
Read the worked answer
The course runs 7 days from the first dose, which is well inside the 14-day post-opening limit, so the treatment itself is safe on that count.
However, the second bottle should not be opened on day 1 if it can be avoided, because its 14-day clock would start unnecessarily early. A better plan is to open the second bottle only when the first is exhausted, around day 5 (after 4 full days plus one dose on day 5).
Either way the remaining doses of the course finish by day 7, so both bottles stay within their windows. The judgement call is logistical: open the second bottle just in time, and record the opening date on the label so the discard date is unambiguous.