Paediatric Dose Calculation for a Compounded Oral Suspension
A 4-year-old child (weight 16 kg) is prescribed oral spironolactone 1.5 mg/kg/day in two divided doses for a cardiac condition. The hospital pharmacy has spironolactone 25 mg tablets and can compound an oral suspension. The standard formula available uses 25 mg tablets crushed and suspended in a 1:1 mixture of simple syrup and methylcellulose to a final concentration of 5 mg/mL. The child's mother has a 5 mL oral syringe and a 1 mL oral syringe at home. The prescription is for 14 days. The pharmacy has 30 tablets of 25 mg in stock.
1. Calculate the daily dose, the dose per administration, and the volume to be measured per dose.
Read the worked answer
Daily dose = 1.5 mg/kg/day x 16 kg = 24 mg/day. Divided into two doses: 12 mg per dose. With a 5 mg/mL suspension, volume per dose = 12 mg / 5 mg/mL = 2.4 mL. So the child needs 2.4 mL twice daily, which is best measured with the 5 mL syringe if it is marked in 0.1 mL increments, or with the 1 mL syringe used three times (not acceptable in practice). Confirm the syringe has 0.1 mL graduations; if not, ask for an appropriate syringe. Total daily volume is 4.8 mL.
2. Determine whether the stock of tablets is sufficient to make the 14-day supply, and calculate the total volume needed.
Read the worked answer
Total volume for 14 days = 4.8 mL/day x 14 days = 67.2 mL. Each 25 mg tablet contains 25 mg of spironolactone, which at 5 mg/mL corresponds to 5 mL of suspension per tablet. Tablets needed = 67.2 mL / 5 mL per tablet = 13.44 tablets, so 14 tablets are required. The pharmacy has 30 tablets, which is sufficient, with 16 tablets spare. A sensible practical approach is to make a slightly larger batch to allow for losses during compounding and for accurate measurement, for example 15 tablets to make 75 mL, and assign a beyond-use date according to local policy, typically 14-28 days refrigerated for an aqueous suspension without preservative. Note that the suspension must be shaken well before each dose because it is not a solution.
3. List the counselling points and safety checks you would complete before dispensing to the mother.
Read the worked answer
Confirm the child's weight and the dose calculation with a second checker, since paediatric doses are high-risk. Confirm no allergy to spironolactone and check for concurrent potassium supplements, ACE inhibitors or potassium-sparing diuretics, which raise hyperkalaemia risk. Explain that the suspension must be shaken well before each dose, and demonstrate measuring 2.4 mL with the oral syringe, not a kitchen spoon. Advise giving with or without food, and to keep the bottle refrigerated and discard after the beyond-use date stated on the label. Explain monitoring: the child may need blood tests for potassium and renal function, and parents should report vomiting, diarrhoea, dehydration, muscle weakness or irregular heartbeat. Provide a clearly labelled dosing chart and a medication administration record. Confirm the mother understands the difference between 2.4 mL and 24 mL, and ask her to teach back the dose.