Compounding an Oral Suspension for a Pediatric Patient
A 4-year-old child (weight 16 kg) is prescribed amoxicillin 250 mg/5 mL oral suspension, 5 mL three times daily for 10 days. The pharmacy has only amoxicillin trihydrate powder for oral suspension 125 mg/5 mL and no commercial 250 mg/5 mL product. The pharmacist decides to compound a suspension from amoxicillin trihydrate powder (potency 90.0% as amoxicillin) using a flavored syrup vehicle. The required volume is 150 mL. The pharmacy has a calibrated electronic balance (readability 0.001 g) and graduated cylinders. The pharmacist must ensure the final suspension contains 250 mg amoxicillin per 5 mL and is stable for 10 days when refrigerated.
1. Calculate the exact amount of amoxicillin trihydrate powder (in grams) needed to prepare 150 mL of the compounded suspension, assuming the powder is 90.0% amoxicillin by mass. Show your reasoning.
Read the worked answer
Step 1: Determine total amoxicillin needed.
The concentration is 250 mg per 5 mL, so per mL: 250 mg / 5 mL = 50 mg/mL.
For 150 mL: 50 mg/mL × 150 mL = 7500 mg = 7.5 g amoxicillin.
Step 2: Adjust for powder potency.
The powder is 90.0% amoxicillin, so the amount of powder needed = 7.5 g / 0.90 = 8.333... g.
Rounding to three decimal places (balance readability 0.001 g): 8.333 g.
Step 3: Practical note.
The pharmacist should weigh 8.333 g of powder, dissolve/suspend in a portion of vehicle, then bring to 150 mL with vehicle. Because the powder is only 90% amoxicillin, the remaining 10% is excipients/impurities that may affect suspension stability; a preservative and suspending agent should be included if not already in the vehicle.
2. Discuss two critical quality attributes the pharmacist must verify for this compounded suspension, and explain how they would be assessed in a community pharmacy setting without advanced instrumentation.
Read the worked answer
1. Content uniformity and dose accuracy:
The pharmacist must ensure the amoxicillin is uniformly dispersed so each 5 mL dose contains 250 mg ± acceptable range (typically 90–110% for compounded preparations). Without a HPLC, the pharmacist can:
- Verify the weighing and volume measurements are accurate (calibrated balance and graduated cylinder).
- After shaking, visually inspect for uniformity (no visible clumps or settling that does not redisperse).
- Use a simple gravimetric check: weigh 5 mL samples from different parts of the suspension; the density should be consistent if the suspension is uniform.
- If available, use a pharmacy balance to check that the total weight of the suspension matches the expected weight based on vehicle density and drug load.
2. Physical stability and microbial quality:
- Physical stability: Observe for phase separation, caking, or crystal growth over 10 days at refrigeration. A well-formulated suspension should redisperse easily upon shaking.
- Microbial quality: Since the vehicle may not contain preservative, the pharmacist should use a preserved vehicle (e.g., containing methylparaben) or prepare fresh weekly. Visual inspection for mold or turbidity is a basic check.
- pH: If pH paper is available, check pH is within stability range for amoxicillin (typically 5–7).
Documentation: Record the compounding process, lot numbers, and beyond-use date (10 days refrigerated) in the compounding log.