Pediatric Dose: Weight Conversion, Daily Cap, and Dispensed Concentration
A pediatric patient weighing 38 lb 8 oz is prescribed amoxicillin at 15 mg/kg/day, divided into two equal doses given 12 hours apart. The pharmacy stocks a suspension labeled 250 mg per 5 mL, and the order carries a hard maximum of 500 mg per day regardless of weight. Clinic policy uses 2.2 lb = 1 kg and requires the final drawn volume to be rounded to the nearest tenth of a milliliter. The parent says the child was weighed at home that morning before eating; the nurse re-weighs on the clinic scale and gets the same reading. The first dose is due at 8:00 a.m. and the second at 8:00 p.m.
1. Convert the weight to kilograms and calculate the prescribed daily dose. Does the 500 mg/day maximum change the order?
Read the worked answer
Convert ounces to a decimal pound: 8 oz = 8 ÷ 16 = 0.5 lb, so the weight is 38.5 lb.
Convert to kilograms: 38.5 ÷ 2.2 = 17.5 kg.
Daily dose: 15 mg/kg/day × 17.5 kg = 262.5 mg/day.
Check the cap: 262.5 mg is well below the 500 mg/day maximum, so the cap does not limit the order. The dose stands as calculated.
Per dose: 262.5 ÷ 2 = 131.25 mg every 12 hours.
2. How many milliliters should be drawn for a single dose at the stocked concentration?
Read the worked answer
Find the concentration in mg/mL: 250 mg ÷ 5 mL = 50 mg/mL.
Set up the proportion: 131.25 mg ÷ 50 mg/mL = 2.625 mL.
Apply the rounding rule: to the nearest tenth gives 2.6 mL.
Sanity check: 2.6 mL × 50 mg/mL = 130 mg, which is 1.25 mg (about 1%) below the exact dose - a clinically acceptable rounding error, and the nearest tenth is the closest available mark on the syringe.
3. The pharmacy substitutes a 100 mg/5 mL suspension. Recalculate the volume and explain the judgement risk this creates.
Read the worked answer
New concentration: 100 mg ÷ 5 mL = 20 mg/mL.
Volume: 131.25 mg ÷ 20 mg/mL = 6.5625 mL → 6.6 mL.
The mg dose is unchanged, but the volume is 2.54 times larger (6.6 vs 2.6 mL). The judgement risk is that a caregiver working from memory may reuse the old 2.6 mL figure with the new bottle, delivering only about 52 mg per dose - roughly 40% of the intended dose. The correct action is to re-label the dosing instructions in mL for the new concentration and to state the volume and the mg together every time the product changes.