Dosage Calculation With a Weight Constraint
A child weighing 28 kg is prescribed an antibiotic at 15 mg/kg per dose, given every 8 hours. The pharmacy stocks an oral suspension of 250 mg per 5 mL. The drug's labeled maximum is 500 mg per dose. The prescriber's order reads "15 mg/kg/dose q8h, do not exceed max." A nurse calculates the volume per dose and prepares to administer it. The medication cup is marked in 1 mL increments, and the suspension must be shaken before measuring.
1. Calculate the prescribed dose in mg and the volume in mL to administer, and state whether the maximum is exceeded.
Read the worked answer
Dose = 15 mg/kg × 28 kg = 420 mg per dose.
Volume: 250 mg / 5 mL = 50 mg/mL. 420 mg ÷ 50 mg/mL = 8.4 mL.
Maximum is 500 mg/dose; 420 mg < 500 mg, so the max is not exceeded. The order is safe to fill as written.
2. The cup is marked in 1 mL increments. How should the nurse handle the 8.4 mL measurement, and why does it matter?
Read the worked answer
The nurse cannot measure 8.4 mL accurately in a 1 mL-increment cup. Rounding to 8 mL delivers 400 mg (a 20 mg shortfall); rounding to 9 mL delivers 450 mg (a 30 mg excess). Neither is acceptable when a more precise tool exists.
Correct action: use an oral syringe graduated to 0.1 mL (or 0.2 mL) to deliver 8.4 mL, and shake the suspension first so the drug is evenly dispersed - an unshaken suspension can deliver substantially more or less than labeled. Do not "eyeball" between cup marks.
3. Over a full 24 hours, how many total mg will the child receive at the calculated dose, and how does that compare to giving the maximum allowable dose each time?
Read the worked answer
Doses per 24 h = 24 ÷ 8 = 3 doses.
At prescribed dose: 420 mg × 3 = 1,260 mg/day. At maximum each dose: 500 mg × 3 = 1,500 mg/day.
Difference = 240 mg/day less than the ceiling. The prescribed regimen stays 80 mg/dose under the cap, leaving margin - but the nurse should still verify the weight (28 kg) is current, since a 5 kg weight error would push the dose to 495 mg, near the maximum.