Synopsis
Calculate medicine doses, concentrations and infusion rates with explicit units, appropriate rounding and an independent check that the final administration delivers the intended treatment.
- Write the required answer unit before calculating: tablets, millilitres, milligrams, micrograms per minute and millilitres per hour are different outputs.
- Convert units explicitly: 1 g = 1,000 mg, 1 mg = 1,000 micrograms and 1 hour = 60 minutes. Carry units through each step so incompatible quantities cannot cancel unnoticed.
- For a weight-based dose, multiply dose per kilogram by the appropriate verified weight; for a volume, divide the required amount by the stock concentration.
Reasoning priorities
Establish the correct patient inputs for dose selection.
Use kilograms and a recent reliable measurement. Check whether the medicine requires actual, ideal or another specified dosing weight, particularly at extremes of body size. Renal impairment, liver risk, dehydration or malnutrition may change a maximum or interval even when the weight calculation is correct.
Worked reasoning
A 44 kg adult temporarily cannot take oral analgesia. The prescribed product is paracetamol 10 mg/mL; renal and liver function are normal, there are no additional hepatotoxicity risks or previous paracetamol doses in 24 hours. The selected dose is 15 mg/kg IV over 15 minutes, with up to four doses in 24 hours at six-hour intervals if required.
- Calculate the medicine amount: 15 mg/kg × 44 kg = 660 mg. Check the daily ceiling separately: 60 mg/kg/day × 44 kg = 2,640 mg/day, which is below the product’s 3 g ceiling for this weight band.
- Convert the single dose into volume using the actual concentration: 660 mg ÷ 10 mg/mL = 66 mL. The 100 mL container is a stock presentation; infusing all 100 mL would give 1,000 mg and exceed this patient’s calculated single dose.
- Convert time to hours: 15 minutes ÷ 60 = 0.25 hours. Divide the intended 66 mL volume by 0.25 hours to obtain a pump rate of 264 mL/hour, using a system that delivers and stops at the prescribed 66 mL volume.
- Give the final order as 660 mg, equivalent to 66 mL of 10 mg/mL solution, intravenously over 15 minutes, with the supplied six-hour minimum schedule and 2,640 mg total daily limit. Reverse-check 264 mL/hour × 0.25 hours × 10 mg/mL = 660 mg, and four such doses equal 2,640 mg.