Question

When Ibuprofen is given for fever to children 6 months of age up to 2 years, the usual dose is 5 milligrams (mg) per kilogram (kg) of body weight when the fever is under 102.5 degrees Fahrenheit. How much medicine would be usual dose for a 18 month old weighing 21 pounds?

Answer 1

This question is throwing a lot of numbers at you. All you need to know is 5 mg / kg (2.2 lbs).

The most straightforward and accurate way to do this is to use dimensional analysis, or multiply by conversion factors until you get to the desired result.

So, start with 21 lbs, and your end goal is number of mg for the dose.

First, multiply 21 lbs by the conversion factor of 1 kg / 2.2 lbs, which cancels the lbs and leaves kg as the unit.

Then, multiply by a conversion factor of 5 mg / 1 kg. This cancels kg and leaves behind mg.

The final result is 47.7 mg

BUT if you pay attention to significant figures, which you should, then based on the information provided, the dose should be rounded to 2 sig figs, which is 48 mg.