The doctor prescribes a daily dose of 25 mg per kg of body weight, to be given in 2 injections 12 hours apart. How many milligrams should be used for each injection? The patient weighs 23 kg.

Question

The answer I got was 580mg per dose

Answer 1

Hi there James!

This question deals with unit conversions, thus using different rates to get the answers you need.

If you're unfamiliar with cancelling with multiplying and dividing, here's how that works:

Looking at 2 * (3 * 5 / 2), you can cancel both 2's because 2 divided by 2 is just 1 and be left with 3 * 5.

Likewise, if it was 4 * (3 * 5 / 2), you can turn that into 2 * 2 * 3 * 5 / 2 and be left with 2 * 3 * 5.

It saves you work to cancel when you can, and cancelling works the same with units!

In this problem, we are given the following rates:

25 mg / 1 kg body weight / 1 day

2 doses / 1 day

What we need is mg / dose. So, let's set up the equation:

25 mg * 1 day * 23 kg = 287.5 mg

1 kg * 1 day 2 doses 1 dose

The days cancel out because "day" divided by "day" equals 1, and same with the kilograms. Notice that the rate of 2 doses per day is flipped so that we can cancel the units and get where we need to go with the numbers.

You don't need to write the 1's, of course, but the idea is that a patient would may take their doses for 1 day or 30, but they will take 2 doses each 1 day. It's good to be thorough with the 1's because sometimes rates could be given in any way, like 35 per 12 or something, and you want to make sure to catch that for the calculations.

Hope this helps! Please reach out with any further questions!

Answer 2

First, you want to determine the total dose the patient needs. You can do this with his weight and the total dose required by weight:

23kg * 25 mg/kg = 575mg

If this is how much the patient needs in total, you can then divide this number by 2 to determine how much he needs at each injection.

575mg/ 2 injections = 287.5mg/injection

hope this helps!