# Dosage calculations

1. I have absolute no clue how to work out this problem, and i need help in working out the steps.

Order: erythromycin oral suspension 300 mg q 6h PO. The child weighs 55lbs. The safe dosage range is 30 mg/kg/day-50 mg/kg/day in 4 divided doses.
Available: erythromycin oral suspension 200 mg/5mL.
What is the safe dose range?
•

Joined: Feb '18; Posts: 2; Likes: 1
Specialty: 1 year(s) of experience

3. Show us what you've tried so far.
4. It looks like they are asking for the safe dose range in mg /dose.
55 lb (1 kg/2.2 lb) (30 mg/kg*day)(1 day/4 doses) =187.5 mg/dose

See if you can calculate the upper limit.
Note :30 mg/kg*day is mathematically equivalent to 30 mg/kg/day, but is easier to work with.
5. So the upper dose would be 55lbs(1 kg/2.2lbs)(50mg/kg*day)(1day/4 doses)=312.5 mg/dose correct?
So the available amount doesn't come into play when trying to determine the safe amount or does it?
6. Quote from SROBISON1
So the upper dose would be 55lbs(1 kg/2.2lbs)(50mg/kg*day)(1day/4 doses)=312.5 mg/dose correct?
[...]
Yes.
Quote from SROBISON1
[...]
So the available amount doesn't come into play when trying to determine the safe amount or does it?
No, not for determining the dosage range. The available concentration would be required if you were asked to calculate the colume of medication required to deliver the 300 mg dose.

This is format that most, if not all of the medication dosing problems you are going to have to complete are going to be written. You are going to be provided a lot of information, not all of which is going to be needed. After reading the problem, take a moment to determine exactly whau you are being asked to solve, and then extract what you need.

Best wishes.
7. Also, once you have calculated the lower limit you can multiply that by 5/3 to get the upper limit. Basically, you are replacing the 30 mg with 50 mg. Don't take this type of shortcut unless you completely understand the math behind it.
8. Quote from bjwojcik
30 mg/kg*day is mathematically equivalent to 30 mg/kg/day, but is easier to work with.
Brad, I beg to differ. As written, they are not mathematically equivalent.

30 mg/(kg*day) is equivalent to 30 mg/kg/day...

Conceptually, I think it makes more sense to think of it as (30 mg/kg)/day
9. Hi Music,
Yes, it is less confusing to write it (30 mg/kg)/day, but I have never seen it listed that way in a dosage. It is perfectly fine to use (30 mg/kg)/day, so the student would know that 30 mg/kg goes on top and day goes on the bottom in a DA problem. I am sure you would agree that when it is written (30 mg/kg)/day, that it is equivalent to 30 mg/(kg*day). I should have included the () in the original post. It is much easier to see when using an equation editor than typing it out like this. Sorry.

Multiply top and bottom by 1/day.

(30 mg/kg)(1/day)/day (1/day) = (30 mg/kg*day)/(day/day)= (30 mg/kg*day)/1=
30 mg/(kg*day)

I think the main thing we are both trying to do is avoid anyone thinking it is 30 mg/(kg/day).