Dosage Calculations

Hi,

I have taught pharmacy calculations in the past and written a book on the subject, and I have looked at that link and find the methods very confusing. Dosage calculations all boil down to this:

The pt is prescribed something in one type of unit (mg, mL, etc) and you are asked to change that into the units of the answer. The problem will always have at least one ratio, which is (are) the tool(s) used to change the units of the prescribed into the units of the answer.

Child is prescribed 220 mg of Acetaminophen. The drug is available as 80 mg/mL. How many mL do you administer?

220 mg (1 mL/80 mg) = 2.75 mL

(I tried to give away my book for free here, but I guess that is not allowed.)

Brad Wojcik, PharmD

Also, if the dose is based on weight, you will be starting off with the pt's weight and ending up with dose.

Pt weighs 14 kg and is prescribed 30 mg/kg. How many mg?

14 kg (30 mg/kg) = 420 mg.

If the dose is 30 mg/kg/day, the easiest thing to do is change that to 30 mg/kg*day, which is mathematically equivalent.

14 kg (30 mg/kg*day) = 420 mg/day

If the drug is available as 100 mg/mL, and you are asked to calculate mL, you just add that ratio into the equation.

14 kg (30 mg/kg*day)(1 mL/100 mg) = 4.2 mL/day.

Suppose it is 30 mg/kg/day divided into 2 doses and you have to calculate mL/dose.

14 kg (30 mg/kg*day)(1 mL/100 mg)(1 day/2 doses) = 2.1 mL/dose.

Hope this helps some.

Brad Wojcik, PharmD