Page #135, Question #6:



SELECTED ADVANCED PHARM MATH COMPUTATIONS

A PRESCRIPTION IS GIVEN AS:

Rx Erythromycin ethylsuccinate 400 mgs / 5 mls

Disp: 100 mls

Sig: ___?____ tsp q.i.d. until all medication is taken

If the dose of Erythromycin ethylsuccinate is given as 40 mg/kg per day.

(a) What would be the proper dose of the medication in the Signa if the prescription is for a 44 Lb. child ?

(b) How many days will the medication last ?

STEP #1: To answer (a), the proper dose in the Signa for a 44 lb child:

First, convert the child lb weight to kg ( 1 kg = 2.2 lbs), in any of the following ways:

1. 1 kg / 2.2 lbs = X / 44 lbs => X = 44 / 2.2 = 20 kg

2. 2.2 lb : 1 kg :: 44 lb : X => X = 44 / 2.2 = 20 kg

3. 44 lb X 0.454 = 20 kg

STEP #2: Find out how many mgs per kg of the child in a day from your 20 kg calculated:

1. 40 mgs / 1 kg = X / 20 kg ==> X = (40 mg/kg) (20 kg) = 800 mgs erythromycin / day

2. 1 kg : 40 mgs :: 20 kg : X ==> X = (40 mg/kg) (20 kg) = 800 mgs erythromycin / day

STEP #3: Find out the proper dose as per Signa ?

The question tells you: "Sig: ___?____ tsp q.i.d. until all medication is taken", which is telling you to compute the amount of teaspoons to be taken four (4) times a day (qid). If the daily total dose computed is 800 mgs, then each dose must be = > 800 mgs / 4 = 200 mg per dose. To compute the number of teaspoons, convert the 400 mgs / 5 mls from the data given to 400 mgs / 1 tsp ( 1 tsp = 5 mls), then compute the following proportion:

1. 400 mgs / 1 tsp = 200 mgs / X tsp ==> X = 0.5 tsp = 1/2 tsp

2. 400 mgs : 1 tsp :: 200 mgs : X tsp ==> X = 0.5 tsp = 1/2 tsp

(b) How many days will the medication last ?

STEP #1: Find the total mgs in the medication in general:

1. 400 mgs / 5 mls = X mgs / 100 mls ==> X = 40000 / 5 = 8000 mgs

2. 5 mls : 400 mgs :: 100 mls : X ==> X = 40000 / 5 = 8000 mgs

STEP #2: Find the total mgs of the medication for a 44 lb child, if the dose of Erythromycin ethylsuccinate is given as 40 mg/kg per day:

1. 40 mgs / 1 kg = X mgs / 20 kg ==> X = 800 mgs per day

2. 1 kg : 40 mgs :: 20 mgs : X mgs ==> X = 800 mgs per day

STEP #3: Find the total days the medication will last:

8000 mgs in an entire bottle / 800 mgs per day = 10 days

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