PHARMACEUTICAL CALCULATIONS III
INTRODUCTION
The therapeutic dose of a medication is based, generally, on its having a desired concentration in the patient s body. What is considered to be an ideal dose for an adult, when placed in the body of a small child, would give a concentration that is much greater than desired and could cause an adverse effect. Therefore, it is normally necessary to calculate a smaller dose for the child. This lesson will explain the methods used in pharmacy to calculate patient dosages.
WHAT IS A DOSE?
a. The term DOSE refers to the amount of medication that a patient must take at one time to produce the optimum therapeutic effect. The terms "average dose," "usual dose," and "adult dose" are based on the amount of medication needed to treat the average size adult (150-154 lbs.) with optimum effect. In order to make it easier to calculate dosages for other than average size patients, many drug manufacturers have established recommended doses based on the patient s weight or Body Surface Area (BSA). With this type of recommended dose, it is easy to calculate a dose for any size person using the same formula.
b. Examples of recommended doses:
(1) Pyrantel pamoate (Antiminth?) has a recommended dose of 11 mg/kg of body weight and is given in a single dose.
(2) Isoniazid (INH?) has a recommended dose of 450 mg/M2/24 hours. The M2 refers to one square meter of body surface area.
(3) Meperidine hydrochloride (Demerol?) has a recommended dose of 6 mg/kg/24 hours for pain and is given in divided doses four to six times daily. No single dose should exceed 100 mg.
PROBLEMS USING MANUFACTURER S RECOMMENDED DOSE
Solution by ratio and proportion: (Example problems)
a. Cyclophoshamide (Cytoxan?) 50 mg tablets are used in the treatment of neoplasms (abnormal growths or tumors) and have a recommended dose of 5 mg/kg of body weight to be given in a single dose daily. How many tablets should be dispensed to a 110-lb patient as a ten-day regimen? (NOTE: A regimen is a treatment plan.).
(1) Find the dose for the patient in milligrams:
5 mg X mg
1 kg (2.2 lbs) = 110 lbs
2.2X = 550
X = 550
2.2
X = 250 mg per dose
NOTE: 1 kg was changed to 2.2 lbs. so that corresponding units of the proportion are the same.
(2) Find the number of tablets to be dispensed per day:
1 tab = X tab
50 mg 250 mg
50 X = 250
X = 5 tablets per day
(3) Times 10 days:
10 x 5 tablets = 50 tablets required for a ten day regimen.
b. Meperidine HCl (Demerol?) is a synthetic narcotic analgesic having a recommended dose of 6 mg/kg/24 hours for pain and is given in divided doses at four to six hour intervals. The maximum single dose is 100 mg. How many milliliters of Demerol??injection (50 mg/ml) should be administered to a 33-pound child, who is suffering the pain of a broken leg, as a single dose every six hours?
(1) Find the daily dose in milligrams:
6 mg = X mg
1 Kg (2.2 lbs) 33 lbs
2.2 X = 198
X = 198
2.2
X = 90 mg of Demerol??per day
NOTE: 1 kg was changed to 2.2 lbs so that corresponding units of the proportion are the same. For a review of this conversion factor,
(2) Find the number of milliliters of Demerol??injection (50 mg/ml) needed
as a daily dose:
50 mg = 90 mg
1 ml Xml
50 X = 90
X = 90
50
X = 1.8 ml every 24 hours.
(3) Find the number of milliliters to be administered every six hours:
1.8 ml = X ml
24 hours 6 hours
24 X = 10.8
X = 108 ÷ 24
X = 0.45 ml of Demerol??injection to administered every six hours
PROBLEMS USING LABEL STRENGTH
In calculating problems involving the label strength, the label strength should be the first ratio of the proportion.