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Calculations
Doses and Dosage Regimens
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Calculations, Doses and Dosage Regimens
Calculations, Doses and Dosage Regimens
Objectives
Young's Rule
Expressions of Dose
Cowling's Rule
Dosage Regimens
Clark's Rule
Calculate Supply Needed
Fried's Rule for Infants
Dosing in Special Cases
References
Usual Pediatric Dose for Children
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Calculations, Doses and Dosage Regimens
Objectives
Review methods of expressing doses and dosage regimens
Learn to calculate the amount of drug product to dispense or days supply from a dosage
regimen
Learn methods to calculate doses for pediatric patients given:
Pediatric dosage information found in reference books
The age or weight of the child and the normal adult dose
Expressions of Dose
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A dose is the quantity of drug taken or given.
Doses are expressed using one of the following formats:
Quantity of drug, e.g. 25 mg, 5 ml, 250 mg/ml.
Amount of drug per kilogram of body weight, e.g. 5mg/kg.
Amount of drug per square meter of body surface (BSA), e.g. 10 mg/m2 BSA.
The latter two methods, based on the patient's body size (weight or body surface area), are
often used for pediatric doses, but they are useful for adult doses for certain drugs. This is
especially true for with narrow therapeutic indices and drugs used in radiopharmaceuticals.
Dosage Regimens
A dosage regimen gives the name and the quantity or concentration of drug with frequency of
schedule of administration.
Name and quantity of drug with frequency of administration:
Example: Diazepam 2 mg three times a day or
Diazepam 6 mg per day in 3 divided doses
NOTE: The technician should use CAUTION when doses are expressed in the second way
as it has been misread as 6 mg/dose to be given in 3 doses/day - 3 times a day is the
intended dose!
Name and concentration of drug with frequency of administration:
Example: Hydrocortisone cream 2.5% apply BID
Name of drug and quantity of drug per kilogram of body weight with frequency of
administration:
Example: Haloperidol 10 mcg per kg three times a day or
Haloperidol 30 mcg per kg per day in 3 divided doses
NOTE: Again, use great CAUTION with this equivalent expression as it has been misread.
Name and quantity of drug per square meter (m2) of body surface area with
frequency of administration:
Example: Haloperidol 1 mg/m2 BSA three times a day or
Haloperidol 3 mg per m2 per day in 3 divided doses
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NOTE: Again use CAUTION with this equivalent expression as it has been misread.
Calculating the amount of drug product to dispense or days supply from a dosage regimen.
Sometimes the pharmacy technician is required to determine the amount or number of units
of a product to dispense from a dosage regimen and a prescribed number of days for therapy.
Example 1:
Cephalexin 500 mg capsules Give 1 cap BID for 10 days.
The patient needs to take 2 capsules each day for 10 days.
2 caps/day x 10 days = 20 capsules
Example 2:
Amoxicillin Susp. 250mg/5ml
Take 1 tsp TID for one week.
Since 1 tsp = 5 ml
Therefore 5ml/dose x 3 doses daily x 7 days = 105 ml
Example 3:
Cefaclor 25mg/kg/day in 2 divided doses
Give for ten days.
The child in this case weighs 44 pounds:
44 lbs 2.2 lb/kg = 20 kg
The daily dose in milligrams of Cefaclor:
25mg/kg/day x 20kg = 500 mg/day
Cefaclor is to be given in two divided doses:
500 mg/day 2 doses/day = 250 mg/dose
The number of 250 mg doses to give is:
2 doses/day x 10 days = 20 doses
If the child can take capsules, this would be twenty 250 mg capsules.
If the child requires a liquid product, Cefaclor is available in a reconstitutable suspension
which has a concentration of 250 mg/5 ml. The volume to dispense would be:
5 ml/dose x 20 doses = 100 ml
Calculate Supply Needed
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Calculate the number of day's supply from the dosage regimen and the number of
units dispensed. This is often required for third - party insurance claims.
Example 1:
Dilantin 100 mg # 100 Give 1 cap BID.
The number of days supply can be calculated by dividing the number of capsules dispensed
by the number of capsules used per day.
100 caps/2 caps per day = 50 days
Example 2:
Albuterol Inhaler 17 g Use two puffs QID.
The product insert states that each Albuterol canister contains ˜ 200 inhalations. Since this
prescription calls for 2 puffs four times a day, this patient will be using 8 inhalations per day.
The number of days supply can be calculated by dividing 200 inhalations by 8 inhalations per
day = 25 days.
Dosing in Special Cases
Geriatric patients often require special consideration when designing dosage regimen.
Pediatric patients are not just little adults. They also need pediatric doses.
In both geriatric and pediatric doses, the technician should use references such as USP/DI,
Drug Facts and Comparison, and product package inserts.
There are also general rules for calculating an infant's or child's dose of medication when
given the age or weight of the patient and the normal adult dose. These general rules are not
drug - specific and should only be used in the absence of more complete information. It is the
duty of pharmacists to be knowledgeable about the limitations of using set formulas or rules
for calculating pediatric doses.
It is best to use dosing based on information in pharmacy reference books or package inserts.
Example:
Jill is a 5 y. o. female patient. Jill weighs 42 pounds and is 43 inches tall. The
prescriber wants to give Diazepam to Jill. What is an acceptable dose?
Checking the USP/DI the technician finds:
"Usual pediatric dose: Children 6 months of age and over -- Oral, 1 to 2.5 mg, 40 to 200 mcg
(0.04 to 0.2 mg) per kg of body weight, or 1.17 to 6 mg per square meter of body surface,
three to four times a day, the dosage being increased gradually as needed and tolerated." 1
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What is the dose of Diazepam for Jill based on the quantity given in the USP/DI?
1 to 2.5 mg given 3 to 4 times a day
What is the dose of Diazepam for Jill based on the mg/kg dose given in the
USP/DI?
Calculate the weight in kg of Jill:
Calculating Weight in kg ( 02080010 )
Calculate the dose of Diazepam based on Jill's weight:
Calculating Dose Based on Weight ( 02080020 )
What is the dose of Diazepam for Jill based on the mg/m2 , body surface area
(BSA) dose given in the USP/DI?
Body Surface Area (BSA) is based on Jill's weight and height and is read from a nomogram
(nomograms for adults and children are included).
Nomograms are used in the following way:
Find the patient's weight in pounds or kilograms on the right hand side of the nomogram.
Find the patient's height in inches or centimeters on the left hand side of the nomogram.
Draw a straight line connecting these two points and red the BSA in m2 at the point the line
intersects with the middle line.
BSA for Jill = 0.76m2
USP/DI said 1.17 - 6 mg per square meter of body surface TID - QID
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Calculation of dose for Jill:
Calculating Dose ( 02080030 )
Calculation of dose using normal adult dose and "rules" for pediatric doses based
on the child's weight or age.
Example:
Tom is a 5 y. o. male patient who weighs 46 pounds. The prescriber wants to give
Captopril to Tom. What is an acceptable dose of Captopril for Tom?
USP/DI: "Usual adult and adolescent dose Antihypertensive:
Initial: Oral, 12.5 mg two or three times a day, the dosage being increased if necessary."
Usual Pediatric Dose for Children
Initial: "Oral, 300 mcg (0.3 mg) per kg of body weight three times a day, the dosage being
increased if necessary." 2
As a reference point, we will first calculate Tom's dose based on the usual pediatric mg/kg
dose given in the USP/DI. Then we will calculate doses for Tom based on the age and weight
rules for pediatric dosing. We can then compare the results.
What is the dose of Captopril for Tom based of the mg/kg dose given in the USP/DI,
the standard of reference?
Calculate the weight in kg for Tom:
Calculating Weight in kg ( 02080040 )
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Calculate the dose of Captopril based on Tom's weight:
Calculating Dose Based on Weight ( 02080050 )
What is the dose of Captopril for Tom based on the normal adult dose
and Tom's age? (Two rules may be used.)
Young's Rule
Young's Rule ( 02080060 )
Cowling's Rule
Cowling's Rule ( 02080070 )
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What is the dose of Captopril for Tom based on the normal adult dose
and Tom's weight?
Clark's Rule
Clark's Rule ( 02080080 )
Notice that in each case the doses for Captopril for Tom are similar. This is not always the
case.
Dosing for an infant based on the normal adult doses and "Fred's Rule" for
pediatric dosing is used when we have no standard of reference (e.g. USP/DI,
Facts and Comparison and package inserts).
Example:
Milli is a four-week-old infant who weighs 8 pounds. The prescriber wants to give
Captopril to Milli. What is an acceptable dose of Captopril for Milli?
USP/DI: "Usual adult and children doses are given."
Usual pediatric dose:
Newborns:
Initial: "Oral, 10 mcg (0.01 mg) per kg of body weight two or three times a day, the dosage
being adjusted as needed and tolerated." 2
Notice the difference in the mg/kg dose for newborns versus children: 10 mcg/kg versus 300
mcg/kg. Why? It may have something to do with the development of the kidneys or liver in
the first weeks of life.
What is the dose of Captopril for Milli based on the mg/kg dose given in the
USP/DI?
Calculation of kg weight of Milli:
Calculating Weight in kg ( 02080090 )
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Calculation of dose:
Calculating Dose ( 02080100 )
What is the dose of Captopril for Milli based on normal adult dose and
age?
Fried's Rule for Infants
Fried's Rule for Infants ( 02080200 )
This dosage regimen is very close to that found in part which is based on the drug - specific
dosage regimen found in the USP/DI, however, the dose of Captopril for Milli based on
normal adult dose and weight as given by Clark's Rule is not so successful. It gives a dose
that is nearly 20 times the correct dose for an infant. Try it for yourself and see!
The general rules are not drug - specific and should be only used in the absence of more
complete information.
References
1.
USP DI Vol. I, 17th ed. Rockville, MD, The United States Pharmacopeial Convention, Inc. 1997, p. 543.
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2.
USP DI Vol. I, 17th ed. Rockville, MD, The United States Pharmacopeial Convention, Inc. 1997, p. 172.
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