Dosage Calculations
CHAPTER
5
Dosage Calculations
L earning Objectives
After completing this chapter, you should be able to:
? Solve one-step pharmaceutical dosage calculations.
? Set up a series of ratios and proportions to solve a single dosage calculation.
? Determine what information you will need to solve for, in addition to any given information, to properly calculate dosages.
? Convert pediatric weights from pounds to kilograms.
? Accurately determine dosages based on mg/kg/day.
? Define common sig (signa) codes used on prescriptions.
INTRODUCTION
Proper dosing of medications is important to ensure patient safety. Calculating dosages, dosage regimens, and compounding formulas involves the use of simple math principles. You can solve many of these problems by setting up ratios and proportions using the information given in the question and keeping like units consistent.
2
Sig Refresher
The sig portion of the prescription order, meaning signa, is where the instructions for the patient are written. Pharmacy technicians enter the information from the prescription order into the computer. The sig is an important value to remember in order to properly determine pediatric dosages. The following are some of the more common sigs you will find on prescriptions:
qd = every day qod = every other day d = daily bid = twice a day tid = three times a day qid = four times a day q4h = every 4 hr q6h = every 6 hr q8h = every 8 hr q4?6h = every 4?6 hr prn = as needed
Depending on the workplace, you may also see sigs such as the following:
q3d = every three days qmwf = every Monday, Wednesday, and Friday qw = every week
Dosage Calculations
Dosage calculations include calculating the number of doses, dispensing quantities, and ingredient quantities; these calculations are performed in the pharmacy on a daily basis. The pharmacy technician must have a full working knowledge of how to perform these calculations.
To perform dosage calculations, you will utilize the information and principles introduced in the previous chapters of this book. You can solve these calculations by setting up ratios and proportions, keeping like units consistent, and cross-multiplying.
CALCULATING THE NUMBER OF DOSES
To calculate the number of doses, you should first determine which information presented is actually applicable to the question. Too often mistakes are made on dosage calculations because we overcomplicate them.
EXAMPLE 5.1
How many 1-tsp doses are in a 4 oz bottle of Prozac? Liquid Solution 20 mg/5 mL?
Rx Prozac? Solution
.. TT
tsp.
po
qd
Disp. # 4 oz
Chapter Five Dosage Calculations 3
Figure 5-1 Drug label for Prozac. (Courtesy of Eli Lilly and Company.)
Let's look at the information that has been provided:
.. TT
tsp
po--the
dose
qd--the frequency
4 oz--the quantity dispensed Prozac? Solution 20 mg/5 mL--the drug name and strength 120 mL--the quantity of the stock bottle
The question is simply asking how many doses make up the total amount being dispensed. The strength of the drug, frequency of dosage and quantity of the stock bottle have no relevance in performing this calculation.
So now we know that we are working with 1 tsp. doses and a total quantity of 4 oz, but to solve this calculation using a ratio/proportion we have to have similar units of measure--in this case mL.
We know that 1 tsp = 5 mL and we should also know that 4 oz. = 120 mL, but if you didn't that could also be solved by using a ratio/proportion.
1 oz 4 oz =
30 mL x mL
Cross-multiply and solve the equation for x.
30 * 4 = 120 and 1 * x = (1) x
(1)x = 120
Now that you have both quantities converted to units in mL, we can set up our ratio/proportion and solve.
1 dose x doses =
5 mL 120 mL
Cross-multiply
5 * x = 5x and 1 * 120 = 120
4 Chapter Five Dosage Calculations
Now set up your equation and solve for x.
5x = 120
To solve for x, divide both sides by 5.
2x 120 =
22 120 , 5 = 24
x = 24
So, there are 24 doses (of 5 mL) in a 4 oz (120 mL) bottle.
CALCULATING THE NUMBER OF DOSES
EXAMPLE 5.2
How many doses are provided in the prescription below? Rx--Ibuprofen 400 mg T. -T.T. po q 6 hr prn pain Disp. # 120
Let's look at the information that has been provided:
T. -T.T. po--the dose q 6 hr prn pain--the frequency 120--the quantity dispensed Ibuprofen 400 mg--the drug name and strength
Workplace Wisdom Always use the higher dosage amount when performing dosage calculations on prescriptions that have a range for the dose, as in Example 5.2. This will provide the most conservative solution and ensure the most accurate potential for days supply.
Using the information provided, set up the ratio/proportion and solve.
Cross-multiply
1 dose x doses =
2 tabs 120 tabs
2 * x = 2x and 1 * 120 = 120
Now set up your equation and solve for x.
2x = 120
To solve for x, divide both sides by 2.
2x 120 =
22
120 , 2 = 60
x = 60
So, there is a minimum of 60 doses prescribed.
Chapter Five Dosage Calculations 5
PRACTICE PROBLEMS 5.1
1. How many dosages are provided in the prescription listed below? ________________
Rx--Cardura? 2 mg
.. TT
po
qd
Disp. # 90
2. How many droppersful (a 2.5 mL dosage) are in a 50 mL bottle of EryPed? Drops? ________________
Figure 5-2 Drug label for EryPed Drops. (Reproduced with permission of Abbott Laboratories.)
3. How many dosages are provided in the prescription listed below? _____________________
Rx--Vicodin? 5/500
.. TT
po
q8hr
Disp. # 50
4. How many 1 tsp doses are in each bottle of Zithromax? 200 mg/5 mL as listed below, when mixed? ________________
Figure 5-3 Drug label for Zithromax. (Registered Trademark of Pfizer Inc. Reproduced with permission.)
5. How many dosages are provided in the prescription listed below? ________________ Rx--Regular Insulin U-100 (100 units/mL) Inject 20 units qAM Disp. # 1 vial (10mL)
6 Chapter Five Dosage Calculations
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