Dosage Calculations

[Pages:23]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

CALCULATING THE QUANTITY TO DISPENSE

EXAMPLE 5.3 How many Biaxin? 250 mg tablets should be dispensed?

Rx--Biaxin? 250 mg

.. TT

po

BID

10d

Figure 5-4 Drug label for Biaxin. (Reproduced with permission of Abbott Laboratories.)

Again, to solve this dosage calculation it is important to first determine which information is necessary; it is also critical to know the common SIG codes to perform dosage calculations.

Let's look at what information that has been provided:

.. TT

po--the

dose

BID--the frequency

x 10d--the duration

Biaxin? 250 mg--the drug name and strength

100 tablets--the quantity of the stock bottle

To calculate the appropriate quantity to dispense, use the following formula:

dose frequency duration quantity to dispense

Using the information provided in the prescription, you can set up the calculation as such:

1 2 10

x

dose frequency duration quantity to dispense

1 2 10 20 20 x

So, 20 tablets of Biaxin? 250 mg should be dispensed.

Chapter Five Dosage Calculations 7

CALCULATING THE QUANTITY TO DISPENSE

EXAMPLE 5.4 How much Promethazine w/Codeine syrup should be dispensed?

Rx--Promethazine w/Codeine Syrup 6.25/10

.. TT

tsp.

po

QID

4d

Let's look at what information has been provided:

.. TT

tsp.

po--the

dose

QID--the frequency

x 4d--the duration

Promethazine w/Codeine Syrup 6.25/10--the drug name and strength

To calculate the appropriate quantity to dispense, use the following formula:

dose frequency duration quantity to dispense

Using the information provided in the prescription, you can set up the calculation as such:

1tsp 4

4

x

dose frequency duration quantity to dispense

1 4 4 16 16 x

So, 16 teaspoonsful, or 80 mL, of promethazine w/codeine should be dispensed.

PRACTICE PROBLEMS 5.2

1. What quantity should be dispensed for the prescription listed below?

________________

Rx--Zoloft? 50 mg

.. TT

po

qd

Disp. 1 mo. supply

2. What quantity should be dispensed, using the stock medication below, to provide 20 mg of diazepam prior to the procedure and 10 mg following? ________________

Figure 5-5 Drug label for Diazepam. (Used with permission of Roxane Laboratories, Inc.)

8 Chapter Five Dosage Calculations

3. What quantity should be dispensed for a three-month supply of the following prescription? ________________

Rx--furosemide 20 mg

.. TT

po

QOD

4. What quantity should be dispensed if 50 mg of amitriptyline has been prescribed daily for 3 weeks, if the medication listed below is all that is available? ________________

Figure 5-6 Drug label for Amitriptyline HCL. (Courtesy of Geneva Pharmaceuticals.)

5. What quantity should be dispensed for a 30-day supply of the following

prescription? ________________

Rx--Soma? 350 mg

.. TT

po

BID

?

TID

prn

muscle

spasms

CALCULATING THE QUANTITY OF INGREDIENT

EXAMPLE 5.5 How much codeine is in each dose of Codeine Phosphate Oral Solution 15 mg/mL?

Rx--Codeine Oral Soln. 15 mg/mL

.. TT

tbsp

po

prn

pain

Disp. # 4 oz

Figure 5-7 Drug label for Codeine Phosphate Oral Solution. (Used with permission of Roxane Laboratories, Inc.)

Chapter Five Dosage Calculations 9

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