chapter 12 methods of dosage calculation

7/14/2019 Chapter 12 Methods of Dosage Calculation

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© 2012 The McGraw-Hill Companies, Inc. All rights reserved.

Chapter 12 Methods of Dosage Calculation

PowerPoint® Presentation to accompany:

Math and Dosage Calculationsfor Healthcare Professionals

Fourth Edition

Booth, Whaley, Sienkiewicz, and Palmunen



12-2


Learning Outcomes

12.1 Determine information needed to

perform dosage calculation.

12.2 Utilize three methods of dosage

calculations.



12-3


Key Terms

Amount to

administer

Conversion factor

Desired dose

Dosage ordered

Dosage strength

Dosage unit

Dose on hand



12-4


Introduction

To calculate dosages you will use:

basic math;

the physician’s order;

drug labels;

methods of converting quantities from oneunit of measurement to another.



12-5


Information Needed for

Dosage Calculations

Desired dose (D)

Desired dose is the amount of the drug to be

administered at a single time.

Desired dose is the ordered dose converted

to the same unit of measurement as thedose on hand.



12-6



Dosage Calculations (cont.)

Dosage ordered (O)

This is the amount of drug ordered and the

frequency of administration.

Dosage s treng th (H/Q)

This is the amount of drug per dosage unit.

Medication may be available in different

dosage strengths.



12-7




Dose on hand (H)

This is the amount of drug contained within

a dosage unit of medication.

Dosage un it (Q)

This is tablet, capsule, mL, etc.

Both can be found on the label



12-8




Before calculating the amount to

adm inister (A ) , first determine the

desired dose (D) .

Convert Dosage ordered (O) to desired

dose having the same units as the dose

on hand (H) .



12-9




Dosage ordered = O

Desired dose = D

Dosage unit = Q

Dose on hand = H

Amount to

administer = A

Dosage strength =

H/Q



12-10


Converting to Like Units of

Measurement

Two methods

Proportion

Fraction Ratio

Dimensional analysis

Method used is a matter of choice



12-11



Measurement (cont.)

Rule 12-1

Before the amount to administer can becalculated, you must convert the unit of measure of the desired dose to thesame as the unit of measurement of thedose on hand.



12-12


Converting to Like Units of Measure – Proportion Method

Procedure Checklist 6-1:

1. Write a conversion factor with the

units needed in the numerator or

before the colon;

units converting from in the denominator or

after the colon.



12-13



Procedure Checklist 6-1 (cont.)

2. Write a proportion

with the unknown, “ x ”, in the numerator or

before the colon;

with the number to convert in the

denominator or after the colon.

x /D = or x :D



12-14


Procedure Checklist 6-1: (cont.)

3. Set the two factors up as a proportion.

4. Cancel units.

5. Cross-multiply, or multiply the meansand extremes and solve for theunknown value.


x/B = C/D or x:B = C:D



12-15


Converting to Like Units of Measure

– Proportion Method

The dosage ordered is0.2 mg once a day.

The dosage strength is 100 mcg/tablet.

Find the desired dose.

D/0.2 mg = 1000 mcg/1 mg

D x 1 = 1000 x 0.2

D = 200 mcg

Desired dose = 200 mcg

Example



12-16


Error Alert!

In proportion method using fractions, units from

the two fractions can be canceled only when

they are the same portion of the fraction.

Always inc lude the uni ts when

perform ing calculations.



12-17



Measure – Proportion Method

The order reads: ASA gr v POdaily.

The drug label indicates 325

mg tablets.Find the desired dose using 1 gr = 65 mg

65mg:1gr = D: 5 gr

1× D = 65× 5

D = 325

Desired dose is 325 mg.

Example



12-18


Error Alert!

In proportion method using ratios, units can

be canceled on ly when they are found inthe same part of each of the ratios.

Always include the units when performing calculations.



12-19


Converting to Like Units of Measure – Dimensional Analysis


1. Determine the unit of measure for the

answer and place it as the unknownon one side of the equation.

x mg =



12-20


Converting to Like Units of Measure – Dimensional Analysis (cont.)


2. On the other side of the equation,

write a conversion factor with the unitsof measure for the answer on top and

the units you are converting from on

the bottom.

g1

mg1000 mg x



12-21




3. Multiply the numerator of the

conversion factor by the number thatis being converted divided by 1.

1

g1.5

1g

1000mg

mg X x



12-22



4. Cancel units on the right side of the

equation.

1

g1.5

1g

1000mg mg Xx




12-23




5. Solve the equation.

1

mg1500 mgx

mg1500mgx



12-24



Measure – Dimensional Analysis (cont.)

Find the desired dose for the following:

Ordered: Nitrostat 800 mcg sublingually PRN chestpain

On hand: See label – must convert mcg to mg.

D mg = 1mg/1000mcg x 800mcg/1

D mg = 800 mg/1000

D mg = 0.8 mg

Desired dose is 0.8 mg

Example



12-25


Error Alert!

In dimensional analysis, units can be canceled

only when they are found in both the

numerator and the denominator of the

fraction.

Always include the units when performing calculations.



12-26


Practice

Determine the desired dose.

Ordered: Clindamycin 0.6 g IVSS

On hand: see label

Desired dose: 600 mg



12-27


Methods of Dosage Calculation

Once the desired

dose is calculated,the amount to

administer must be

calculated.



12-28


Methods of Dosage

Calculation (cont.)

Three methods

Proportion

method

Dimensional

analysis

Formula method

Three steps

A. Convert

B. Calculate

C. Think!...Is It

Reasonable?



12-29


Methods of Dosage

Calculation (cont.)

Rule 12 –2 To calculate the Amount to

Administer

Step A ConvertConvert desired dose to same unit of

measurement as dose on hand.



12-30


Calculating the Amount to

Administer (cont.)

Rule 12-2 (cont.)

Step B CalculatePerform the dosage calculation using

method of choice.



12-31



Administer (cont.)

Rule 12-2 (cont.)

Step C Think!…Is It Reasonable? Think about the answer.

Apply common sense and rules of dosage calculation.



12-32



Administer - Proportion

Procedure 12-1

1. Set up the proportion as follows:

a. Fraction

b. Ratio

A

D

Q

H

A DQ H ::

12 33



12-33



Administer - Proportion (cont.)

Procedure 12-1 (cont.)

2. Cancel units.

3. Cross multiply or multiply means and

extremes.

4. Solve for the unknown value.

12 34



12-34


Ordered: Unasyn 250 mg IM

Dosage strength available: 0.5 g/mL

Find the amount to administer.

Step A Convert grams to milligrams

D : 0.5 g = 1000 mg / 1 g

0.5 x 1000 mg = D X 1

500 mg = D



Example

12 35



12-35


H = 500 mg Q = 1 mL D = 250 mg

Step B Calculate

500 X A = 1 mL X 250

500

250

500

500 mL A

mg250A

mg500mL1



Example

A X 500 = 250 mL

Amount to administer= 0.5 mL

12 36



12-36


Step C Think!…Is It Reasonable?

250 mg is half dose on hand (0.5 g = 500 mg)

Since the dose on hand = 500 mg/mL, 0.5 mL

is reasonable for the amount to administer to

deliver 250 mg (dose ordered).



Example

12 37



12-37



Administer – Dimensional Analysis

Procedure 12-3

With dimensional analysis you will notneed to calculate the desired dose and

amount to administer separately.

12 38



12-38


Calculating the Amount to Administer

– Dimensional Analysis (cont.)

Procedure 12-3 (cont.):

1. Determine the units of measure for the

answer.

Place on one side of the equation

Answer = the amount to administer

x tab=

12 39



12-39





2. Write a conversion factor.

Place the unit of measure for the desireddose on top.

Place the unit of measure for the dosageordered on the bottom.

g1

mg1000 tab?

12 40



12-40





3. Multiply the conversion factor by a

second factor. Dosage unit over dose on hand

mg250tab1

1g1000mg tabx X

12 41



12-41





4. Multiply by a third factor.

Dose ordered over the number one

1

g0.5

250mg

tab1

1g

1000mg

tabxXX

12 42



12-42



5. Cancel units on the right side of the

equation.

1

g0.5

250mg

tab1

1g

1000mg tabx XX



12-43



12-43





6. Solve the equation.

1g0.5

250mgtab1

1g1000mg tabx XX

1X250X10.5Xtab1X1000 tabx A tab = 2

12-44



12-44




The order reads: Prozac Liquid

40mg daily

On hand: Prozac 20 mg/5 mL


H = 20mg Q = 5mL D = 40 mg

Step A ConvertNo conversion needed.

Example

12-45



12-45


H = 20 mg Q = 5 mL D = 40mg

Step B Calculate

1. A mL =

2. S

3. A

4. A mL = 10 mL

1mg40

mg20mL5AmL



Example

20mg

5mLmL A

12-46



12-46


Step C Think!...Is It Reasonable?

What is the relationship between the doseon hand and the dose ordered?

The dose ordered (40 mg) is twice that of

the dose on hand (20 mg) and 10 mL is

twice 5 mL so it is reasonable.



Example

12-47



12 47



Administer – Formula Method (cont.)

Procedure 12-4

1. Determine components of the formula

method: D, H, Q, and A.

2. Fill the formula with known data.

AQxHD

12-48

C l l i h A



12 48





3. Cancel the units.

4. Solve for the unknown.

12-49

C l l ti th A t t



12 49




The dosage ordered is Famvir

500 mg PO q 8 h

On and: Famvir 250 mg/tabs


H = 250 mg Q = tablet D = 250 mg

Step A Convert

No conversion necessary.

Example

12-50

C l l ti th A t t



12 50


Step B Calculate

2 x 1 tablet = A

2 tablets = A

A1tabletmg250500mg



Example

AQxH

D

12-51

C l l ti th A t t



12 51


Step C Think!...Is It Reasonable?

What is the relationship between the dose

ordered and dose on hand?

Dose ordered is twice dose on hand, and twotablets is twice 1, so it is reasonable.



Example

12-52




Practice

Ordered: Targretin 0.3g po daily

On hand: see label

Calculate the amount to administer:

Is it reasonable?

Why?

A = 4 capsules

Dose ordered (300 mg)

is 4 times the dose onhand (75 mg), and 4

capsules is 4 times 1,

so it is reasonable.

12-53




In Summary

In this chapter you learned to

determine information needed toperform dosage calculation.

12-54




In Summary

In this chapter you learned to

utilize three methods of dosagecalculations.

12-55




Apply Your Knowledge

The amount of the drug to be given at a

single time is the:

a. dosage ordered

b. dosage unit

c. desired dose

d. dose on hand

12-56





True or False

If a medication has 300 mg of drug per

tablet, the dose on hand is 300 mg.

Answer: True

12-57





The MAR reads Glucotrol 10 mg PO qd.

The desired dose is ____ mg.

Answer: 10 mg

12-58





Calculate the amount to administer.

Ordered: Prednisone 10 mg PO qid

On hand: Prednisone 5 mg tablets

Is the answer reasonable? Why?

Answer : Amount to administer = 2 tablets

Answer : Dose ordered (10 mg) is 2 times the

dose on hand (5 mg), and 2 tablets is twice the

dose on hand, so it is reasonable.

12-59



End of Chapter 12

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