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Chapter 12 Methods of Dosage Calculation
PowerPoint® Presentation to accompany:
Math and Dosage Calculationsfor Healthcare Professionals
Fourth Edition
Booth, Whaley, Sienkiewicz, and Palmunen
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Learning Outcomes
12.1 Determine information needed to
perform dosage calculation.
12.2 Utilize three methods of dosage
calculations.
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Key Terms
Amount to
administer
Conversion factor
Desired dose
Dosage ordered
Dosage strength
Dosage unit
Dose on hand
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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.
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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.
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Information Needed for
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.
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Information Needed for
Dosage Calculations (cont.)
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
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Information Needed for
Dosage Calculations (cont.)
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) .
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Information Needed for
Dosage Calculations (cont.)
Dosage ordered = O
Desired dose = D
Dosage unit = Q
Dose on hand = H
Amount to
administer = A
Dosage strength =
H/Q
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Converting to Like Units of
Measurement
Two methods
Proportion
Fraction Ratio
Dimensional analysis
Method used is a matter of choice
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Converting to Like Units of
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.
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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.
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Converting to Like Units of Measure – Proportion Method
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
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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.
Converting to Like Units of Measure – Proportion Method
x/B = C/D or x:B = C:D
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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
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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.
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Converting to Like Units of
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
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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.
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Converting to Like Units of Measure – Dimensional Analysis
Procedure Checklist 6-2:
1. Determine the unit of measure for the
answer and place it as the unknownon one side of the equation.
x mg =
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Converting to Like Units of Measure – Dimensional Analysis (cont.)
Procedure Checklist 6-2 (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
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Converting to Like Units of Measure – Dimensional Analysis (cont.)
Procedure Checklist 6-2 (cont.)
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
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Procedure Checklist 6-2 (cont.)
4. Cancel units on the right side of the
equation.
1
g1.5
1g
1000mg mg Xx
Converting to Like Units of Measure – Dimensional Analysis (cont.)
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Converting to Like Units of Measure – Dimensional Analysis (cont.)
Procedure Checklist 6-2:
5. Solve the equation.
1
mg1500 mgx
mg1500mgx
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Converting to Like Units of
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
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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.
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Practice
Determine the desired dose.
Ordered: Clindamycin 0.6 g IVSS
On hand: see label
Desired dose: 600 mg
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Methods of Dosage Calculation
Once the desired
dose is calculated,the amount to
administer must be
calculated.
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Methods of Dosage
Calculation (cont.)
Three methods
Proportion
method
Dimensional
analysis
Formula method
Three steps
A. Convert
B. Calculate
C. Think!...Is It
Reasonable?
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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.
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Calculating the Amount to
Administer (cont.)
Rule 12-2 (cont.)
Step B CalculatePerform the dosage calculation using
method of choice.
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Calculating the Amount to
Administer (cont.)
Rule 12-2 (cont.)
Step C Think!…Is It Reasonable? Think about the answer.
Apply common sense and rules of dosage calculation.
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Calculating the Amount to
Administer - Proportion
Procedure 12-1
1. Set up the proportion as follows:
a. Fraction
b. Ratio
A
D
Q
H
A DQ H ::
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Calculating the Amount to
Administer - Proportion (cont.)
Procedure 12-1 (cont.)
2. Cancel units.
3. Cross multiply or multiply means and
extremes.
4. Solve for the unknown value.
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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
Calculating the Amount to
Administer - Proportion (cont.)
Example
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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
Calculating the Amount to
Administer - Proportion (cont.)
Example
A X 500 = 250 mL
Amount to administer= 0.5 mL
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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).
Calculating the Amount to
Administer - Proportion (cont.)
Example
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Calculating the Amount to
Administer – Dimensional Analysis
Procedure 12-3
With dimensional analysis you will notneed to calculate the desired dose and
amount to administer separately.
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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=
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Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Procedure 12-3 (cont.):
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?
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Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Procedure 12-3 (cont.):
3. Multiply the conversion factor by a
second factor. Dosage unit over dose on hand
mg250tab1
1g1000mg tabx X
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Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Procedure 12-3 (cont.):
4. Multiply by a third factor.
Dose ordered over the number one
1
g0.5
250mg
tab1
1g
1000mg
tabxXX
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Procedure 12-3 (cont.):
5. Cancel units on the right side of the
equation.
1
g0.5
250mg
tab1
1g
1000mg tabx XX
Calculating the Amount to Administer
– Dimensional Analysis (cont.)
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Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Procedure 12-3 (cont.):
6. Solve the equation.
1g0.5
250mgtab1
1g1000mg tabx XX
1X250X10.5Xtab1X1000 tabx A tab = 2
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Calculating the Amount to Administer
– Dimensional Analysis (cont.)
The order reads: Prozac Liquid
40mg daily
On hand: Prozac 20 mg/5 mL
Find the amount to administer.
H = 20mg Q = 5mL D = 40 mg
Step A ConvertNo conversion needed.
Example
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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
Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Example
20mg
5mLmL A
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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.
Calculating the Amount to Administer
– Dimensional Analysis (cont.)
Example
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Calculating the Amount to
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
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C l l i h A
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Calculating the Amount to
Administer – Formula Method (cont.)
Procedure 12-4 (cont.):
3. Cancel the units.
4. Solve for the unknown.
12-49
C l l ti th A t t
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Calculating the Amount to
Administer – Formula Method (cont.)
The dosage ordered is Famvir
500 mg PO q 8 h
On and: Famvir 250 mg/tabs
Find the amount to administer.
H = 250 mg Q = tablet D = 250 mg
Step A Convert
No conversion necessary.
Example
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C l l ti th A t t
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Step B Calculate
2 x 1 tablet = A
2 tablets = A
A1tabletmg250500mg
Calculating the Amount to
Administer – Formula Method (cont.)
Example
AQxH
D
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C l l ti th A t t
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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.
Calculating the Amount to
Administer – Formula Method (cont.)
Example
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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.
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In Summary
In this chapter you learned to
determine information needed toperform dosage calculation.
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In Summary
In this chapter you learned to
utilize three methods of dosagecalculations.
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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
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Apply Your Knowledge
True or False
If a medication has 300 mg of drug per
tablet, the dose on hand is 300 mg.
Answer: True
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Apply Your Knowledge
The MAR reads Glucotrol 10 mg PO qd.
The desired dose is ____ mg.
Answer: 10 mg
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Apply Your Knowledge
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.
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End of Chapter 12
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