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Chapter 9
Calculating Doses of
Nonparenteral Medications
Nonparenteral Medications
Calculating Solid Doses
• Three methods used
– ratio and proportion
– formula method
– dimensional analysis
• Two parts
– dose ordered (DO)
– dosage available (DA)
Calculating Doses
• Identify available unit to be used for desired
dose
– tablets, capsules, mL, ounces
• Ensure all units are alike
– conversions must be done prior to calculation
of the dose
• Verify dose calculation with medication order
Ratio and Proportion Terminology
Proportional Method
• First set of comparisons
– dosage available (DA)
– located right side
• Other side of equation
– dose ordered (DO)
– located left side
• Dosage available (DA) : Dosage form = Dose
ordered (DO) : Dose to be given
• Always perform dose calculations in writing
Proportional Method
• Ordered: Glucophage 850 mg qd po with meal
• Dosage available: Glucophage 850 mg tablets
• Are the units in the same measurement?
Proportional Method
• Ordered: Glucophage 850 mg qd po with meal
l
• Dosage available: Glucophage 850 mg tablets
• Are the units in the same measurement?
Yes
Proportional Method
• Ordered: Glucophage 850 mg qd po with meal
• Dosage available: Glucophage 850 mg tablets
• Are the units in the same measurement?
Yes
• 850 mg (DA) : 1 tablet (form) :: 850 mg (DO) :
# tablet(s) (dose to be given)
Proportional Method
• 850 mg (DA) : 1 tablet (form) :: 850 mg (DO) :
# tablet(s) (dose to be given)
• x stands for dose to be given
• 850 is to x as 850 is to 1
• 850 : 1 :: 850 : x
• 850x = 850
• x=1
• Answer: one 850 mg tablet daily
Proportional Method
• Ordered: phenobarbital gr 1/2 po tid
• Dosage available (DA): phenobarbital gr
1/4 tablet
• Are the units in the same measurement?
Proportional Method
• Ordered: phenobarbital gr 1/2 po tid
• Dosage available (DA): phenobarbital gr
1/4 tablet
• Are the units in the same measurement?
Yes
Proportional Method
• Ordered: phenobarbital gr 1/2 po tid
• Dosage available (DA): phenobarbital gr
1/4 tablet
• Are the units in the same measurement?
Yes
• gr 1/4 (DA) : 1 tablet (dosage form) :: gr 1/2
(DO) : x # tablet(s) (dose to be given)
Proportional Method
• gr 1/4 (DA) : 1 tablet (dosage form) :: gr 1/2
(DO) : x # tablet(s) (dose to be given)
• 1/4 : 1 :: 1/2 : x
• 1/4 is to x as 1/2 is to 1
• 1/4 x = 1/2
Invert fraction before multiplying
• x = 1/2 x 4/1
• x=2
• Desired dose is two gr 1/4 tablets three times
daily
Proportional Method
• Ordered: phenobarbital gr iii po hs
• Dosage available (DA): phenobarbital 100 mg
scored tablet
• Are the units in the same measurement?
Proportional Method
• Ordered: phenobarbital gr iii po hs
• Dosage available (DA): phenobarbital 100 mg
scored tablet
• Are the units in the same measurement?
No
Proportional Method
• Ordered: phenobarbital gr iii po hs
• Dosage available (DA): phenobarbital 100 mg
scored tablet
• Are the units in the same measurement?
No
• What is the equivalency?
Proportional Method
• Ordered: phenobarbital gr iii po hs
• Dosage available (DA): phenobarbital 100 mg
scored tablet
• Are the units in the same measurement?
No
• What is the equivalency?
• 60 mg = 1 grain (gr i)
Proportional Method Conversion
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60 mg = 1 grain (gr i)
60 mg : 1 gr :: 100 mg : x
100 mg = 1½ grains gr iss
100 mg : gr iss :: x mg : gr iii
100 is to 1½ as x is to 3
1½ x = 300
Change 1½ to 3/2
100 mg : 3/2 = x : 3
3/2x = 300
x = 300 x 2/3
x = 200 mg
Proportional Method
• 100 mg (DA) : 1 tablet (form) :: 200 mg (DO) : x
(dose to be given)
• 100 : 1 :: 200 : x
• 100 is to x as 200 is to 1
• 100x = 200
• x=2
• Desired dose is two 100 mg tablets every day
at bedtime
Proportional Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: 5 mg scored tablets
• Are the units in the same measurements?
Proportional Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: 5 mg scored tablets
• Are the units in the same measurements?
Yes
Proportional Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: 5 mg scored tablets
• Are the units in the same measurements?
Yes
• 5 mg (DA) : 1 tablet (form) :: 10 mg (DO) : #
tablet(s) (dose to be given)
Proportional Method
• 5 mg (DA) : 1 tablet (form) :: 10 mg (DO) : #
tablet(s) (dose to be given)
• 5 is to 1 as 10 is to x
• 5x = 10
• x=2
• Desired dose is two 5 mg tablets, twice daily
Formula Method
• Dosage ordered (DO) / Dosage available
(DA) x Quantity or Form (Qty) = Dose to be
given
• DO / DA x Qty = Dose to be given
• Always perform dosage calculations in
writing
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Yes
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Yes
• What is the desired dose?
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Yes
• What is the desired dose?
• 850 mg tablet daily
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Yes
• What is the desired dose?
• 850 mg tablet daily
• DO (850 mg) / DA (850 mg) x Qty (1 tablet) =
1 tablet (dose to be given)
Formula Method
• Ordered: Glucophage 850 mg po qd with meal
• Available: Glucophage 850 mg tablets
• Are the units in the same measurements?
Yes
• What is the desired dose?
• 850 mg tablet daily
• DO (850 mg) / DA (850 mg) x Qty (1 tablet) =
1 tablet (dose to be given)
• Answer: One 850 mg tablet
Formula Method
• Ordered: phenobarbital gr iii po qd hs
• Available: phenobarbital 100 mg scored tablets
• Are the units in the same measurement?
Formula Method
• Ordered: phenobarbital gr iii po qd hs
• Available: phenobarbital 100 mg scored tablets
• Are the units in the same measurement?
No
Formula Method
• Ordered: phenobarbital gr iii po qd hs
• Available: phenobarbital 100 mg scored
tablets
• Are the units in the same measurement?
No
• What is the equivalency for conversion?
Formula Method
• Ordered: phenobarbital gr iii po qd hs
• Available: phenobarbital 100 mg scored
tablets
• Are the units in the same measurement?
No
• What is the equivalency for conversion?
• gr I = 60 mg
Formula Method Conversion
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60 mg = 1 grain (gr i)
60 mg : 1 gr :: 100 mg : x
100 mg = 1½ grains (gr iss)
100 mg : gr iss :: x mg : gr iii
100 is to 1½ as x is to 3
1½ x = 300
Change 1½ to 3/2
100 mg : 3/2 = x : 3
3/2x = 300
x = 300 x 2/3
x = 200 mg
Formula Method
• 200 mg (DO) / 100 mg
(DA) x 1 tablet (Qty) =
2 tablets (Dose to be
given)
• Answer: Two 100 mg
tablets
Formula Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: oxybutynin HCl 5 mg scored tablets
• Are the units in the same measurement?
Formula Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: oxybutynin HCl 5 mg scored tablets
• Are the units in the same measurement?
Yes
Formula Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: oxybutynin HCl 5 mg scored tablets
• Are the units in the same measurement?
Yes
• 10 mg (DO) / 5 mg (DA) x 1 tablet (Qty)
Formula Method
• Ordered: oxybutynin HCl 10 mg po bid
• Available: oxybutynin HCl 5 mg scored tablets
• Are the units in the same measurement?
Yes
• 10 mg (DO) / 5 mg (DA) x 1 tablet (Qty)
• Answer: Two 5 mg tablets
Calculating Liquid Doses
• Must convert to household system—patients
more familiar with household than metric
• Two numbers for drug dose calculation
– amount of active ingredients (usually mg)
– amount of liquid mixed with drug (usually mL)
– mg/mL
Reconstitution
• Must reconstitute some medications from
powder to liquid form prior to administering
• Label gives total volume in bottle, medication
strength
Figure 9-1. Label for Zithromax.
Courtesy of Pfizer Labs, New York.
Reconstitution
Different dosage strengths created with
different amounts of water
Always document strength, date, time of
reconstitution on medication label, and initial
Two Active Ingredients
• Protuss
• Each 5 mL (1 teaspoon) contains:
– hydrocodone bitartrate
– potassium guaiacolsulfonate
5 mg
300 mg
• Remember: 5 mL = 1 tsp = 1 fluid dram (f )
Liquid Medications
• Ordered: Protuss f iss po prn
• Available: Protuss, 5 mg hydrocodone
bitartrate, and 300 mg potassium
guaiacolsulfonate in each 5 mL.
• Are the units in the same measurement?
Liquid Medications
• Ordered: Protuss f iss po prn
• Available: Protuss, 5 mg hydrocodone
bitartrate, and 300 mg potassium
guaiacolsulfonate in each 5 mL.
• Are the units in the same measurement?
No
Liquid Medications Conversion
• 1 fluid dram (f ) = 1 teaspoon (tsp) = 5 mL
• f (DA) : 5 ml (form) :: f iss (DO) : x mL
(dose to be given)
• 1 : 5 :: 1½ : x
• 1 is to 5 as 1½ is to x
• 5 x 3/2 = x
• x = 7.5 mL
• Desired dose is 7.5 mL = 1½ tsp
Liquid Medications
• Ordered: Augmentin 400 mg po q 12 hr
• Available: Augmentin 400 mg/5 mL oral
suspension
• Are the units in the same measurement?
Liquid Medications
• Ordered: Augmentin 400 mg po q 12 hr
• Available: Augmentin 400 mg/5 mL oral
suspension
• Are the units in the same measurement?
Yes
Liquid Medications
• 400 mg (DA) : 5 mL (form) :: 400 mg (DO) : x
mL (dose to be given)
• 400 : 5 :: 400 : x
• 400 is to 5 as 400 is to x
• 400x = 2000
• x = 5 mL
• Desired dose is 5 mL = 1 tsp
Liquid Medications
• Ordered: cetirizine HCl 10 mg po
• Available: cetirizine HCl 1 mg/1 mL
• Are the units in the same measurement?
Liquid Medications
• Ordered: cetirizine HCl 10 mg po
• Available: cetirizine HCl 1 mg/1 mL
• Are the units in the same measurement?
Yes
Liquid Medications
• Ordered: cetirizine HCl 10 mg po
• Available: cetirizine HCl 1 mg/1 mL
• Are the units in the same measurement?
Yes
• 1 mg (DA) : 1 mL (form) :: 10 mg (DO) : x mL
(dose to be given)
• 1 : 1 :: 10 : x
• 1 is to 1 as 10 is to x
• x = 10
• Answer: desired dose is 10 mL = 2 tsp
Formula Method
• Ordered: Protuss f iss po qid prn
• Available: Protuss 5 mg hydrocodone
bitartrate and 300 mg potassium
guaiacolsulfonate in each 5 mL
• Are the units in the same measurement?
Formula Method
• Ordered: Protuss f iss po qid prn
• Available: Protuss 5 mg hydrocodone
bitartrate and 300 mg potassium
guaiacolsulfonate in each 5 mL
• Are the units in the same measurement?
No
Formula Method
• 1 dram = 5 mL
• i / 5 mL = iss / x mL
• Cross multiply 1x = 1 1/2 x 5
• x = 7.5 mL
Formula Method
• 5 mL/1 tsp = 7.5 mL / x
tsp
• Cross multiply
• 1 1/2 tsp
• 7.5 mL = 1 1/2 tsp
• Answer: Administer 1
1/2 tsp up to four times
a day as needed
Formula Method
• Ordered: Augmentin susp 400 mg po q12hr
• Available: 400 mg/5 mL
• Are the units in the same measurement?
Formula Method
• Ordered: Augmentin susp 400 mg po q12hr
• Available: 400 mg/5 mL
• Are the units in the same measurement?
Yes
Formula Method
• Ordered: Augmentin susp 400 mg po q12hr
• Available: 400 mg/5 mL
• Are the units in the same measurement?
Yes
• 400 mg (DO) / 400 mg (DA) x 5 mL (Qty) = 5
mL (Dose to be given)
• 1 tsp every 12 hours
Formula Method
• Ordered: cetirizine HCl syr 10 mg po qd
• Available: cetirizine HCl syrup 1 mg/1 mL
• Are the units in the same measurement?
Formula Method
• Ordered: cetirizine HCl syr 10 mg po qd
• Available: cetirizine HCl syrup 1 mg/1 mL
• Are the units in the same measurement?
Yes
Formula Method
• Ordered: cetirizine HCl syr 10 mg po qd
• Available: cetirizine HCl syrup 1 mg/1 mL
• Are the units in the same measurement?
Yes
• 10 mg (DO) / 1 mg (DA) x 1 mL = 10 mL
• 5 mL/ 1 tsp = 10 mL/ x tsp
• 2 tsp daily dose to be given
Measuring Liquid Medications
• Pour at eye level
• Read bottom of meniscus
Meniscus
Figure 9-3. The meniscus
Liquid Measurement Instruments
• Oral dispenser (A)
• Oral syringe (B)
• Liquid dropper (C)
Figure 9-4.
Body Surface Area (BSA)
• Calculate precise pediatric and geriatric
dosages
• Based on weight and height using nomogram
• Nomogram—set up as graph; shows
relationships among numerical values
– height—left side, cm and inches
– weight—right side, lb and kg
West BSA Nomogram
Figure 9-5.
From Behrman RE et al
(2000). Nelson textbook
of pediatrics (16th ed.).
Philadelphia: Saunders.
Calculating BSA
• Determine BSA
• BSA of adult weighing 140 lb
is 1.7
• BSA of patient in m2 / 1.7 x
adult dose = desired dose
Calculating BSA
Child weighs 55 lbs and is 85 cm.
• Find and mark height in first column
• Find and mark weight in second column
• Use ruler to align marks
• Read BSA
• 0.82 m2
Calculating BSA
Child weighs 70 lbs and is 45 in. tall. Physician
orders albuterol based on the BSA. Normal
adult dose is 4 mg.
Formula: BSA in m2 x adult dose = desired dose
1.7
• BSA = 1.2 m2
• 1.2 m2 / 1.7 m2 x 4 mg = 2.8 mg (3 mg)
Calculating BSA
• Albuterol syrup
available in 2 mg/5 mL
• 3 mg (DO) / 2 mg (DA)
x 5 mL = 7.5 mL
• Answer: administer
7.5 ml or 1 1/2 tsp of
albuterol