Calculating Doses of Nonparenteral Medications 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 Other side of equation 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 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 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 5 mg potassium guaiacolsulfonate 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 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 Formula: BSA = 1.2 m2 1.2 m2 / 1.7 m2 x 4 mg = 2.8 mg (3 mg) Child weighs 70 lbs and is 45 in. tall. Physician orders albuterol based on the BSA. Normal adult dose is 4 mg. BSA in m2 x adult dose = desired dose  1.7

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