PHARMACY TECHNICIAN CHAPTER FOURTEEN

Dosage Calculation Determine which information presented is actually applicable to the question Always use the higher dosage amount when performing dosage calculations on prescriptions that have a range for the dose This will provide the most conservative solution and ensure the most accurate potential days supply 2

(Courtesy of Eli Lilly and Company.) Drug label for Prozac®. (Courtesy of Eli Lilly and Company.) 3

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Drug label for EryPed Drops®. (Reproduced with permission of Abbott Laboratories.) 5

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Quantity to Disperse Determine which information is necessary Know the common SIG codes to perform dosage calculations 7

SIG Code Refresher qd = every day qod = every other day bid = twice a day tid = three times a day qid = four times a day q4h = every 4 hours, or 6 times a day q6h = every 6 hours, or 4 times a day q8h= every 8 hours, or 3 times a day q12h = every 12 hours, or twice a day q4–6h = every 4 to 6 hours, or 4 to 6 times a day prn = as needed 8

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Drug label for diazepam. (© Copyright Boehringer Ingelheim Roxane, Inc. and/or affiliated companies 2009. All Rights Reserved.) 10

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Active Ingredient Calculations Typically calculated when preparing compounded preparations Determine which information is necessary Use ratio/proportion equation with the dose and strength Units of measurement (mL, etc.) must be the same 13

Drug label for Codeine Phosphate Oral Solution. (© Copyright Boehringer Ingelheim Roxane, Inc. and/or affiliated companies 2009. All Rights Reserved.) 14

Drug label for Fentanyl®. (Courtesy of Baxter Health Care Corporation.) 15

Drug label for dexamethasone oral solution. (© Copyright Boehringer Ingelheim Roxane, Inc. and/or affiliated companies 2009. All Rights Reserved.) 16

Calculating the Correct Days Supply The information required to solve this equation is: Quantity dispensed Dose dispensed Dose frequency The days supply is equal to the total dose, divided by the product of the dose and the dose frequency 17

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(Courtesy of Eli Lilly and Company.) Drug label for Ceclor®. (Courtesy of Eli Lilly and Company.) 19

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Multiple Dosage Calculations What is the appropriate quantity to dispense? How many total doses are to be dispensed? What is the total amount of amoxicillin, in mg, to be dispensed? How many days should this prescription last? 23

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Multiple Dosage Calculations (cont.) What is the appropriate quantity to dispense? How many total doses are to be dispensed? What is the total amount of dexamethasone, in mg, to be dispensed? How many days should this prescription last? 25

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Multiple Dosage Calculations (cont.) What is the appropriate quantity to dispense? How many total doses are to be dispensed? What is the total amount of albuterol, in mg, per inhalation? How many days should this prescription last? 27

Figure 14.28 28

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Multiple Dosage Calculations (cont.) What is the appropriate quantity to dispense? How many total doses are to be dispensed? What is the total amount of methylprednisolone, in mg, to be dispensed in the pak? How many days should this prescription last? What is the maximum number of doses available if 60 tablets are dispensed? 30

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Multiple Dosage Calculations (cont.) What is the appropriate quantity to dispense? What is the maximum number of doses available if 60 tablets are dispensed? What is the maximum amount of alprazolam, in mg, to be taken daily? How many days could this prescription last, if 60 tablets are dispensed? 32

Dosages for Pediatric Patients Three primary systems for determining accurate pediatric doses: Fried’s Rule—calculates pediatric dosages based on the child's age in months Young’s Rule—uses a formula based on the child’s age expressed in years (rather than months) Clark’s Rule—based on using the patient's weight, expressed in pounds 33

Fried’s Rule An infant, 15 months old and weighing 20 pounds, needs streptomycin sulfate, which is usually administered to adults as 1 gm (1,000 mg) in a daily IM injection. What is the appropriate dosage for the infant? To calculate the pediatric dosage based on a child’s age in months, simply use the formula for Fried’s Rule. 34

Fried’s Rule (cont.) Using the information provided, set up the calculation as follows: Pediatric dose Pediatric dose = 0.1 x 1,000 Pediatric dose = 100 mg So, according to Fried’s Rule, the pediatric dosage appropriate for a 15-month-old would be 100 mg. 35

Young’s Rule Let’s reexamine Example 14.9 in light of Young’s Rule, which uses the child’s age in years. The age of a 15-month-old could be expressed as 1.25 years, since the child has lived for 12 months (1 year) + 3 months (1/4 or 0.25 of a year). 36

Young’s Rule Using an age of 1.25 years and the information provided in Example 14.9, set up the calculation, using Young’s Rule, as follows: Pediatric dose Pediatric dose = 0.094 x 1,000 Pediatric dose = 94 mg So, according to Young’s Rule, the pediatric dosage appropriate for a 15-month-old would be 94 mg. 37

Clark’s Rule (cont.) Let’s reexamine Example 14.9 in light of Clark’s Rule, which uses the child’s weight in pounds. Using a weight of 20 pounds and the information provided in Example 14.9, set up the calculation using Clark’s Rule as follows: Pediatric dose Pediatric dose = 0.133 x 1,000 Pediatric dose = 133 mg So, according to Clark’s Rule, the pediatric dosage appropriate for a 15-month-old who weights 20 pounds would be 133 mg. 38

Weight Conversions Pounds are converted to kilograms by dividing by 2.2 Example: If the infant weighs 20 pounds, what is her weight in kg? 20 ÷ 2.2 = 9.09 The infant weighs 9.09 kg Kilograms are converted to pounds by multiplying by 2.2 Example: If the infant weighs 8 Kg, what is his weight in pounds? 8 x 2.2 = 17.6 The infant weighs 17.6 pounds 39

Mg/Kg/Day Most precise system of dosage Based on the number of milligrams suggested per kilogram per day 40

Practice A dose of 4mg/kg/day of Plaquenil® can be recommended for certain children suffering from lupus. What would be the appropriate dosage for a patient weighing 47 kg? The daily dose of Omnicef®, for children, is 14 mg/kg, up to a maximum dose of 600 mg/day. What is the appropriate daily dosage for a patient who weighs 98 pounds? 41

Practice (cont.) The recommended dosage of fluconazole is 6 mg/kg on day one, followed by 3 mg/kg/day for oropharyngeal candidiasis. What are the appropriate dosages for a child weighing 30 kg? A child who weighs 76 pounds is prescribed the antibiotic cefaclor. It is recommended that children receive 20 mg/kg/day in divided doses every 8 hours. How many mg should the child take per dose? 42