Calculation of Medications for Special Populations Based on Body Weight and Patient Age

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Chapter 10

Calculation of Medications for Special Populations Based on Body Weight and Patient Age

Objectives

• Calculate doses of medications for children and adults using body weight

• Calculate doses of medications for children using Clark’s rule

• Calculate doses of medication for special populations based on body surface area

• Calculate doses of medication for infants using Fried’s rule

• Calculate doses of medication for children using Young’s rule

Key Words

Pretest

Complete the following calculations for pediatric patients on the basis of age and body weight. The nomogram necessary for calculating some of these exercises can be found on p. 359. Show your calculations. Show your answers in tenths for strengths calculated for weights in the metric, apothecary or household system unless otherwise noted. Be sure your answer is a measurable dose for solids or volume measurements as appropriate unless otherwise noted.

image 1 The adult dose of amoxicillin is 500 mg tid.

image 2 The adult dose for Augmentin is 500 mg tid.

image 3 A child weighing 54 lb is to take phenobarbital. The physician orders 1 mg/kg tid.

image 4 An 8-month-old infant is ordered Demerol following surgery. The normal adult dose is meperidine 50 mg.

image 5 A child weighs 55 lb and has an order for acetaminophen on the basis of body weight. The adult dose is 325 mg.

image 6 A child has an order for digoxin based on 8 mcg per kilogram of body weight. The child weighs 35 lb.

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image 7 An 8-year-old child is to receive phenobarbital, and the adult dose is phenobarbital gr image.

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image 8 A physician orders Zantac syrup for a 10-month-old child. The adult dose is 150 mg bid.

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image image 9 A child weighs 66 lb and is of normal height for weight. The physician orders Benadryl elixir to be given based on body surface area. The adult dose is 50 mg q8h prn.

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image 10 A child 10 years old has bronchitis, and the physician orders Keflex suspension. The normal adult dose is 500 mg bid. Use age of the child and the normal adult dose as the basis for your answer.

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image 11 A child age 5 years 4 months is ordered Colace syrup. The adult dose is Colace 100 mg hs.

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image 12 A child weighs 88 lb, and the physician orders Compazine syrup 0.5 mg/kg tid.

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image 13 A child weighs 84 lb. The physician orders Biaxin oral suspension 10 mg/kg bid in divided doses.

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image 14 A child weighs 56 lb and is 40″ tall. He has been diagnosed as having epilepsy. The physician orders Dilantin suspension for this child to be based on BSA. The normal adult dose is Dilantin 300 mg per day in three divided doses.

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image 15 A 9-month-old child is to receive Dilantin according to the following label. The adult dose is Dilantin 100 mg.

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image 16 A child is 12 years old and is to receive Dilantin according to label A. The adult dose is Dilantin 200 mg qam.

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image 17 A child weighing 75 lb has otitis media. The physician orders Keflex to be given tid.

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image 18 A child weighing 45 lb has been prescribed Lorabid 15 mg/kg/day in divided doses bid.

Round to the whole number for the total dosage per day.

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image 19 A child weighing 33 lb has an order for Epivir bid. The adult dose is Epivir 150 mg bid.

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image 20 An 8-year-old child is being treated for a urinary tract infection. The usual adult dose for Furadantin is 100 mg.

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Introduction

Children are not small adults but rather distinct individuals who have different medication absorption, distribution, metabolism, and excretion rates compared with adults. The growth and development of the child, or the focus of the field of pediatrics, significantly affect the prescription for medication for the child. Pediatric patients include neonates or newborns—from birth to 1 month; infants—from 1 month to 1 year; early childhood—from age 1 to 5 years; late childhood—from age 6 to 12 years; and adolescence—from age 13 through 17 years or 20 years by some professionals. Geriatric persons also have differences in pharmacokinetics—drug absorption, metabolism, and excretion—thus requiring recalculation of drug dosages for many drugs. With toxic medications such as chemotherapeutics, even non-elderly adults with normal organ function may require use of body surface area (BSA) based on height and weight for drug calculations. Patients who have excessively low body weight or muscle mass (e.g., emaciated) or those who are obese may also require dose adjustments for some medications. Individuals with renal or liver dysfunction, whether adults or children, often also require dose adjustments of medications. All of these groups are considered special populations because of the need for special dosage calculations. These calculations may be based on height, weight, and overall physical condition for determining prescribed doses and dosages of a drug. Although as pharmacy technicians you will not be personally responsible for the calculation of the amount of medication to be given to these persons, you do need to understand how such determinations are made.

In these cases, the dose of the prescribed medication takes into account BSA which is based on patient height and weight relative to a typical adult. Remember that most usual dosages are provided in adult standards, so the dose for special populations must be altered to meet the patient’s needs according to age and/or body weight/height.

image Tech Alert

Remember that if the manufacturer provides a pediatric dose/dosage for a medication, the dose/dosage indicated by the manufacturer should be used. Under these conditions, the dose/dosage does not need to be calculated using BSA or any of the specific rules for estimating dosage. If a physician’s prescription does not agree with the manufacturer’s indication, the physician should be consulted by the pharmacist.

Calculating Medication Doses By Body Weight

Pediatric doses of medications may be calculated by using body weight or body mass. One method of calculation uses micrograms, milligrams, or grams of drug per kilogram of body weight. The formula therefore appears as mg (g or mcg)/kg. With this in mind, the child must be weighed with each visit to the hospital or physician’s office to be sure the correct amount of medication can be ordered. Because most physician’s office scales weigh in pounds, pounds must first be converted to kilograms before calculating the dose to be given.

To convert pounds to kilograms, 2.2 lb = 1 kg is the basis for the conversion. So to convert pounds to kilograms, the number of pounds is divided by 2.2 (or number of lb ÷ 2.2). For ratio and proportion, the formula is as follows for a child who weighs 22 lb:

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