Pediatric and Adult Dosage Calculation Based on Weight

CHAPTER 25


Pediatric and Adult Dosage Calculation Based on Weight



Objectives


After reviewing this chapter, you should be able to:


1. Convert body weight from pounds to kilograms


2. Convert body weight from kilograms to pounds


3. Calculate dosages based on milligram per kilogram


4. Determine whether a dosage is safe


5. Determine body surface area (BSA) using the West nomogram


6. Calculate BSA using formulas according to units of measure


7. Determine dosages using the BSA


8. Calculate the flow rates for pediatric IV therapy


9. Calculate the safe dosage ranges and determine if within normal range for medications administered IV in pediatrics


Accuracy in dosage calculation becomes even more of a priority when calculating and administering medications to infants and children. Snyderman (2008) reported that 1 in 15 hospitalized children is harmed as a result of medication errors. Snyderman stressed that the margin of error is much less in children. Cohen (2007) indicated that the rate of actual, potential, and preventable adverse drug events is three times higher for pediatric patients. Causes of errors included the following:



These are just a few reasons for dosing errors that occur in the administration of medications to children.


Before administering medications to children, the nurse should know whether the ordered dosage is safe. Accuracy is always important when calculating medication dosages. For infants and children, exact and careful mathematics takes on even greater importance. A miscalculation, even small discrepancies, may be dangerous because of the size, weight, and body surface area (BSA) of the infant or child. In addition, infants and children’s physiological capabilities (e.g., a lessened ability to metabolize medications, immaturity of systems, differences in rate of medication absorption and excretion) differ when compared with adults. Therefore it is vital that the nurse adhere to pediatric protocols and guidelines and always use a reference to verify medication orders to ensure that medication dosages are correct.


Body weight is an important factor used to calculate medication dosages for children and adults, although it is used more frequently with children. Medications may be prescribed based on body weight or body surface area.


The safe administration of medications to infants and children requires knowledge of the methods used in calculating doses. In addition, the nurse must apply the principles of the “six rights” of medication administration (right medication, right dosage, right time, right route, right client, right documentation) when working with pediatric clients and families. Nurses are also responsible for educating the families regarding medication administration.


Dosages for infants and children are based on their unique physiological differences. The prescriber must consider weight, height, body surface area, age, and condition of the child when ordering dosages. As mentioned previously, the two methods used for calculating safe pediatric dosages are body weight (e.g., mg/kg) and body surface area (BSA, measured in square meters [m2]). The body weight method is more common in pediatrics and is emphasized in this chapter, as well as the BSA. As stated previously, body weight and BSA methods are also used for adults, especially in critical care, and the calculation methods used are the same.


Although the prescriber is responsible for ordering the medication and dosage, the nurse remains responsible for verifying the dosage to be sure it is correct and safe for administration. If a dosage is higher than normal, it may be unsafe, and a dosage lower than normal may not have the desired therapeutic effect, which is also unsafe. It is imperative that nurses check medication labels or package inserts for specific dosage details. Other references that may be checked for more in-depth and additional information on a medication are drug formularies at the institution, the Physician’s Desk Reference (PDR), drug reference books, and the hospital pharmacist. Various pocket-size pediatric medication handbooks are also available; one that is widely used is The Harriet Lane Handbook (Johns Hopkins Hospital, 2005).


The two methods currently being used to calculate pediatric dosages are as follows:




CAUTION


To ensure safe practice, when in doubt consult a reliable source. Always double-check dosages by comparing the prescribed dose with the recommended safe dose.



Determining medication dosages according to body weight and BSA is a means of individualizing medication therapy. Although body weight and BSA are common determinants for medication dosing in children, they are also used to calculate adult medication dosages, particularly for those who are very old or grossly underweight. Body weight and BSA are also used in the administration of cancer medications. Of the two methods, the BSA method has been determined to be the most accurate for calculating dosages.



PRINCIPLES RELATING TO BASIC CALCULATIONS


Before calculating medications for the child or infant, some guidelines are helpful to know.




1. Calculation of pediatric dosages, as with adult dosages, involves the use of ratio and proportion, the formula method, or dimensional analysis to determine the amount of medication to administer.


2. Pediatric dosages are much smaller than those for an adult. Micrograms are used a great deal. The tuberculin syringe (1-mL capacity) is used to administer very small dosages.


3. Intramuscular (IM) dosages are usually not more than 1 mL for small children and older infants; however, this can vary with the size of the child. The recommended IM dosage for small infants is not more than 0.5 mL.


4. The recommended subcutaneous (subcut) dosage for children is not more than 0.5 mL.


5. Dosages that are less than 1 mL may be measured in tenths of a milliliter, or with a tuberculin syringe in hundredths of a milliliter.


6. Medications in pediatrics generally are not rounded off to the nearest tenth but may be administered with a tuberculin syringe (measured in hundredths) to ensure accuracy.


7. All answers must be labeled.



CALCULATION OF DOSAGES BASED ON BODY WEIGHT


Let’s begin our discussion with calculation of dosages according to milligrams per kilogram (mg/kg) of body weight. Before calculating dosages according to mg/kg of body weight, it is essential that you be able to convert a child’s weight. Most recommendations for medication dosages are based on weight in kilograms.


Therefore the most common conversion you will encounter involves the conversion from pounds to kilograms. To do this, remember the conversion 1 kg = 2.2 lb. Conversions of weights are presented in Chapter 9. The methods of converting presented in Chapter 8 can also be used. It is, however, essential that we review this again.



RULE


To convert from pounds to kilograms, use the conversion 1 kg = 2.2 lb. To convert from pounds to kilograms divide by 2.2, and express your answer to the nearest tenth.




CONVERTING POUNDS TO KILOGRAMS


Let’s do some sample problems with the conversion of weights.








CONVERTING KILOGRAMS TO POUNDS



RULE


To convert from kilograms to pounds, use the conversion 1 kg = 2.2 lb. To convert from kilograms to pounds multiply by 2.2, and express your weight to the nearest tenth.








Example 3:

Convert the weight of a 10.2-kg child to pounds.


2.2 lb=1 kg               (conversion factor)


image


imageSolution Using Ratio and Proportion



2.2 lb:1 kg=x lb:10.2 kgx=10.2×2.2x=22.44 lb=22.4 lb (rounded to the nearest tenth)Child’s weight=22.4 lb


image



imageSolution Using Dimensional Analysis



image



image Practice Problems


Convert the following weights in kilograms to pounds. Round to the nearest tenth.





Answers on p. 680



NOTE


Decimal movement may be preferred for converting grams to kilograms; however, the other methods presented in Chapter 8 can also be used.



Infants 0 to 4 weeks old (neonates) and premature infants may also be given medications. The scale will convert the child’s weight in grams, or the weight may be reported in grams, rather than kilograms. Therefore it may be necessary for the nurse to convert the weight in grams to kilograms because most dosage recommendations are commonly given in kilograms.


As discussed in the chapter on metric conversions, 1 kg = 1,000 g; therefore to convert grams to kilograms, divide by 1,000 or move the decimal point three places to the left.



RULE


To convert grams to kilograms use the conversion 1 kg = 1,000 g. Divide the number of grams by 1,000, or move the decimal point three places to the left. Round kilograms to the nearest tenth.




CONVERTING GRAMS TO KILOGRAMS








Remember






Medication dosages can be calculated based on mg/kg/day, mg/lb/day, or sometimes mcg/kg. References often state the safe amount of the drug in mg/kg/day (24-hour period). Once you have determined the child’s weight in kilograms, you are ready to calculate the medication dosage. Calculating the dosage involves three steps:



Before beginning to calculate dosages based on weight, let’s review some concepts that may be used.




Recommended dosage (also referred to as the safe dosage)—This information comes from a reputable resource, such as a medication reference written especially for pediatrics or another medication reference book. Recommended dosages may also be indicated on the medication label under children’s dosages. This is usually expressed in mg/kg for a 24-hour period to be given in one or more divided doses. This may also be seen as mcg/kg and occasionally mg/lb. Recommended dosage can also be stated as a range and referred to as safe dosage range (SDR). This is the upper and lower limits of the dosage as stated by an approved medication reference.


Total daily dosage—Dosage obtained by multiplying the child’s weight after it is converted to kilograms with the use of a reputable medication reference: multiply child’s weight in kilograms by dosage expressed as mg/kg.


Divided dosage—This represents the dosage a child should receive each time the medication is administered. The recommended daily dosage may be stated as mg/kg/day to be divided into a certain number of individual dosages, such as “three divided dosages,” q6h, and so on. “Three divided dosages” means that the total daily dosage is divided equally and administered three times per day. q6h means the total daily dosage is divided equally and administered every 6 hours, for a total of four dosages per day (24 hr ÷ 6 hr). This is the dosage for 24 hours divided by the frequency, or the number of times, the child will receive the medication.



NOTE


The dosage per kilogram may be mg/kg, mcg/kg, and so on.



Deciding if the dosage is safe—This is done by comparing the ordered dosage with the recommended dosage. In other words, this is decided by comparing and evaluating the 24-hour ordered amount with the recommended dosage.




Example 1:

Refer to Dilantin label.


Available:


image

Order: Dilantin 30 mg p.o. q8h. Child weighs 18 kg. Is the dosage safe?


Recommended dosage: 5 mg/kg/day in two or three equally divided dosages. (Notice the information written sideways on the left of the Dilantin label.)


Now that we have the dosage information and the child’s weight, we can calculate the safe total daily dosage for the child. Note: The child’s weight is in kilograms (18 kg), and the average dose range is 5 mg/kg/day. No conversion of weight is required.



Step 1:

Start by calculating the safe total daily dosage for this child. Multiply the recommended dosage in milligrams by the child’s weight in kilograms.


image


The safe dosage for this child (total) is 90 mg/day.



NOTE


Per day could be stated as daily or every 24 hours.



The calculation of the safe daily dosage could also be done by setting up a ratio and proportion using the format of fractions or colons. Using this example, the setup as a ratio and proportion might be as follows:



imageSolution Using Ratio and Proportion


5 mg:1 kg=x mg:18 kgx=90 mg/dayor5 mgx mg=1 kg18 kgx=90 mg/day


image


Calculation of the safe daily dosage could also be done using dimensional analysis. To do this, use the information regarding the recommended dosage range as the starting fraction (in this case, 5 mg/kg).



imageSolution Using Dimensional Analysis



image






Example 2:

Order: Gentamicin 50 mg IVPB q8h for a child weighing 40 lb. The recommended dosage for a child is 6 to 7.5 mg/kg/day divided q8h. Is the dosage ordered safe?




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Feb 11, 2017 | Posted by in PHARMACY | Comments Off on Pediatric and Adult Dosage Calculation Based on Weight

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