Learning Outcomes
52.1 Explain the role of the medical assistant to ensure safe dosage calculations.
52.2 Identify systems of measurements and their common uses.
52.3 Convert among systems of measurements.

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52Dosage Calculation52.1 Explain the role of the medical assistant to ensure safe dosage calculations.52.2 Identify systems of measurements and their common uses. 52.3 Convert among systems of measurements.Learning Outcomes52.4 Execute dosage calculations accurately.52.5 Calculate dosages based upon body weight and body surface area.Learning OutcomesIntroductionAdminister medicationsClose attention to detailDosage calculation must be accurate Basic mathSystems of measurementConversions within or between measurement systemsCalculations for special patient populationsEnsuring Safe Dosage CalculationsMinor math errors major error in dosageEach time you calculate a dosageAnalyze the problemEstimate a range Is the answer reasonable?Measurement SystemsThree systemsMetric ~ most commonApothecary Household Focus on the basic units of Volume Weight Metric SystemBased on multiples of 10Basic units Volume = liter (L)Weight = gram (g)Prefixes indicate multiples of eachMetric SystemPrefixKilo-Base unitCenti-Milli-Micro-Value× 1000--÷ 100 ÷ 1000 ÷ 1,000,000Weightkilogram (kg)1000 ggram (g)1 gcentigram (cg)0.01 gmilligram (mg)0.001 gmicrogram (mcg)0.000001 gVolumekiloliter (kL)1000 Lliter (L)1 Lcentiliter (cL)0.01 Lmilliliter (mL)0.001 Lmicroliter (mcL)0.000001 LApothecary and Household SystemsApothecary Volume Fluid ouncesFluid dramsPint quartWeight DramsOuncesPounds Apothecary and Household SystemsOuncesCups Pints, quarts, and gallons Household – only volumeDrops Teaspoons and tablespoonsApothecary and Household SystemsApothecary UnitsEquivalentMeasures of Volume8 fluid drams (fl dr) 1 fluid ounce (fl oz)16 fl oz1 pint (pt)2 pt 1 quart (qt)4 qt 1 gallon (gal)Measures of Weight60 gr1 dram (dr)8 dr 1 ounce (oz)16 oz 1 pound (lb)Apothecary and Household SystemsHousehold UnitsEquivalentMeasures of Volume60 drops* (gtt) = 1 teaspoon (tsp)3 tsp = 1 tablespoon (tbsp)6 tsp = 1 ounce (oz) or 2 tbsp8 oz = 1 cup (c)2 c = 1 pint (pt)4 c = 1 quart (qt) or 2 ptApply Your KnowledgeMatch the following: units of measurements are the same as in the household system drops multiples of ten only uses units of volume fluid dram microliterMetric ApothecaryHousehold ACCBBAANSWER:Conversions Within and Between Measurement SystemsRules for converting within the Metric SystemMove the decimal point to the right when you convert from a larger to a smaller unit. This is dividing.Move the decimal point to the left when you convert from a smaller to a larger unit. This is multiplying.Converting Within the Metric SystemDetermine the direction and number of places to move the decimal pointA milliliter is threedecimal places to the right of literA kilogram is threedecimal places to the left of a gramConverting Within the Metric System (cont.)LEFT3Converting Between SystemsBe familiar with equivalent measuresConversion charts – check carefully before administering a drug1 teaspoon = 5 mL1 tablespoon = 15 mL1 pint = 480 mL1 kg = 2.2 lbs1 fl oz = 30 mL1 fl oz = 2 tbspConverting Between Systems (cont.)Converting Between Systems (cont.)Apply Your KnowledgeWhich measuring system is most commonly used?ANSWER: The metric system.Convert 25 grams to milligrams.ANSWER:Add a decimal point to the measurement: 25. gAdd 3 zeros so you can move the decimal point three places to the right: 25.000 gMove the decimal point to the right three places: 25,000Change the unit: 25,000 mgRight!Dosage CalculationsMust be accurateDouble checkHave coworker checkUse Proportion methodFormula methodProportion Method for Dosage CalculationsProportion Method for Dosage Calculations (cont.)Proportion Method for Dosage Calculations (cont.)Formula Method for Dosage CalculationsFormula MethodDosage Calculations (cont.)Prevent errorsDuring calculationsWhen selecting medicationWhen reading the labelApply Your KnowledgeThe physician has ordered ampicillin 500 mg, on hand 250 mg capsules. How much would you give?You have 50 mg metoprolol as a scored tablet on hand and the doctor tells you to give 25 mg. How much would you give?2 capsules½ tabletSuper!Body Weight and Body Surface Area CalculationsMore common for pediatric or geriatric dosages Greater risk of harmCalculations must be preciseDosages Based on WeightOrder Dosage of medication / weight of patient Over a unit of timeWeight must be calculated in kilogramsDosages Based on Weight (cont.)Dosages Based on Weight (cont.)Dosages Based on Weight (cont.)Dosages Based on Weight (cont.)Calculate the amount to administer. Refer to label for medication on handDosages Based on Weight (cont.)Dosages Based on Weight (cont.)Dosages Based upon Body Surface AreaPrecise dosagesPediatric patientsBurn patientsChemotherapy patientsNomogram Plot height and weightThird scale will give the BSAApply Your KnowledgeA 5-year-old child weighs 44 lbs. The physician orders him to receive Zinacef 50 mg/kg/day IM q6h. What is the child’s weight in kg? How many milligrams of medication should the child receive in one dose?20 kg250 mg Very Good!In Summary52.1 A medical assistant must be proficient in math and determine if the answer to every calculation he performs is reasonable. 52.2 The metric system is based on 10 and is the most common system of measurement for dosage calculations. Metric units commonly used for dosage calculations include g, mg, mcg, and mL. The apothecary and household systems have some equal measures, but they are used rarely. In Summary (cont.)52.3 To convert among systems of measurements, you can refer to a conversion chart or perform a proportion method calculation. Keep in mind that measurements between the metric and apothecary and household systems are only approximationsIn Summary (cont.)52.4 Use the formula method or proportion method to perform dosage calculations. Formula Method D/H × Q Proportion Method D/X = H/Q 52.5 Dosages based on body weight and BSA are used when precise amounts of medication must be administered. Body weight calculations are usually ordered in mg/kg/day. BSA calculations use special formulas or a nomogramEnd of Chapter 52Math is like love -- a simple idea but it can get complicated.~ Unknown