Dosage Calculation

CHAPTER

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52Dosage Calculation



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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.

Learning Outcomes



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52.4 Execute dosage calculations accurately.

52.5 Calculate dosages based upon body weight and body surface area.

Learning Outcomes



52-4

Introduction

• Administer medications– Close attention to detail

– Dosage calculation must be accurate

• Basic math

• Systems of measurement

• Conversions within or between measurement systems

• Calculations for special patient populations



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Ensuring Safe Dosage Calculations

• Minor math errors major error in dosage

• Each time you calculate a dosage– Analyze the problem

– Estimate a range

– Is the answer reasonable?



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Measurement Systems

• Three systems– Metric ~ most common– Apothecary – Household

• Focus on the basic units of – Volume – Weight



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Metric System

• Based on multiples of 10

• Basic units – Volume = liter (L)– Weight = gram (g)

• Prefixes indicate multiples of each



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Metric System

Prefix Kilo- Base unit

Centi- Milli- Micro-

Value × 1000 -- ÷ 100 ÷ 1000 ÷ 1,000,000

Weight kilogram

(kg)

1000 g

gram

(g)

1 g

centigram

(cg)

0.01 g

milligram

(mg)

0.001 g

microgram

(mcg)

0.000001 g

Volume kiloliter

(kL)

1000 L

liter

(L)

1 L

centiliter

(cL)

0.01 L

milliliter

(mL)

0.001 L

microliter

(mcL)

0.000001 L



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Apothecary and Household Systems

• Apothecary – Volume

• Fluid ounces

• Fluid drams

• Pint quart

– Weight • Drams

• Ounces

• Pounds



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– Ounces

– Cups

– Pints, quarts, and gallons

• Household – only volume– Drops

– Teaspoons and tablespoons



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Apothecary Units Equivalent

Measures of Volume

8 fluid drams (fl dr) 1 fluid ounce (fl oz)

16 fl oz 1 pint (pt)2 pt 1 quart (qt)4 qt 1 gallon (gal)

Measures of Weight60 gr 1 dram (dr)8 dr 1 ounce (oz)

16 oz 1 pound (lb)



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Household Units

Equivalent

Measures of Volume60 drops* (gtt) = 1 teaspoon (tsp)

3 tsp = 1 tablespoon (tbsp)

6 tsp = 1 ounce (oz) or 2 tbsp

8 oz = 1 cup (c)2 c = 1 pint (pt)4 c = 1 quart (qt) or 2 pt



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Apply Your Knowledge

Match the following:

units of measurements are the same as in the household system

drops

multiples of ten

only uses units of volume

fluid dram

microliter

A. Metric

B. Apothecary

C. Household

A

C

C

B

B

A

ANSWER:



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Conversions Within and Between Measurement Systems

• Rules for converting within the Metric System1. Move the decimal point to the right when you

convert from a larger to a smaller unit.

This is dividing.

2. Move the decimal point to the left when you convert from a smaller to a larger unit.

This is multiplying.



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Converting Within the Metric System

• Determine the direction and number of places to move the decimal point

– A milliliter is threedecimal places to the right of liter

– A kilogram is threedecimal places to the left of a gram



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Converting Within the Metric System (cont.)

LEFT

3



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Converting Between Systems

• Be familiar with equivalent measures

• Conversion charts – check carefully before administering a drug

1 teaspoon = 5 mL 1 tablespoon = 15 mL

1 pint = 480 mL 1 kg = 2.2 lbs

1 fl oz = 30 mL 1 fl oz = 2 tbsp



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Converting Between Systems (cont.)



52-19

Converting Between Systems (cont.)



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1.Which measuring system is most commonly used?

ANSWER: The metric system.

2. Convert 25 grams to milligrams.

ANSWER:1. Add a decimal point to the measurement: 25. g2. Add 3 zeros so you can move the decimal point

three places to the right: 25.000 g3. Move the decimal point to the right three places:

25,0004. Change the unit: 25,000 mg



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Dosage Calculations

• Must be accurate– Double check– Have coworker check

• Use – Proportion method– Formula method



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Proportion Method for Dosage Calculations



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Proportion Method for Dosage Calculations (cont.)



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Proportion Method for Dosage Calculations (cont.)



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Formula Method for Dosage Calculations



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Formula Method



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Dosage Calculations (cont.)

• Prevent errors– During calculations

– When selecting medication

– When reading the label



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1. The physician has ordered ampicillin 500 mg, on hand 250 mg capsules. How much would you give?

2. 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 capsules

½ tablet



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Body Weight and Body Surface Area Calculations

• More common for pediatric or geriatric dosages – Greater risk of harm

– Calculations must be precise



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Dosages Based on Weight

• Order – Dosage of medication / weight of patient – Over a unit of time

• Weight must be calculated in kilograms



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Dosages Based on Weight (cont.)



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4. Calculate the amount to administer. Refer to label for medication on hand



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Dosages Based upon Body Surface Area

• Precise dosages– Pediatric patients– Burn patients– Chemotherapy patients

• Nomogram – Plot height and weight– Third scale will give

the BSA



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A 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 kg

250 mg



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In Summary

52.1 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.



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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 approximations



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In 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 nomogram



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End of Chapter 52

Math is like love -- a simple idea but it can get complicated.

~ Unknown

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Dosage Calculation