Medical assisting Administrative and clinical procedures (5e) Chapter 52 Dosage calculation

After completing this unit, you should be able to: Explain the role of the medical assistant to ensure safe dosage calculations; identify systems of measurements and their common uses; convert among systems of measurements;...

Dosage Calculation

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

52.4 Execute dosage calculations

accurately.

52.5 Calculate dosages based upon body

weight and body surface area.

Learning Outcomes

• Administer medications

– Close attention to detail

– Dosage calculation must be accurate

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?

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

Apothecary and Household Systems

Apothecary and Household Systems

– Ounces – Cups

– Pints, quarts, and gallons

• Household – only volume

– Drops

– Teaspoons and

tablespoons

Apothecary and Household Systems

Apothecary Units Equivalent

Apothecary and Household Systems

Apply Your Knowledge

Match the following:

units of measurements are the

same as in the household system

Measurement Systems

• Rules for converting within the Metric

System

1 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

Converting Within the Metric System

• Determine the direction and number of

places to move the decimal point

Converting Within the Metric System (cont.)

LEFT 3

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

Converting Between Systems (cont.)

Converting Between Systems (cont.)

Apply Your Knowledge

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 g

2 Add 3 zeros so you can move the decimal point

three places to the right: 25.000 g

3 Move the decimal point to the right three places:

25,000

Calculations

Calculations (cont.)

Calculations (cont.)

Formula Method for Dosage Calculations

Formula Method

Dosage Calculations (cont.)

• Prevent errors

– During calculations

– When selecting medication

– When reading the label

Apply Your Knowledge

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

Area Calculations

• More common for pediatric or geriatric

dosages

– Greater risk of harm

– Calculations must be precise

Dosages Based on Weight

• Order

– Dosage of medication / weight of patient

– Over a unit of time

• Weight must be calculated in kilograms

Dosages Based on Weight (cont.)

Dosages Based on Weight (cont.)

Dosages Based on Weight (cont.)

Dosages Based on Weight (cont.)

4 Calculate the amount to administer Refer to

label for medication on hand

Dosages Based on Weight (cont.)

Dosages Based on Weight (cont.)

Dosages Based upon Body Surface Area

– Plot height and weight

– Third scale will give

the BSA

Apply Your Knowledge

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

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

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

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