Chapter 9

Calculating Drug Dosages

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Learning Objectives

Use formulas to determine the dosages of tablets, capsules, or liquids

Use formulas to determine the total number of tablets or capsules or the amount of liquid to be ordered for a specified time

Use information about the apothecaries', metric, and household measurements systems to accurately calculate drug dosages

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Learning Objectives (cont.)

Calculate dosages for parenteral injections, including those for special preparations such as insulin

Calculate flow rates for infusions

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Calculating Medication Dosages

Three Steps1. Verify that the drug available is the same

measurement system as the drug

dosage desired (convert if needed)

2. Reduce to lowest terms

3. Calculate the dosage quantity to be

administered

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Drug Calculation Methods

Fraction Method 600 mg = 200 mg

x tablets 1 tablet

Solve for x Ratios or Proportion Method

600 mg : x tablets :: 200 mg : 1 tablet Solve for x

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Drug Calculation Methods (cont.)

Desired over Available Method

Desired units (conversion factor) ×

Quantity of drug form = Quantity to give

Quantity available (× conversion factor)

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Forms of Oral Medications

Capsules Cannot be broken or divided If amount to be given is more than 0.5, round to next

whole number Tablets

Only divide if scored Coated tablets are not to be broken

Liquids May be measured in a medication cup, syringe, or

calibrated dropper

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Scored and Unscored Tablets

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Parenteral Medications

Medication available in three forms: Prefilled syringe labeled with specific dosage

• For example: meperidine (Demerol) 100 mg in 1 mL

Single-dose ampule or multiple-dose vial labeled with a specific dosage per volume

• For example: epinephrine (Adrenalin) 1:1000 in 0.1 mL

A vial with powder that requires a specific fluid be added to it to obtain a specific dosage (Reconstitution)

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Insulin

A critical medication that replaces the insulin not being produced by the patient’s pancreas

Insulin comes in a standardized measure called a “Unit”

Smallest amounts may be given; errors are critical

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U-100 Vial

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Insulin (cont.)

Strengths U-100 (100 Units of insulin per 1 mL) U-500 (500 Units of insulin per 1 mL)

• Preparation 5 times stronger, rarely used

Syringe Calibrated in Units also Tuberculin syringe used in emergency

• Minims used; 16 minims = 1 mL

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U-100 Syringe

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Tuberculin Syringe

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Intravenous Medications

Medications administered into the vein IV push IV hanging by gravity (flow rate formula) IV pump (mL/min or hr)

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Flow Rate Formula

Gtts/min =

Volume to be administered × gtt factor Time in minutes

Drop factor of tubing:Macrodrip = 10, 15, or 20 gtt/mL

Microdrip = 60 gtt/mL

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Question 1

The patient is prescribed prednisone 60 mg PO once daily. The pharmacist sends 20 mg tablets to the nursing unit. How many tablets should the LPN give?

1. 1 tablet

2. 2 tablets

3. 3 tablets

4. 4 tablets

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Question 2

Ancef 750 mg is ordered. The vial is labeled 500 mg/1mL. How many mL are needed?

1. 0.5 mL

2. 1 mL

3. 1.5 mL

4. 2 mL

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Question 3

When mixing regular and NPH insulin:

1. Draw up the NPH insulin first.

2. Draw up the regular insulin first.

3. Inject air into the regular insulin first.

4. These two insulins should not be mixed.

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Learning Objectives

List the rule used to calculate medication dosages for children

Calculate flow rates for infusions for children

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Clark’s Rule

Formula

Weight of the child × Adult dose = Child’s doseWeight of the adult

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Body Surface Area

Body surface area (BSA) = the total tissue area A nomogram is used to easily calculate the BSA

in square meters BSA formula

Surface area of the child (M2) × Usual adult doseSurface area of an adult (1.73 M2) = Child’s dose

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Dimensional Analysis

Steps Numbers in the dosage calculation problem are

placed on a grid along with their labels The labels are cross-canceled to assure only one

label is left (one for answer) Numbers in calculation are placed along grid next

to their labels

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Dimensional Analysis (cont.)

Numbers are cross-canceled Numbers are multiplied across the top and bottom

of the grid to yield a fraction The fraction is divided, and the remaining label is

applied to the answer

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