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Chapter 9Chapter 9

Calculating Drug Dosages

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Chapter 9

Lesson 9.1

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Learning ObjectivesLearning 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.)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

Calculating Medication Dosages

Three Steps1. Verify the drug available is the same

measurement system as the drug

dosage desired (convert if needed)

2. Reduce to lowest terms

3. Calculate dosage quantity to be

administered

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Drug Calculation MethodsDrug 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.)

Drug Calculation Methods (cont.)

• Desired over Available Method

Desired units (conversion factor) x

Quantity of drug form = Quantity to give

Quantity available (x conversion factor)

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Forms of Oral MedicationsForms 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 TabletsScored and Unscored Tablets

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Parenteral MedicationsParenteral 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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InsulinInsulin

• 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 VialU-100 Vial

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Insulin (cont.)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 SyringeU-100 Syringe

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Tuberculin SyringeTuberculin Syringe

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Intravenous MedicationsIntravenous 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 FormulaFlow 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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Chapter 9

Lesson 9.2

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Learning ObjectivesLearning Objectives

• List the rule used to calculate medication dosages for children

• Calculate flow rates for infusions for children

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Clark’s RuleClark’s Rule

• Formula

Weight of the child

________________ x Adult dose = Child’s dose

Weight of the adult

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Body Surface AreaBody Surface Area

• Body surface area (BSA) = the total tissue area

• A nomogram is used to easily calculate the BSA in square meters

• BSA formulaSurface area of the child (M2) × Usual adult dose

Surface area of an adult (1.73 M2) = Child’s dose

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Dimensional AnalysisDimensional 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.)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