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Chapter 9
Calculating Drug Dosages
Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 1
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
2Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Learning Objectives (cont.)
Calculate dosages for parenteral injections, including those for special preparations such as insulin
Calculate flow rates for infusions
3Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
4Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
5Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Drug Calculation Methods (cont.)
Desired over Available Method
Desired units (conversion factor) ×
Quantity of drug form = Quantity to give
Quantity available (× conversion factor)
6Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
7Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Scored and Unscored Tablets
8Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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)
9Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
10Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
U-100 Vial
11Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
12Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
U-100 Syringe
13Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Tuberculin Syringe
14Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Intravenous Medications
Medications administered into the vein IV push IV hanging by gravity (flow rate formula) IV pump (mL/min or hr)
15Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
16Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
17Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
18Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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.
19Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Learning Objectives
List the rule used to calculate medication dosages for children
Calculate flow rates for infusions for children
20Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
Clark’s Rule
Formula
Weight of the child × Adult dose = Child’s doseWeight of the adult
21Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
22Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
23Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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
24Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.
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