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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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 2 Learning Objectives (cont.) Calculate dosages for parenteral injections, including those for special preparations such as insulin Calculate flow rates for infusions Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 3 Calculating Medication Dosages Three Steps 1. 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 4 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 5 Drug Calculation Methods (cont.) Desired over Available Method Desired units (conversion factor) × Quantity of drug form = Quantity to give Quantity available (× conversion factor) Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 6 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 7 Scored and Unscored Tablets Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 8 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) Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 9 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 10 U-100 Vial Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 11 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 12 U-100 Syringe Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 13 Tuberculin Syringe Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 14 Intravenous Medications Medications administered into the vein IV push IV hanging by gravity (flow rate formula) IV pump (mL/min or hr) Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 15 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 16 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. 2. 3. 4. 1 tablet 2 tablets 3 tablets 4 tablets Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 17 Question 2 Ancef 750 mg is ordered. The vial is labeled 500 mg/1mL. How many mL are needed? 1. 2. 3. 4. 0.5 mL 1 mL 1.5 mL 2 mL Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 18 Question 3 When mixing regular and NPH insulin: 1. 2. 3. 4. Draw up the NPH insulin first. Draw up the regular insulin first. Inject air into the regular insulin first. These two insulins should not be mixed. Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 19 Learning Objectives List the rule used to calculate medication dosages for children Calculate flow rates for infusions for children Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 20 Clark’s Rule Formula Weight of the child × Adult dose = Child’s dose Weight of the adult Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 21 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 dose Surface area of an adult (1.73 M2) = Child’s dose Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 22 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 23 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 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc. 24