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 2 Copyright © 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 3 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.

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 4 Copyright © 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 5 Copyright © 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) 6 Copyright © 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 7 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.

Scored and Unscored Tablets 8 Copyright © 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) 9 Copyright © 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 10 Copyright © 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 11 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.

U-100 Syringe 12 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.

Tuberculin Syringe 13 Copyright © 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 14 Copyright © 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 15 Copyright © 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? mL 2.1 mL mL 4.2 mL 16 Copyright © 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. 17 Copyright © 2013, 2010, 2006, 2003, 2000, 1995, 1991 by Mosby, an imprint of Elsevier Inc.

Practice  Pharmacy sends you Tagamet (cimetidine) 1 g IV liquid / 1 mL.  The order requests 300 mg IV be given.  How do you calculate this does?  How much Tagamet do you draw up?

Practice  Pharmacy sends esomeprazole 20mg extended release capsule.  Ordered: Nexium (esomeprazole) 20 mg/capsule  How do you calculate this does?  How much Nexium do you give?

Practice  Ordered: Ampicillin 750 mg IVPB every 6 hours.  Pharmacy sent Ampicillin 1 g/ 5mL.  How do you calculate this does?  How much Ampicillin do you give?

Practice  IV 0.9% solution to infuse 1000 mL over 8 hr.  How much infuses over 1 hour?  If your gtt factor is 12 gtts/mL, how many gtts/ mins will infuse the solution at the correct rate?

Practice  Insulin ordered: 25 units NPH and 12 units regular subcutaneous every AM.  What supplies do you need?  What is the total amount in the syringe?

WARNING!!!!!  Your calculation labels must be correct or the answer is wrong!  Your zero’s placement must be correct in nursing terms or it’s wrong!  Accuracy is the issue.  Safety is the result.