Lecture Math for the pharmacy technician Concepts and calculations Chapter 6 – Lynn M. Egler, Kathryn A. Booth

Chapter 6, Dosage calculations, teaches the techniques for calculating doses. Building on information from earlier chapters, students are taught how to calculate the amount to dispense, using all four methods of dosage calculations: ratio proportion, fraction proportion, dimensional analysis, and formula. Students will also learn to calculate estimated days supply.

Math for the Pharmacy Technician: Concepts and Calculations Egler • Booth Chapter 6: Dosage Calculations McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-2 METHODS OF DOSAGE CALCULATIONS McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-3 Learning Objectives When you have successfully completed Chapter 6, you will have mastered skills to be able to:  Identify the information on a medication order and drug label needed to calculate the desired dose  Convert the dosage ordered to the desired dose McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-4 Learning Objectives (con’t)    Calculate the amount to dispense of a drug Recognize common errors that occur during dose calculations Calculate estimated days supply McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-5 Introduction   This chapter will bring all of the previous information together to calculate the amount of medication to dispense to a patient You will use:     Basic math Information from the physician’s order Drug labels Method of converting quantities from one unit of measurement to another McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-6 Dosages and Doses To calculate medication dosages you must know: Desired Dose:  The amount of the drug to be dispensed at a single time  Its unit of measurement must be the same as the dosage unit McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-7 Dosages and Doses (con’t) Dosage ordered:  The total amount of drug the physician has ordered, along with the frequency  Its unit of measurement may not be the same as that of the dosage unit McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-8 Dosages and Doses (con’t) Dose on hand:  The amount of drug contained within a dosage unit of medication  If a medication has 250 mg of drug per tablet, the dose on hand is 250 mg McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-9 Dosages and Doses (con’t)   The unit of measurement for the desired dose must be the same as the unit of measurement of the dose on hand before the amount to dispensed can be calculated This is calculated by converting the dose ordered into the same unit of measurement as the dose on hand; once converted it becomes the desired dose McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-10 Calculating the Desired Dose    Before calculating the amount to be dispensed, you must first determine the desired dose The dosage ordered is not always written in the same units that are found on the drug label You must convert the dosage ordered into a desired dose having the same units as the dose on hand McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved Calculating the Amount to Dispense (con’t) Example:  Ordered is Famvir 500 mg PO q 8h  On hand: Famvir 250 mg tabs  Find the amount to dispense Amount to dispense = tablets McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-30 Calculating the Amount to Dispense (con’t) Procedure 6-6 Calculating the Amount to Dispense by Dimensional Analysis: With dimensional analysis you will not need to calculate the desired dose and amount to dispense separately You will place your unknown (amount to dispense) on one side of the equation then multiply a series of factors on the right side of the equation Canceling units will help you determine the equation has been set up correctly McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-31 Calculating the Amount to Dispense (con’t) Procedure 6-6 Calculating the Amount to Dispense by Dimensional Analysis (con’t): Determine the units of measure for the answer and place it as the unknown on one side of the equation On the right side of the equation, write a conversion factor with the units of measurement for the desired dose on top and the unit of measurement for the dose on hand McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-32 Calculating the Amount to Dispense (con’t) Procedure 6-6 Calculating the Amount to Dispense by Dimensional Analysis (con’t): Multiply the conversion factor by a second factor: the dosage unit over the dose on hand Multiply by a third factor: dose ordered over the number one Cancel units on the right side of the equation The remaining unit of measure on the right side of the equation should match the unknown unit of measure on the left side of the equation Solve the equation McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-33 Calculating the Amount to Dispense (con’t) Example:  The order reads: Prozac Liquid 40 mg daily  On hand: Prozac 20 mg/5 mL  Find the amount to dispense Amount to dispense = 10 mL McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-34 Calculating the Amount to Dispense (con’t) 6-35 Procedure 6-7 Calculating Amount to Dispense Using the Formula Method: Determine the desired dose Determine the dose on hand (H) and dosage unit (Q) D xQ= A H Fill in the formula •D for the desired dose •H for the dose on hand •Q for the dosage unit in the formula •A for the unknown or the amount to dispense McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved Calculating the Amount to Dispense (con’t) Procedure 6-7 Calculating Amount to Dispense Using the Formula Method (cont.): Cancel the units Solve for the unknown McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-36 Calculating the Amount to Dispense (con’t) Example:  The dosage ordered is 500 mg PO q h  On hand: Famvir 250 mg/tabs  Find the amount to dispense Amount to dispense = tablets McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-37 6-38 Estimated Days Supply  As a pharmacy technician you may need to determine the estimated days supply of a prescription, which is how long the medication will last the patient if taken correctly McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-39 Estimated Days Supply  (con’t) To determine estimated days supply you will multiply amount of medication to dispense by days needed over the number of dosage units per day Amount to dispense McGraw-Hill × ? days = estimated days supply dosage units per day ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-40 Review and Practice The amount of the drug to be dispensed at a single time is called the: a dosage ordered b dosage unit c desired dose d dosage strength Answer: c desired dose McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-41 Review and Practice True or False If a medication has 300 mg of drug per tablet, the dose on hand is 300 mg Answer: True McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-42 Review and Practice The MAR reads Glucotrol 10 mg PO qd The desired dose is mg Answer: 10 mg McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-43 Review and Practice Calculate the amount to dispense  Ordered: Prednisone 10 mg PO qid  On hand: Prednisone mg tablets Answer (Using ratio proportion) mg : tablet : : 10 mg : ? tablet x 10 = x ? ?=2 Amount to dispense: tablets McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved Methods of Dosage Calculations Performing dosage calculations is a large part of the pharmacy technician’s daily responsibilities It is important to always use any resources available to you when performing dosage calculations THE END McGraw-Hill ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved 6-44 ... measure for the answer and place it as the unknown on one side of the equation On the other side of the equation, write a conversion factor with the units of measure for the answer on top and the. .. they are the same portion of the fraction Units in the denominator of one fraction cannot be canceled with units found in the numerator of the other Always include the units when performing calculations. .. ©2010 by the McGraw-Hill Companies, Inc All Rights Reserved Calculating the Amount to Dispense (con’t) 6- 26 The dosage strength or the dose on hand (H) per the dosage unit (Q) The dose on hand (H)