# Y khoa - Dược - Chapter 6: Dosage calculations

Identify the information on a medication order and drug label needed to calculate the desired dose. Convert the dosage ordered to the desired dose. Calculate the amount to dispense of a drug. Recognize common errors that occur during dose calculations. Calculate estimated days supply.

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Math for the Pharmacy Technician: Concepts and CalculationsChapter 6: Dosage CalculationsEgler • BoothMETHODS OF DOSAGE CALCULATIONS2Learning ObjectivesIdentify the information on a medication order and drug label needed to calculate the desired dose.Convert the dosage ordered to the desired dose.When you have successfully completed Chapter 6, you will have mastered skills to be able to:3Learning Objectives (con’t)Calculate the amount to dispense of a drug.Recognize common errors that occur during dose calculations.Calculate estimated days supply. 4IntroductionThis chapter will bring all of the previous information together to calculate the amount of medication to dispense to a patient.You will use:Basic mathInformation from the physician’s orderDrug labels Method of converting quantities from one unit of measurement to another5Dosages and DosesTo 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.6Dosages 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.7Dosages and Doses (con’t)The amount of drug contained within a dosage unit of medication.Dose on hand:If a medication has 250 mg of drug per tablet, the dose on hand is 250 mg.8Dosages 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.9Calculating the Desired DoseBefore 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.10Calculating the Desired Dose (con’t)Three methods to calculate desired dose:Fraction proportionRatio proportion Dimensional analysisEach gives the same resultUse the method that you likeOnce you identify your favorite method, follow the color coding of that method in the book11Fraction Proportion MethodProcedure Checklist 6-1Write the conversion factor with the units that you are converting to in the numerator and the units you are converting from in the denominator.Write a fraction with the unknown,”?,” in the numerator and the number that you need to convert in the denominator.12Fraction Proportion Method (con’t)3. Set the two fractions up as a proportion.4. Cancel units.5. Cross multiply, then solve for the unknown value.Example:The dosage ordered is 0.2 mg once a day.The dosage strength is 100 mcg/tablet.Find the desired dose.Desired Dose = 200 mcg 13CAUTION!In a fraction proportion, units from the fraction can be canceled only when 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.14Ratio Proportion MethodProcedure Checklist 6-2Write the conversion factor as a ratio A : B so that A has the units of the value that you are converting (the dosage ordered) and B has the unit of value of the dose on hand.Write a second C : D so that C is the missing value (desired dose) and D is the number that is being converted (the dosage ordered).15Ratio Proportion Method (con’t)3. Write the proportion in the form A : B :: C : D.Note: When using the ratio proportion method to calculate the desired dose, C indicates the unknown value (desired dose).4. Cancel units. 5. Solve the proportion by multiplying means and extremes.16Ratio Proportion Method (con’t)ExamplePractice by determining the desired dose.The desired dose is 325 mg. The order reads: ASA gr v PO daily.The drug label indicates 325 mg tablets.Find the desired dose.17CAUTION!In a ratio proportion, units can be canceled only when they are found in the same part of each of the ratios.Always include the units when performing calculations.18Dimensional AnalysisProcedure Checklist 6-3Determine the units of 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 units you are converting from on the bottom.19Dimensional Analysis (con’t)3. Multiply the conversion factor by the number that is being converted over one.4. 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.5. Solve the equation.20Dimensional Analysis (con’t)ExampleFind the desired dose for the following:Ordered: Levsinex timecaps ER 1500 mcg once dailyOn hand: Levsinex timecaps 0.375 mgDesired dose = 1.5 mg21CAUTION!In dimensional analysis, units can be canceled only when they are found in both the numerator and the denominator of the fraction.Always include the units when performing calculations.22Review and Practice Desired dose: 250 mg Determine the desired dose. Ordered: Penicillin VK 0.25 gOn hand: Penicillin VK 500 mg23Calculating the Amount to DispenseOnce you have determined the desired dose, you still must calculate the amount to dispense.24Calculating the Amount to Dispense (con’t) To calculate the amount of medication to dispense (A), the following information must be known:The desired dose (D) or the amount of drug to be given at a single time. This is the dosage ordered converted to the same units as the dose on hand, if necessary.25Calculating the Amount to Dispense (con’t)The dosage strength or the dose on hand (H) per the dosage unit (Q).The dose on hand (H) is the amount of drug contained in a dosage unit.The dosage unit (Q) is the unit by which you will measure the medication-tablets, capsules, milliliters, teaspoons, etc. (Find this on the medication label).26Calculating the Amount to Dispense (con’t)Procedure 6-4 Calculating the Amount to Dispense by Fraction Proportion:1. Set up the proportion as follows: dosage unit amount to dispense dose on hand desired dose2. Cancel units3. Cross multiply, then solve for the unknown value27Calculating the Amount to Dispense (con’t)Example:Ordered: 250 mg IMDosage strength available: 0.5 g/mLFind the amount to dispenseAmount to dispense = 0.5 mL28Procedure 6-5 Calculating the Amount to Dispense by Ratio Proportion:The proportion will be set up as follows:dosage unit : dose on hand :: amount to dispense : desired dose Or Q : H :: A : D2. Cancel units3. Multiply the means and extremes then solve for the missing valueCalculating the Amount to Dispense (con’t)29Calculating the Amount to Dispense (con’t)Example:Ordered is Famvir 500 mg PO q 8hOn hand: Famvir 250 mg tabsFind the amount to dispenseAmount to dispense = 2 tablets30Calculating 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.31Calculating 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.32Calculating 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.33Calculating the Amount to Dispense (con’t)Example:The order reads: Prozac Liquid 40 mg dailyOn hand: Prozac 20 mg/5 mLFind the amount to dispenseAmount to dispense = 10 mL34Calculating the Amount to Dispense (con’t)Procedure 6-7 Calculating Amount to Dispense Using the Formula Method:1. Determine the desired dose. Determine the dose on hand (H) and dosage unit (Q).2. Fill in the formulaD 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 35Calculating the Amount to Dispense (con’t)Procedure 6-7 Calculating Amount to Dispense Using the Formula Method (cont.):3. Cancel the units.4. Solve for the unknown.36Calculating the Amount to Dispense (con’t)Example:The dosage ordered is 500 mg PO q 8 hOn hand: Famvir 250 mg/tabsFind the amount to dispenseAmount to dispense = 2 tablets37Estimated Days SupplyAs 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. 38Estimated 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  39Review and PracticeThe 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 strengthAnswer: c. desired dose40Review and PracticeTrue or FalseIf a medication has 300 mg of drug per tablet, the dose on hand is 300 mg.Answer: True41Review and PracticeThe MAR reads Glucotrol 10 mg PO qd.The desired dose is ____ mg.Answer: 10 mg42Review and PracticeCalculate the amount to dispense.Ordered: Prednisone 10 mg PO qidOn hand: Prednisone 5 mg tabletsAnswer (Using ratio proportion) 5 mg : 1 tablet : : 10 mg : ? tablet 1 x 10 = 5 x ? ? = 2 Amount to dispense: 2 tablets43Methods of Dosage CalculationsTHE ENDPerforming 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. 44