Dosage Calculations

1 Dosage CalculationsAfter completing this chapter, youshould be able to: Solve one-step pharmaceuticaldosage Calculations . Set up a series of ratios andproportions to solve a singledosage calculation . Determine what informationyou will need to solve for, inaddition to any giveninformation, to properlycalculate dosages. Convert pediatric weights frompounds to kilograms. Accurately determine dosagesbased on mg/kg/day. Define common sig (signa)codes used on dosing of medications is important to ensure patientsafety. Calculating dosages, Dosage regimens, andcompounding formulas involves the use of simple mathprinciples.

2 You can solve many of these problems by settingup ratios and proportions using the information given in thequestion and keeping like units 8/31/07 2:39 PM Page 2 Chapter Five Dosage Calculations3 Sig RefresherThe sigportion of the prescription order, meaning signa, is where the instruc-tions for the patient are written. Pharmacy technicians enter the informationfrom the prescription order into the computer. The sig is an important valueto remember in order to properly determine pediatric dosages. The followingare some of the more common sigs you will find on prescriptions:qdevery dayqodevery other dayddaily bidtwice a daytidthree times a dayqidfour times a dayq4hevery 4 hrq6hevery 6 hrq8hevery 8 hrq4 6hevery 4 6 hrprnas neededDepending on the workplace, you may also see sigs such as the following:q3devery three daysqmwfevery Monday, Wednesday, and Fridayqwevery weekDosage CalculationsDosage Calculations include calculating the number of doses, dispensingquantities, and ingredient quantities.

3 These Calculations are performed in thepharmacy on a daily basis. The pharmacy technician must have a full work-ing knowledge of how to perform these perform Dosage Calculations , you will utilize the information andprinciples introduced in the previous chapters of this book. You can solvethese Calculations by setting up ratios and proportions, keeping like unitsconsistent, and THE NUMBER OF DOSESTo calculate the number of doses, you should first determine which informa-tion presented is actually applicable to the question.

4 Too often mistakes aremade on Dosage Calculations because we overcomplicate many 1-tsp doses are in a 4 oz bottle of Prozac LiquidSolution 20 mg/5 mL?Rx Prozac Solutiontsp. po qdDisp. # 4 8/31/07 2:39 PM Page 34 Chapter Five Dosage CalculationsLet s look at the information that has been provided: tsp po the dose qd the frequency 4 oz the quantity dispensed Prozac Solution 20 mg/5 mL the drug name and strength 120 mL the quantity of the stock bottleThe question is simply asking how many doses make up the total amountbeing dispensed. The strength of the drug, frequency of Dosage and quantityof the stock bottle have no relevance in performing this now we know that we are working with 1 tsp.

5 Doses and a total quan-tity of 4 oz, but to solve this calculation using a ratio/proportion we have tohave similar units of measure in this case know that 1 tsp = 5 mL and we should also know that 4 oz. = 120 mL,but if you didn t that could also be solved by using a and solve the equation for .Now that you have both quantities converted to units in mL, we can setup our ratio/proportion and *x=5x and 1*120=1201 dose5 mL=x doses120 mL(1)x=12030*4=120 and 1*x=(1) xx1 oz30 mL=4 ozx 5-1 Drug label for Prozac.(Courtesy of Eli Lilly and Company.) 8/31/07 2:39 PM Page 4 Chapter Five Dosage Calculations5 Now set up your equation and solve for solve for x, divide both sides by , there are 24 doses (of 5 mL) in a 4 oz (120 mL) THE NUMBER OF DOSESEXAMPLE many doses are provided in the prescription below?

6 Rx Ibuprofen 400 mg-po q 6 hr prn painDisp. # 120 Let s look at the information that has been provided: - po the dose q 6 hr prn pain the frequency 120 the quantity dispensed Ibuprofen 400 mg the drug name and strengthWorkplace WisdomAlways use the higher Dosage amount when perform-ing Dosage Calculations on prescriptions that have a range for the dose, as inExample This will provide the most conservative solution and ensurethe most accurate potential for days the information provided, set up the ratio/proportion and set up your equation and solve for.

7 To solve for , divide both sides by , there is a minimum of 60 doses prescribed. x=60 120,2=60 2x2=1202x2x=120x2*x=2x and 1*120=1201 dose2 tabs=x doses120 x=24 120,5=24 2x2=12025x= 8/31/07 2:39 PM Page 56 Chapter Five Dosage CalculationsPRACTICE PROBLEMS many dosages are provided in the prescription listed below?_____Rx Cardura 2 mgpo qdDisp. # many droppersful (a mL Dosage ) are in a 50 mL bottle ofEryPed Drops? many dosages are provided in the prescription listed below?_____Rx Vicodin 5/500po q8hrDisp. # many 1 tsp doses are in each bottle of Zithromax 200 mg/5 mL aslisted below, when mixed?

8 Many dosages are provided in the prescription listed below?_____Rx Regular Insulin U-100 (100 units/mL)Inject 20 units qAMDisp. # 1 vial (10mL)Figure 5-2 Drug label for EryPed Drops.(Reproduced with permission of Abbott Laboratories.)Figure 5-3 Drug label for Zithromax.(Registered Trademark of Pfizer Inc. Reproduced with permission.) 8/31/07 2:39 PM Page 6 Chapter Five Dosage Calculations7 CALCULATING THE QUANTITY TO DISPENSEEXAMPLE many Biaxin 250 mg tablets should be dispensed?Rx Biaxin 250 mgpo BID 5-4 Drug label for Biaxin.(Reproduced with permission of Abbott Laboratories.)

9 Again, to solve this Dosage calculation it is important to first determinewhich information is necessary; it is also critical to know the common SIGcodes to perform Dosage s look at what information that has been provided: po the dose BID the frequency x10d the duration Biaxin 250 mg the drug name and strength 100 tablets the quantity of the stock bottleTo calculate the appropriate quantity to dispense, use the following formula:dose frequency duration quantity to dispenseUsing the information provided in the prescription, you can set up thecalculation as such.

10 1 2 10 xdose frequency duration quantity to dispense1 2 10 2020 xSo, 20 tablets of Biaxin 250 mg should be 8/31/07 2:39 PM Page 78 Chapter Five Dosage CalculationsCALCULATING THE QUANTITY TO DISPENSEEXAMPLE much Promethazine w/Codeine syrup should be dispensed?Rx Promethazine w/Codeine Syrup po QID 4dLet s look at what information has been provided: tsp. po the dose QID the frequency x4d the duration Promethazine w/Codeine Syrup the drug name and strengthTo calculate the appropriate quantity to dispense, use the following formula:dose frequency duration quantity to dispenseUsing the information provided in the prescription, you can set up thecalculation as such.