Module 10 - Dosing Calculations

Module 10 Outcomes Outcome 1: Interpret prescription orders Outcome 2: Solve oral and parenteral dosage problems including pediatric and weight-based dosing Outcome 3: Create dimensional analysis formulas to solve for necessary units

10.1 Pharmacy Math Introduction This module will cover the basic mathematics involved in most medication dosage calculations. Medication dosage calculations are often necessary for a variety of reasons. Examples include when a medication is dosed based on a patient’s weight, when the dose needed for a patient is not commercially available, and when the rate at which a medication is to be administered intravenously needs to be calculated. These are a few common examples of why pharmacy math is so important. Before discussing the calculation portion of this module, some helpful information regarding prescription orders will be introduced. It is critical that one be able to interpret the prescription, or order (as it is referred to in the hospital setting), to know what the provider is ordering for the patient and to ensure the dose administered is the intended dose. Table 1.1 below contains a list of commonly used abbreviations in prescriptions. Table 1.1 Abbreviations Commonly Used in Prescriptions Abbreviation Interpretation Abbreviation Interpretation Route Frequency IM Intramuscular ac Before meals IV Intravenous pc After meals IVPB Intravenous piggyback prn As needed SC Subcutaneous hs Bedtime SL Sublingual stat Immediately, at once PO By mouth qd Once a day PR Rectally qod Every other day OD Right eye bid Twice a day OS Left eye tid Three times a day

OU Both eyes qid Four times a day AD Right ear qh Every hour AS Left ear q4h Every 4 hours AU Both ears q6h Every 6 hours General General a before mEq Milliequivalent c with npo Nothing by mouth cap capsule qs A sufficient quantity et and Sig Write on label dc discontinue Tbs, tbsp Tablespoon disp dispense tsp teaspoon gtt drop Ud As directed IU International Units Rx Prescription Misc. Drugs and Conditions IV Products ASA Aspirin DW Distilled Water APAP Acetaminophen D5LR Dextrose 5% in Lactated Ringers Solution BP Blood Pressure D5NS Dextrose 5% in normal saline BS Blood Sugar D10W Dextrose 10% in water CA Cancer NS Normal Saline

CHF Congestive Heart Failure HA Headache HC Hydrocortisone NTG Nitroglycerin SOB Shortness of breath Note: The abbreviation tsp is usually lowercase so as not to be confused with Tbs. Table 1.2 below outlines some common prescription sigs, or directions for use of the medicine, that are seen in the clinical setting. This will help illustrate how the abbreviations above are used together to make a complete prescription order that can be interpreted by the healthcare professional to make sure the patient receives the appropriate dose. Table 1.2 Common Prescription Sigs Directions as written by prescriber “Sig” Interpretation 1 tab po q 4-6h prn Take 1 tablet by mouth every 4-6 hours as needed 1 cap po bid Take 1 capsule by mouth twice daily 2 gtts OU HS Place 2 drops in both eyes at bedtime Prn sob Use as needed for shortness of breath Figures 1.1 and 1.2 provide examples of prescriptions. Pay attention to the information included in each prescription. Figure 1.1 identifies the different components of the order that must be present. These include the following: patient name, date of birth, name of the medication, strength of the medication, amount of medication, route and frequency, how much total medication should be dispensed (this is not necessary in a hospital setting),

refills given (also not necessary in a hospital setting), and prescriber signature and date. Figure 1.1 Example outpatient prescription for a liquid medication highlighting the necessary components of a prescription. The Sig indicates for the patient to take 2.5 ml by mouth every 4 hours. Figure 1.2 Example prescription ordering 30 tablets of Lipitor 20mg. The Sig indicates for the patient to take 1 tablet by mouth every day.