PHARMACY CALCULATIONS •Explain Importance of Calculations in Determining Days Supply •Describe Consequences of Calculation Errors

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THE OBJECTIVES INCLUDE THE FOLLOWING: •Discuss importance of calculations in a pharmacy practice. •Outline steps to avoid calculation errors 2016 ANNUAL MEETING •Explain importance of reading the prescription carefully then performing calculations PHARMACY CALCULATIONS •Explain importance of calculations in determining days supply •Describe consequences of calculation errors. •Perform calculations during the presentation

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ADDITIONAL LEARNING OBJECTIVES •Understand how the concentration of electrolyte 2016 ANNUAL MEETING solutions are expressed and calculations of m.Eq. •Understand the importance of iso-tonicity in SUNIL S. JAMBHEKAR, B. PHARM.; M.S., PH.D ophthalmic solutions. PROFESSOR, PHARMACEUTICAL SCIENCES LECOM BRADENTON, SCHOOL OF PHARMACY •Perform calculations, using a prescription, to BRADENTON, FL34202 determine the quantity of a liquid product required [email protected] to fill the prescription accurately.

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ADDITIONAL LEARNING OBJECTIVES

•Perform calculations, using a prescription, to 2016 ANNUAL MEETING determine the # of day supply for a specific DISCLOSURE product. I DO NOT HAVE A VESTED INTEREST IN OR AFFILIATION •Perform calculations to determine m.Eq WITH ANY CORPORATE ORGANIZATION OFFERING FINANCIAL SUPPORT OR GRANT MONIES FOR THIS CONTINUING strength of an electrolyte solution. EDUCATION ACTIVITY, OR ANY AFFILIATION WITH AN ORGANIZATION WHOSE PHILOSOPHY COULD POTENTIALLY •Perform calculations to prepare an isotonic BIAS MY PRESENTATION ophthalmic solution.

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IMPORTANCE OF CALCULATIONS IN THE PHARMACY PRACTICE: •It is consistent with the practice and philosophy of pharmaceutical care. •Insurance companies will reimburse you or •Patient safety is of utmost importance. your employer only for the quantity written on the prescription and not for the quantity •Your pharmacy is the last check point for the dispensed. prescription written by a physician. Therefore, pharmacy team is responsible for all the final checks.

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Electrolytes Concentration •The final check points will include Its solution contains ions. •accuracy of the dose and dosage regimen; Examples include, •the quantity prescribed or required; sodium chloride, calcium chloride, potassium chloride, •# of refills; and # of days supply. zinc sulfate, magnesium sulfate, and many more. Electrolytes dissociate into particles known as ions. For •Your Pharmacists depend on you to input the example, prescription accurately to avoid editing and + - save time. NaCl ------Na (cation) + Cl (anion) These ions carry electric charge

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•The internal and external audit is another good Electrolyte ions (cations) commonly present in the blood reason for maintaining accuracy of the include: prescription. sodium (Na+), potassium (K+), calcium (Ca++), •Federal and state regulations will also play an magnesium (Mg++), etc. important role. The anions commonly found in the blood are Chloride (Cl-), bicarbonate (HCO --), phosphate (HPO --), and •When there is a reimbursement issue, accuracy 3 4 sulfate (SO --). in the quantity of the medication dispensed 4 becomes an important matter. These ions maintain acid-base balance in the body.

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For example: Potassium chloride (KCl) •Electrolyte solutions are, therefore, used in the treatment of disturbances in the electrolytes. Molecular weight = 74.5 grams •The concentrations of these solutions can be can be Equivalent weight = 74.5 grams expressed as % W/V or mg %; however, these expressions Potassium (K+) = 39 grams and chloride (Cl-) = 35.5 do not take into consideration chemical equivalence and, grams hence, they do not give any direct information as to the number ions or charges they carry. Therefore, 39 + 35.5 = 74.5 grams • A chemical unit, mili-equivalent (m.Eq.) is almost 74.5/1000 = 0.0745 grams or 74.5 mgs exclusively used to express the concentration of an Therefore, 74.5 milligrams of KCl =1 m.Eq of KCl electrolyte in solution. 149 milligrams of KCl = 2 m.Eq of KCl

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For Calcium Chloride (CaCl2.2H2O) •This unit is related to the number of ionic charges in Molecular weight = 147 grams solution and the valence of ions or m.Eq unit is a Its equivalent weight is measurement of the amount of chemical activity of an electrolyte. = Molecular weight/total valence (2) • Under normal conditions, blood plasma contains 155 Equivalent weight = 147/2 = 73.5 grams and 73.5 m.Eq of cations and same number of anions. grams/1000 = 0.0735 grams or 73.5 mgs. •The total concentration of cations always equals the total Therefore, 73.5 mg of calcium chloride represent 1 mEq concentration of anions. of calcium chloride (CaCl2.2H2O) and 147.0 mg of calcium chloride (CaCl2.2H2O) represent 2 m.Eq of calcium chloride

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The expression mEq represents the amount of solute in mg equal to 1/1000 th gram of the equivalent weight of the substance. Equivalent weight (grams) Two Tables in your handout provide values for some 1 mEq = ------important cations and anions. 1000

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Ions Valence At. weight mEq. (mg) •Patient will need 74.5 mg/mEq. x 15 mEq. X 2/day = 2235 mg of potassium chloride daily + Na 1 23 23 •Potassium chloride is available as 10% w/v (i.e., 10 K+ 1 39 39 g/100 ml or 10,000 mg/100 mL) or 100 mg/mL ++ Ca 24020 •100 mg of KCl is provided by 1 mL of 10% w/v Mg++ 22412 potassium chloride solution + NH4 11818 •1117.5 mg (15 mEq.) will be provided by X mL of Li+ 177 solution

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Ions Valence At. weight mEq. (mg) •1117.5 mg/100 mg/mL =

Cl- 1 35.5 35.5 11.175 mL of 10% w/v solution will provide 15 -- mEq. of potassium chloride SO4 2 96.0 48.0 - •11.175 mL x 2/day = 22.35 mL per day x 21 days = HCO3 1 61.0 61.0 -- 469.35 ml quantity for 3 weeks CO3 2 60.0 30.0 - •Sig: 11.175 ml by mouth twice a day for three H2PO4 1 97.0 97.0 -- weeks HPO4 2 96.0 48.0

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POTASSIUM CHLORIDE PRESCRIPTION Isotonic Solution Potassium chloride liquid A solution that has same osmotic pressure as body fluids. Sig: 15 mEq;po;bid;for 3 weeks Hypotonic Solution Dis; QS Refill x2 A solution that has lower osmotic pressure than the body Potassium chloride is available as 10% w/v solution fluid. Molecular weight of potassium chloride is 74.5 grams Hypertonic Solution Therefore 74.5 mg (Molecular weight in milligrams is A solution that has higher osmotic pressure than that of equal to 1 mEq. for monovalent compounds) body fluids.

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• It has been determined that 0.9% w/v solution of sodium chloride exhibits same osmotic pressure as the human body fluids • Therefore, while preparing solution to be administered in the body (ophthalmic solution and intravenous solutions), it is imperative that these solutions have same osmotic pressure as body fluid or as 0.9% sodium chloride solution. • Therefore, most frequently, sodium chloride is used to make solution isotonic. Boric acid and dextrose may also be used when it is necessary.