THE OBJECTIVES INCLUDE THE FOLLOWING: PHARMACY CALCULATIONS Discuss importance of calculations in a pharmacy practice. Outline steps to avoid calculation errors Explain importance of reading the prescription carefully then performing calculations Explain importance of calculations in determining days supply Describe consequences of calculation errors. Perform calculations during the presentation 4 ADDITIONAL LEARNING OBJECTIVES SUNIL S. JAMBHEKAR, B. PHARM.; M.S., PH.D PROFESSOR, PHARMACEUTICAL SCIENCES LECOM BRADENTON, SCHOOL OF PHARMACY BRADENTON, FL34202 SJAMBHEKAR@LECOM.EDU Understand how the concentration of electrolyte solutions are expressed and calculations of m.eq. Understand the importance of iso-tonicity in ophthalmic solutions. Perform calculations, using a prescription, to determine the quantity of a liquid product required to fill the prescription accurately. 5 ADDITIONAL LEARNING OBJECTIVES DISCLOSURE I DO NOT HAVE A VESTED INTEREST IN OR AFFILIATION WITH ANY CORPORATE ORGANIZATION OFFERING FINANCIAL SUPPORT OR GRANT MONIES FOR THIS CONTINUING EDUCATION ACTIVITY, OR ANY AFFILIATION WITH AN ORGANIZATION WHOSE PHILOSOPHY COULD POTENTIALLY BIAS MY PRESENTATION Perform calculations, using a prescription, to determine the # of day supply for a specific product. Perform calculations to determine m.eq strength of an electrolyte solution. Perform calculations to prepare an isotonic ophthalmic solution. 6 1
IMPORTANCE OF CALCULATIONS IN THE PHARMACY PRACTICE: It is consistent with the practice and philosophy of pharmaceutical care. Patient safety is of utmost importance. Your pharmacy is the last check point for the prescription written by a physician. Therefore, pharmacy team is responsible for all the final checks. Insurance companies will reimburse you or your employer only for the quantity written on the prescription and not for the quantity dispensed. 7 10 The final check points will include accuracy of the dose and dosage regimen; the quantity prescribed or required; # of refills; and # of days supply. Your Pharmacists depend on you to input the prescription accurately to avoid editing and save time. Electrolytes Concentration Its solution contains ions. Examples include, sodium chloride, calcium chloride, potassium chloride, zinc sulfate, magnesium sulfate, and many more. Electrolytes dissociate into particles known as ions. For example, NaCl Na + (cation) + Cl - (anion) These ions carry electric charge 8 11 The internal and external audit is another good reason for maintaining accuracy of the prescription. Federal and state regulations will also play an important role. When there is a reimbursement issue, accuracy in the quantity of the medication dispensed becomes an important matter. Electrolyte ions (cations) commonly present in the blood include: sodium (Na + ), potassium (K + ), calcium (Ca ++ ), magnesium (Mg ++ ), etc. The anions commonly found in the blood are Chloride (Cl - ), bicarbonate (HCO 3 ), phosphate (HPO 4 ), and sulfate (SO 4 ). These ions maintain acid-base balance in the body. 9 12 2
Electrolyte solutions are, therefore, used in the treatment of disturbances in the electrolytes. The concentrations of these solutions can be can be expressed as % W/V or mg %; however, these expressions do not take into consideration chemical equivalence and, hence, they do not give any direct information as to the number ions or charges they carry. A chemical unit, mili-equivalent (m.eq.) is almost exclusively used to express the concentration of an electrolyte in solution. 13 For example: Potassium chloride (KCl) Molecular weight = 74.5 grams Equivalent weight = 74.5 grams Potassium (K + ) = 39 grams and chloride (Cl - ) = 35.5 grams Therefore, 39 + 35.5 = 74.5 grams 74.5/1000 = 0.0745 grams or 74.5 mgs Therefore, 74.5 milligrams of KCl =1 m.eq of KCl 149 milligrams of KCl = 2 m.eq of KCl 16 This unit is related to the number of ionic charges in solution and the valence of ions or m.eq unit is a measurement of the amount of chemical activity of an electrolyte. Under normal conditions, blood plasma contains 155 m.eq of cations and same number of anions. The total concentration of cations always equals the total concentration of anions. 14 For Calcium Chloride (C a Cl 2.2H 2 O) Molecular weight = 147 grams Its equivalent weight is = Molecular weight/total valence (2) Equivalent weight = 147/2 = 73.5 grams and 73.5 grams/1000 = 0.0735 grams or 73.5 mgs. Therefore, 73.5 mg of calcium chloride represent 1 meq of calcium chloride (CaCl 2.2H 2 O) and 147.0 mg of calcium chloride (CaCl 2.2H 2 O) represent 2 m.eq of calcium chloride 17 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) 1 meq = 1000 Two Tables in your handout provide values for some important cations and anions. 15 18 3
Ions Valence At. weight meq. (mg) N + a 1 23 23 K + 1 39 39 C ++ a 2 40 20 Mg ++ 2 24 12 NH + 4 1 18 18 Li + 1 7 7 Patient will need 74.5 mg/meq. x 15 meq. X 2/day = 2235 mg of potassium chloride daily Potassium chloride is available as 10% w/v (i.e., 10 g/100 ml or 10,000 mg/100 ml) or 100 mg/ml 100 mg of KCl is provided by 1 ml of 10% w/v potassium chloride solution 1117.5 mg (15 meq.) will be provided by X ml of solution 19 22 Ions Valence At. weight meq. (mg) Cl - 1 35.5 35.5 SO 4 2 96.0 48.0 HCO - 3 1 61.0 61.0 CO 3 2 60.0 30.0 H 2 PO - 4 1 97.0 97.0 HPO 4 2 96.0 48.0 1117.5 mg/100 mg/ml = 11.175 ml of 10% w/v solution will provide 15 meq. of potassium chloride 11.175 ml x 2/day = 22.35 ml per day x 21 days = 469.35 ml quantity for 3 weeks Sig: 11.175 ml by mouth twice a day for three weeks 20 23 POTASSIUM CHLORIDE PRESCRIPTION Potassium chloride liquid Sig: 15 meq;po;bid;for 3 weeks Dis; QS Refill x2 Potassium chloride is available as 10% w/v solution Molecular weight of potassium chloride is 74.5 grams Therefore 74.5 mg (Molecular weight in milligrams is equal to 1 meq. for monovalent compounds) Isotonic Solution A solution that has same osmotic pressure as body fluids. Hypotonic Solution A solution that has lower osmotic pressure than the body fluid. Hypertonic Solution A solution that has higher osmotic pressure than that of body fluids. 21 24 4
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. 25 5