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Joy A. Awoniyi, PharmD. PGY1 Pharmacy Practice Resident   Miami VA Healthcare SystemFor The FAMU COPPS Class of 2013
Objectives• To emphasize the importance of calculations• To review important calculation concepts related to     •   Patie...
WHY SHOULD YOU CARE?         • You can’t escape it         • You need it to pass the NAPLEX         • It can be applied to...
PATIENTPARAMETERS Important Calculation Concepts               3
BSA and IBW• Body Surface Area (BSA), m2  • BSA = Weight (kg)0.425 x Height (cm)0.725 x 0.007184  • BSA =             cm ´...
BODY MASS INDEX (BMI)• Measure of body fat to help categorize patients and  assess risk of morbidity for certain diseases•...
Creatinine Clearance (CrCl)• Formula used to estimate renal function• Not useful when renal function is fluctuating rapidl...
EXAMPLE        Mack “Big Mack” Donald          is a 55 year old man       recently admitted to your       service after ch...
EXAMPLE    Body Surface Area                            Body Mass Index• Height in centimeters: 6’2” = 74           • Heig...
EXAMPLE        Mack had a DVT during        admission. The doctors          wish to start him on        Enoxaparin (Loveno...
EXAMPLE  Ideal Body Weight             Creatinine Clearance• Inches over 60               • Use IBW of 82.2kg  • 74 – 60 =...
MEASUREMENTS  Important Calculation Concepts                11
The Basics Of                   Measurement• Irrelevant information is often given• Fundamental information is often left ...
EXAMPLE A cough syrup contains 10mg of dextromethorphan per 5 mL. Your pharmacy has 6 bottles left in stock. Each bottle  ...
EXAMPLE A cough syrup contains 10mg of dextromethorphan per 5 mL. Your pharmacy has 6 bottles left in stock. Each bottle  ...
Specific Gravity          Definition                            Equations• A ratio of the weight of any        • Grams = m...
EXAMPLE    There is 10 grams of glycerin in a 500mL solution.    300 mL of glycerin weighs 165 grams. What is the         ...
EXAMPLE There is 10 grams of glycerin in a 500mL solution. 300 mL of glycerin weighs 165 grams. What is the            spe...
EXAMPLE   What is the weight, in grams of a 2 fluid ounces of a          liquid with a specific gravity of 1.118?• Grams =...
DOUBLE CHECK• DOES THE ANSWER MAKE  SENSE?• If the SG is 1.118, the substance is  • Heavier than water?  • Lighter than wa...
CONCENTRATIONS   Important Calculation Concepts                 20
PERCENTAGEPREPARATIONS  Weight in volume  • Amount of grams in 100mL of solution  Volume in volume  • Amount of mL in 100m...
EXAMPLE         Weight in Volume   How many grams of dextrose are required to prepare              4000 mL of a 5% solutio...
ALLIGATION• Alligation is a method of solving  problems that involves the mixing  of solutions or substances with  differe...
EXAMPLE              ALLIGATION  What is the percentage of zinc oxide in an ointment prepared by mixing 200 grams of a 10%...
EXAMPLE   ALLIGATION ALTERNATE A pharmacist needs to prepare 50 mL of 3% hydrogen peroxide solution. He has 30% and 1.5% s...
EXAMPLE  ALLIGATION ALTERNATEA pharmacist needs to prepare 50 mL of 3% hydrogenperoxide solution. He has 30% and 1.5% solu...
EXAMPLE     ALLIGATION ALTERNATE   A pharmacist needs to prepare 50 mL of 3% hydrogen   peroxide solution. He has 30% and ...
ELECTROLYTE SOLUTIONS Important Calculation Concepts               28
MILLIEQUIVALENTS• Unit of measure related to the total number of ionic  charges in a solution  • Measures the chemical act...
MILLIEQUIVALENTS   Ion        Abbr.    Valence   Atomic   Milliequivalent                                 Weight        we...
EXAMPLE   What is the concentration, in milligrams per milliliter, of a          solution containing 2mEq of NaCl per mL?•...
MOLARITY• Millimoles  • 1 mol = molecular weight in grams, therefore…  • mmol = molecular weight in milligrams• Molarity i...
OSMOLARITY• mOsmoles represent the number of particles in a  solution when the substance dissociates  • NaCl = 2 mOsmol  •...
CLINICALLABORATORY TESTS    Important Calculation Concepts                  34
CALCIUM-ALBUMIN• Almost 50% of calcium is bound to plasma proteins• If levels of proteins are low, then the serum calcium ...
SODIUM-GLUCOSE• In patients with hyperglycemia, glucose does not enter  the cell, causing a shift of fluid from intracellu...
INFUSION FLOW    RATES  Important Calculation Concepts                37
EXAMPLE     A physician orders 2 liters of D5W to be administered   over 8 hours. The IV administration set in your pharma...
TOTAL PARENTERAL    NUTRITION   Important Calculation Concepts                 39
TOTAL PARENTERAL           NUTRUTION• Provides a patient with all  nutritional requirements• Composition:  • Fluids  • Car...
TPN HIGHLIGHTS• Fluid Requirements:  • Generally 30 – 40 mL/kg/day  • Calculated: 1500 mL + 20mL (kg over 20)• Protein Req...
TPN HIGHLIGHTS                     Component            Units• Each TPN  component          Dextrose             3.4 kcal ...
PHARMACOKINETICS    Important Calculation Concepts                  43
PHARMACOKINETCS• Pharmacokinetics describes what happens to a drug  or substance inside of the body         Absorption    ...
FIRST-ORDER KINETICS• The amount of drug given is proportional to the change  in concentration  • The change in drug conce...
HALF-LIFE• The half-life (t1/2) is the time required for the  concentration of the drug to decrease by one-half  • t1/2 = ...
EXAMPLE   A patient is receiving an antibiotic for the treatment of a  respiratory infection. The initial concentration of...
QUESTIONS?    48
THANK YOU!• Joy A. Awoniyi• Email your questions, comments or concerns to  Joy.awoniyi@va.gov                        49
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Basic Pharmacy Calculations and Pharmacokinetes

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This was a lecture presented to the Davie campus of Florida A&M University College of Pharmacy P4 students. The lecture was presented during their final semester, at a course geared towards NAPLEX preparation. During the lecture, students wer asked to participate. We worked through each problem together and questions were encouraged.

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Basic Pharmacy Calculations and Pharmacokinetes

  1. 1. Joy A. Awoniyi, PharmD. PGY1 Pharmacy Practice Resident Miami VA Healthcare SystemFor The FAMU COPPS Class of 2013
  2. 2. Objectives• To emphasize the importance of calculations• To review important calculation concepts related to • Patient Parameters • Measurements • Concentrations • Electrolyte Solutions • Clinical Laboratory Tests • IV infusions flow rates • Parenteral Nutrition• To practice using the reviewed calculation concepts• To review important pharmacokinetic concepts 1
  3. 3. WHY SHOULD YOU CARE? • You can’t escape it • You need it to pass the NAPLEX • It can be applied to any pharmacy-related specialty • It may save your patient’s money • It may save your patient’s life (and your license) 2
  4. 4. PATIENTPARAMETERS Important Calculation Concepts 3
  5. 5. BSA and IBW• Body Surface Area (BSA), m2 • BSA = Weight (kg)0.425 x Height (cm)0.725 x 0.007184 • BSA = cm ´ kg 3600• Ideal Body Weight, kg • Males: 50 +( 2.3 x inches greater 60) • Females: 45.5 + (2.3 x inches greater than 60) 4
  6. 6. BODY MASS INDEX (BMI)• Measure of body fat to help categorize patients and assess risk of morbidity for certain diseases• BMI = weight(kg) ÷ [height (m)2] BMI Classification < 18.5 Underweight 18.5 to <25 Normal 25 to <30 Overweight >30 Obese 5
  7. 7. Creatinine Clearance (CrCl)• Formula used to estimate renal function• Not useful when renal function is fluctuating rapidly• Used in the dosing of several medications • Tamiflu, Antibiotics, Low-molecular weight heparins CrCl (mL/min) Interpretation(140 - Age) ´ IBW 60 to 90 30 to < 60 Mild renal impairment Moderate Renal impairment 72´ SCr 15 to <30 Severe Renal impairment Multiply by 0.85 if female < 15 Kidney failure 6
  8. 8. EXAMPLE Mack “Big Mack” Donald is a 55 year old man recently admitted to your service after choking on a Zebra Cake. He is 6’2” and 980 pounds. Calculate his BSA and BMI. 7
  9. 9. EXAMPLE Body Surface Area Body Mass Index• Height in centimeters: 6’2” = 74 • Height in meters = 1.88 m inches • 74 inches x 2.54 cm/inch = • Answer: 126 187.96 cm • How do we classify Big Mack’s• Weight in kilograms: 980 lbs BMI? • 980 lbs x 1 kg/2.2lbs = 445.45 kg • Underweight • Normal• Answer: 4.82 • Overweight • Obese 8
  10. 10. EXAMPLE Mack had a DVT during admission. The doctors wish to start him on Enoxaparin (Lovenox), a renally dosed medication. His SCr is 2.3. Calculate his IBW and Creatinine Clearance 9
  11. 11. EXAMPLE Ideal Body Weight Creatinine Clearance• Inches over 60 • Use IBW of 82.2kg • 74 – 60 = 14 inches • Answer: 42.19 mL/min• IBW = 50kg + 2.3 (14) • Dose of Lovenox is 50% if CrCl is less• Answer: 82.2 kg than 30 mL/min 10
  12. 12. MEASUREMENTS Important Calculation Concepts 11
  13. 13. The Basics Of Measurement• Irrelevant information is often given• Fundamental information is often left out • How many grams are in a kilogram? • How many milliliters are in a fluid ounce?• Start each problem by assessing what you know and what you want to know• Double check! 12
  14. 14. EXAMPLE A cough syrup contains 10mg of dextromethorphan per 5 mL. Your pharmacy has 6 bottles left in stock. Each bottle contains 120mL of the syrup. How many grams of the drug are in one bottle?• What information is irrelevant?• What fundamental information do you need to know to complete this problem?• What do you know?• What do you want to know? 13
  15. 15. EXAMPLE A cough syrup contains 10mg of dextromethorphan per 5 mL. Your pharmacy has 6 bottles left in stock. Each bottle contains 120mL of the syrup. How many grams of the drug are in one bottle? 120mL/ 1 1 0.24 grams/ 110mg/5mL bottle gram/1000mg bottle 14
  16. 16. Specific Gravity Definition Equations• A ratio of the weight of any • Grams = mL x SG substance in relation to the weight of an equal volume of • mL = Grams ÷ SG water • SG = Grams ÷ mL• Water is used because 1g of • Do not confuse with density or water is equal to 1mL concentration • If SG > 1 the substance is heavier than water • If SG < 1 the substance is lighter 15
  17. 17. EXAMPLE There is 10 grams of glycerin in a 500mL solution. 300 mL of glycerin weighs 165 grams. What is the specific gravity of glycerin?• Specific Gravity = grams / mL • 10 g / 500mL ? • 165 g/ 300mL ?• Think of SG as a conversion factor • 10 grams of glycerin does not EQUAL 500 mL of glycerin • 165 grams of Glycerin EQUALS 300 mL of glycerin 16
  18. 18. EXAMPLE There is 10 grams of glycerin in a 500mL solution. 300 mL of glycerin weighs 165 grams. What is the specific gravity of glycerin? 165 300 0.55grams mL 17
  19. 19. EXAMPLE What is the weight, in grams of a 2 fluid ounces of a liquid with a specific gravity of 1.118?• Grams = mL x SG 30mL/ 67.08 2 fl oz 1.118 1 fl oz grams 18
  20. 20. DOUBLE CHECK• DOES THE ANSWER MAKE SENSE?• If the SG is 1.118, the substance is • Heavier than water? • Lighter than water?• 2 fluid ounces of water (60mL) = 60 grams• The answer is 67.08 grams which is > 60 grams• Any answer below 60 would be WRONG 19
  21. 21. CONCENTRATIONS Important Calculation Concepts 20
  22. 22. PERCENTAGEPREPARATIONS Weight in volume • Amount of grams in 100mL of solution Volume in volume • Amount of mL in 100mL of solution Weight in weight • Amount of grams in 100 grams of substance 21
  23. 23. EXAMPLE Weight in Volume How many grams of dextrose are required to prepare 4000 mL of a 5% solution?• 5% Dextrose by definition means 5 grams in 100mL 54000mL grams/ 200 grams 100mL 22
  24. 24. ALLIGATION• Alligation is a method of solving problems that involves the mixing of solutions or substances with different percentage strengths• Alligation alternate is a method to calculate the number of parts of two or more components of different strengths mixed to prepare a desired strength. 23
  25. 25. EXAMPLE ALLIGATION What is the percentage of zinc oxide in an ointment prepared by mixing 200 grams of a 10% ointment, 50 grams of a 20% ointment, and 100 grams of a 5% ointment 0.10 x 200 g = 20 grams 0.20 x 50 g = 10 grams 0.05 x 100 g = 5 grams 350 g 35 grams35 grams ÷ 350 grams = 0.10 x 100% = 10% 24
  26. 26. EXAMPLE ALLIGATION ALTERNATE A pharmacist needs to prepare 50 mL of 3% hydrogen peroxide solution. He has 30% and 1.5% solutions in stock. How many mL of each should he use? • A– C =Y Percent Percent ProportionsAvailable Desired required • C–B=X A X • X and Y are C proportions of A and B (respectively) B Y needed for the entire preparation 25
  27. 27. EXAMPLE ALLIGATION ALTERNATEA pharmacist needs to prepare 50 mL of 3% hydrogenperoxide solution. He has 30% and 1.5% solutions in stock. How many mL of each should he use? Percent Percent ProportionsAvailable Desired required • 1.5 parts of 30% 30% 1.5 • 27 parts of 1.5% 3% • Total Parts: 28.5 1.5% 27 26
  28. 28. EXAMPLE ALLIGATION ALTERNATE A pharmacist needs to prepare 50 mL of 3% hydrogen peroxide solution. He has 30% and 1.5% solutions in stock. How many mL of each should he use?• We need 1.5 of all 28.5 parts to contain 30% Hydrogen peroxide • 1.5/28.5 = x/50mL • X = 2.63mL of 30%• We need 27 parts of all 28.5 parts to contain 3% Hydrogen peroxide • 27/28.5 = x/50mL • X = 47.37mL of 1.5% 27
  29. 29. ELECTROLYTE SOLUTIONS Important Calculation Concepts 28
  30. 30. MILLIEQUIVALENTS• Unit of measure related to the total number of ionic charges in a solution • Measures the chemical activity of an electrolyte relative to 1mg of hydrogen• 1 mEq represents the milligrams equal to its equivalent weight, taking into account the valency mg ´ valence mEq ´ Molecular.Weight mEq = mg = Molecular.Weight Valence 29
  31. 31. MILLIEQUIVALENTS Ion Abbr. Valence Atomic Milliequivalent Weight weightAluminum Al3+ 3 27 9Magnesium Mg2+ 2 24 12 Sodium Na+ 1 23 23Bicarbonate HCO32- 2 60 ??? 30
  32. 32. EXAMPLE What is the concentration, in milligrams per milliliter, of a solution containing 2mEq of NaCl per mL?• Molecular weight of Sodium = 23 2mEq ´ 58.3• Molecular weight of Chloride = 35.5 1• Molecular weight of Sodium chloride = 58.5• Valency = 1 (Na+, Cl+) mEq´ Molecular.Weight 2mEq ´ 58.5mg = Valence 1 31
  33. 33. MOLARITY• Millimoles • 1 mol = molecular weight in grams, therefore… • mmol = molecular weight in milligrams• Molarity is the number of mmols in a solution How many milligrams would 3 mmol of monobasic sodium phosphate (MW = 138) weigh? 138 mg/3 mmol 414 mg 1 mmol 32
  34. 34. OSMOLARITY• mOsmoles represent the number of particles in a solution when the substance dissociates • NaCl = 2 mOsmol • CaCl2 = 3 mOsmol • Anhydrous dextrose = 1 mOsmol• Osmolality is the milliosmoles of solute in a solution 1mmol mOsmol = mg ´ Molecular.Weight(mg) 33
  35. 35. CLINICALLABORATORY TESTS Important Calculation Concepts 34
  36. 36. CALCIUM-ALBUMIN• Almost 50% of calcium is bound to plasma proteins• If levels of proteins are low, then the serum calcium may be inaccurate• If albumin is low, calcium will appear to be low, when it levels are actually within normal limits • Correct levels when Albumin is less than 4 g/dL• Corrected Ca2+ = Ca2+ + 0.8 [4 – albumin(g/dL)] 35
  37. 37. SODIUM-GLUCOSE• In patients with hyperglycemia, glucose does not enter the cell, causing a shift of fluid from intracellular to extracellular• The shift of fluid dilutes the concentration of sodium in the extracellular fluid• This type of hyponatremia (translational) does not need to be treated, instead control of glucose levels is indicated• Corrected Na+ = Na+ + 0.016(Serum Glucose – 100) 36
  38. 38. INFUSION FLOW RATES Important Calculation Concepts 37
  39. 39. EXAMPLE A physician orders 2 liters of D5W to be administered over 8 hours. The IV administration set in your pharmacy delivers 10 drops/mL. How many drops per minute should the patient be set to receive?2 Liters/ 1000mL/ 1 hour/ 10 drops/ 42 drops/8hours 1 Liter 60 min 1mL min 38
  40. 40. TOTAL PARENTERAL NUTRITION Important Calculation Concepts 39
  41. 41. TOTAL PARENTERAL NUTRUTION• Provides a patient with all nutritional requirements• Composition: • Fluids • Carbohydrates (Dextrose) • Protein (Amino acids) • Fats • Vitamins, minerals, trace elements • Electrolytes 40
  42. 42. TPN HIGHLIGHTS• Fluid Requirements: • Generally 30 – 40 mL/kg/day • Calculated: 1500 mL + 20mL (kg over 20)• Protein Requirements • Ambulatory: 0.8 – 1 g/kg/day • Hospitalized: 1.2 – 2 g/kg/day• Non-Protein Requirements (Total Energy Expenditure) • Male: 66.47 + 13.75(kg) + 5.0(cm) – 6.76 (yrs) • Female: 655.1 + 9.6(kg) + 1.85(cm) – 4.68(yrs) • Multiply by 1.2 if confined to a bed, and 1.3 if out of bed 41
  43. 43. TPN HIGHLIGHTS Component Units• Each TPN component Dextrose 3.4 kcal per gram provides a defined Amino Acids 4 kcal per gram amount of calories Lipids 9 kcal per gram• COMMIT THESE 10% lipid emulsion 1.1 kcal per mL TO MEMORY!! 20% lipid emulsion 2 kcal per mL 42
  44. 44. PHARMACOKINETICS Important Calculation Concepts 43
  45. 45. PHARMACOKINETCS• Pharmacokinetics describes what happens to a drug or substance inside of the body Absorption Distribution Metabolism Excretion 44
  46. 46. FIRST-ORDER KINETICS• The amount of drug given is proportional to the change in concentration • The change in drug concentration with respect to time will create a rate constant (k) dC -k = dt• C = C0e-kt 45
  47. 47. HALF-LIFE• The half-life (t1/2) is the time required for the concentration of the drug to decrease by one-half • t1/2 = 0.693/k (k is the rate constant) A patient is receiving an antibiotic for the treatment of a respiratory infection. The initial concentration of the drug was 17.9mg/L. The drug has an elimination half-life of 2 hours. How much of the drug is present after 8 hours? 46
  48. 48. EXAMPLE A patient is receiving an antibiotic for the treatment of a respiratory infection. The initial concentration of the drug was 17.9mg/L. The drug has an elimination half-life of 2 hours. How much of the drug is present after 8 hours?• C = C0e-kt SOLVE FOR “C”• t1/2 = 0.693/k (k is the rate constant) • 2h = 0.693/k • k = 0.347h-1• C = 17.90e-0.347(8) = 1.11mg/L 47
  49. 49. QUESTIONS? 48
  50. 50. THANK YOU!• Joy A. Awoniyi• Email your questions, comments or concerns to Joy.awoniyi@va.gov 49

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