Career Corner: Pharmaceutical Calculations for Technicians Ashlee Mattingly, PharmD, BCPS
Disclosure I have no actual or potential conflict(s) of interest in relation to this program.
Learning Objectives 1. Accurately calculate a patient s body mass index (BMI), ideal and adjusted body weight, creatinine clearance (CrCl), and body surface area (BSA). 2. Utilize CrCl and BSA to calculate patient specific dosing for various medications. 3. Identify four methods used to express concentration and utilize these concentrations to calculate dosing information.
Useful Equivalents Equivalent Lengths Equivalent Volumes Equivalent Weights 1 inch = 2.54 cm 1 tsp = 5 ml 1 lb = 454 g 1 m = 38.37 in 1 tbsp = 15 ml 1 kg = 2.2 lb 1 fl oz = 29.6 ml 1 gr = 64.8 mg 1 pint = 473 ml 1 quart = 946 ml 1 gallon = 3785 ml
Fundamentals Proportions Two fractions that are set equal to each other One variable is unknown and labeled x Method 1 Match both numerators to each other and both denominators to each other 176 lb 2.2 lb = x kg 1 kg Method 2 Match numerator and denominator of each fraction 176 lb 2.2 lb = x kg 1 kg
Fundamentals Dimensional Analysis Allows multiple proportion calculations to be completed quickly Diagonal units that are the same can be canceled out 176 lb 1 kg 2.2 lb
Body Mass Index (BMI) Measure of body fat based on height and weight Units = kilogram (kg)/meter (m) 2 BMI = Remember weight (kg) [height (m)] 2 1 meter = 39.37 inches 1 kilogram = 2.2 pounds OR weight (lb) [height (in)] 2 704. 5 BMI Categories BMI (kg/m 2 ) Underweight < 18.5 Normal weight 18.5 24.9 Overweight 25 29.9 Obese > 30
Example Calculate the BMI of a patient weighing 182 lb and 5 5 in height. BMI = 182 lb 1 kg 2.2 lb weight (kg) [height (m)] 2 5 5 = 65 in 1 m 39.37 in BMI = = 82. 7 kg = 1. 65 m 82.7 kg (1.65 m) 2 = 30. 4 kg/m2 BMI = BMI = weight (lb) [height in ] 2 704.5 182 lb (65 in) 2 704.5 = 30. 3 kg/m2
Ideal and Adjusted Body Weight If a patient is under or over weight, actual body weight may not be the best measure to dose a medication Ideal body weight (IBW) The ideal body weight based on height and gender Units = kg IBW male = 50 kg + 2.3 kg IBW female = 45.5 kg + 2.3 kg Adjusted body weight (ABW) # of inches > 5 feet Accounts for the actual (TBW) and ideal body weights Units = kg # of inches > 5 feet ABW = IBW + 0.4(TBW IBW)
Example IBW Calculate the ideal body weight for a male patient weighing 164 lb and measuring 68 inches in height. Step 1: Convert lb to kg IBW male = 50 kg + 2.3 kg 164 lb 1 kg 2.2 lb Step 2: Determine # of inches > 5 feet 68 in = 5 8 Step 3: Plug into IBW equation for a male # of inches > 5 feet = 74. 5 kg IBW male = 50 kg + 2.3 kg 8 = 68. 4 kg
Example IBW Calculate the ideal body weight for a female patient weighing 60 kg and measuring 160 cm in height. IBW female = 45.5 kg + 2.3 kg Step 1: Convert cm to inches 160 cm 1 inch 2.54 cm Step 2: Determine # of inches > 5 feet 63 inches = 5 3 Step 3: Plug into IBW equation for a female IBW female = 45.5 kg + 2.3 kg # of inches > 5 feet = 63 inches 3 = 52. 4 kg
Example ABW Calculate the adjusted body weight of a male patient weighing 248 lb and measuring 188 cm in height. ABW = IBW + 0.4(TBW IBW) Step 1: Convert lb to kg Step 2: Convert cm to inches 248 lb 1 kg 2.2 lb = 112. 7 kg 188 cm 1 inch = 74 inches 2.54 cm Step 3: Determine # of inches > 5 feet 74 inches = 6 feet 2 inches = 14 inches > 5 feet Step 4: Calculate IBW IBW male = 50 kg + 2.3 kg IBW male = 50 kg + 2.3 kg # of inches > 5 feet 14 = 82. 2 kg Step 5: Plug into ABW equation ABW = IBW + 0.4(TBW IBW) ABW = 82.2 + 0.4 112.7 82.2 = 94. 4 kg
Body Surface Area (BSA) The calculated surface of a person s body May be a more accurate measure of metabolic mass since it is less affected by irregular amounts of fat tissue Units = m 2 BSA m 2 = ht cm wt(kg) 3600
Example BSA Calculate the BSA for a patient weighing 124 lb and measuring 5 3 in height. Step 1: Convert weight to kg Step 2: Convert height to cm Step 3: Plug into BSA equation 124 lb 1 kg 2.2 lb 5 3 = 63 inches = 56. 4 kg 2.54 cm 1 inch = 160 cm BSA m 2 = ht cm wt(kg) 3600 BSA m 2 = 160 cm 56.4 kg 3600 = 1. 58 m 2
Renal Function The kidneys play an important role in the absorption, distribution, elimination, and metabolism of various medications If a patient has kidney dysfunction, whether acute or chronic, this may lead to a decrease in efficacy of the drug or an increase in toxicity Some medications require dose adjustments if a patient has kidney dysfunction Creatinine clearance (CrCl) is one method used to estimate kidney function
Renal Function Creatinine is a waste product from normal muscle breakdown and is predominately eliminated by the kidneys If the kidneys are not working properly, creatinine can build up in the blood Normal = 0.7 1.4 mg/dl Elevated SCr Kidney damage or failure, infection, reduced blood flow Dehydration Muscle problems Decreased SCr Conditions affecting the muscles (i.e. multiple sclerosis) Malnutrition Age Serum creatinine (SCr) Blood test to measure the amount of creatinine in the body
Creatinine Clearance Cockcroft Gault Equation Units = ml/min CrCl = 140 age weight (kg) 72 SCr ( mgτdl) 0.85 if female If SCr is < 0.5 mg/dl then creatinine clearance is OVER estimated If TBW is < IBW, then use TBW If TBW is 120 130% > than IBW, then use ABW Normal values Men = 97 137 ml/min Women = 88 128 ml/min
Example Calculate the creatinine clearance of an 82 year old female weighing 105 lb, 5 1 in height, with a SCr of 1.4 mg/dl. Step 1: Determine which weight to use 105 lb 1 kg 2.2lb = 47. 7 kg IBW = 45.5 + 2.3 inches > 5 IBW = 45.5 + 2.3 Step 2: Plug into CrCl equation CrCl = CrCl = 1 = 47. 8 kg 140 age weight (kg) 72 SCr ( mgτdl) 140 82 47.7 kg 72 1.4 mgτdl 0.85 0.85 = 23 ml/min
Example Calculate the creatinine clearance of a 47 year old male weighing 204 lb, measuring 5 10 in height, with a SCr of 1 mg/dl. Step 1: Determine which weight to use 204 lb 1 kg = 92. 7 kg 2.2lb IBW = 50 + 2.3 inches > 5 IBW = 50 + 2.3 92.7 kg 73 kg 10 = 73 kg 100% = 127%, use ABW Step 2: Plug into CrCl equation ABW = 73 + 0.4 92.7 73 = 81 kg CrCl = 140 age weight (kg) 72 SCr ( mgτdl) CrCl = 140 47 81 kg 72 1 mgτdl = 105 ml/min
Concentration concentration = quantity of solute quantity of preparation Weight-in-volume (w/v) Weight-in-weight (w/w) Volume-in-volume (v/v)
Concentration Percent Strength percent strength = Weight-in-volume = g/100 ml Weight-in-weight = g/100 g Volume-in-volume = ml/100 ml Ratio Strength 1 unit of solute:total amount of preparation quantity of solute 100 units of preparation 1:25 = 1 part in 25 parts Weight-in-volume = g/ml Weight-in-weight = g/g Volume-in-volume = ml/ml
Example Epinephrine is available as a concentration of 1:10,000. How many milliliters are needed to provide a dose of 4 mg? 1 unit of solute:total amount of preparation 1 g epinephrine:10,000 ml product 1 g 4 mg = 0.004 g 1000 mg 1 g 10,000 ml = 0.004 g x ml 1 g x ml = 0.004 g 10,000 ml x ml = 0.004 g 10,000 ml 1 g = 40 ml
Concentration Parts per million (PPM) and parts per billion (PPB) Used for very dilute products Number of parts of the drug per 1 million (or 1 billion) parts of the whole Weight-in-weight = g/g Weight-in-volume = g/ml Volume-in-volume = ml/ml
Example If a sample of a solution is found to contain 1.4 ppm of lead, how much of the solution will contain 50 μg of lead? 50 μg 1 g 1,000,000 μg = 0.00005 g 1.4 parts 1,000,000 parts = 0.00005 g x ml x = 35. 7 ml
Example The pharmacy receives an order for Zosyn for a 72 year old male patient with nosocomial pneumonia. He is 6 2 tall, weighs 258 lb, and has a SCr of 1.8 mg/dl. Zosyn must be renally adjusted as follows: CrCl > 40 ml/min: 4.5g IV q6 hours CrCl 20 40 ml/min: 3.375 g IV q6 hours CrCl < 20 ml/min: 2.25 g IV q6 hours What dose should this patient receive?
Example Step 1: Calculate CrCl Age = 72 Height = 6 2 Weight = 258 lb SCr = 1.8 mg/dl CrCl = 140 age weight (kg) 72 SCr ( mgτdl) 258 lb 1 kg 2.2 lb = 117 kg IBW = 50 + 2.3 inches > 5 IBW = 50 + 2.3 117 kg 82 kg 14 = 82 kg 100% = 143%, use ABW ABW = IBW + 0.4(TBW IBW) ABW = 82 + 0.4 117 82 = 96 kg
Example Step 1: Calculate CrCl Age = 72 Wt = 96 kg SCr = 1.8 mg/dl CrCl = CrCl = 140 age weight (kg) 72 SCr ( mgτdl) 140 72 96 72 1.8 CrCl > 40 ml/min: 4.5g IV q6 hours CrCl 20 40 ml/min: 3.375 g IV q6 hours CrCl < 20 ml/min: 2.25 g IV q6 hours Dose = 4.5 g IV q6 hours = 50 ml/min
Example JS is a 75 year-old female scheduled to receive cisplatin 80mg/m 2. She weighs 185 lb and is 5 9 in height. What dose of cisplatin should she receive? Step 1: Calculate BSA BSA m 2 = ht cm wt(kg) 3600 185 lb 1 kg 2.2 lb 5 9 = 69 inches 2.54 cm 1 inch = 84 kg = 175. 26 cm BSA m 2 = 175.26 84 3600 = 2. 02 m 2
Example JS is a 75 year-old female scheduled to receive cisplatin 80mg/m 2. She weighs 185 lb and is 5 9 in height. What dose of cisplatin should she receive? Step 2: Calculate dose 80 Τ mg m 2 2.02 m 2 = 161. 6 mg
Example The dose of cisplatin is typically reduced if the CrCl is less than 60 ml/min. If JS has a SCr of 1.5 mg/dl, would her dose need to be reduced? Height = 5 9 Age = 75 TBW = 185 lb = 84 kg CrCl = 140 age weight (kg) 72 SCr ( mgτdl) 0.85 Should we use TBW or ABW? IBW female = 45.5 kg + 2.3 kg IBW female = 45.5 kg + 2.3 kg # of inches > 5 feet 9 = 66. 2 kg 84 kg 100% = 127%, use ABW 66.2 kg ABW = IBW + 0.4(TBW IBW) ABW = 66.2 + 0.4 84 66.2 = 73. 3 kg
Example The dose of cisplatin is typically reduced if the CrCl is less than 60 ml/min. If JS has a SCr of 1.5 mg/dl, would her dose need to be reduced? CrCl = CrCl = 140 age weight (kg) 72 SCr ( mgτdl) 140 75 73.3 72 1.5 0.85 0.85 = 37. 5 Τ ml min YES, the dose should be reduced
Example The pharmacist determines that the dose of cisplatin for JS is 80 mg. Cisplatin comes in 0.1% vials. How many milliliters of cisplatin are needed to equal the dose for JS? 80 mg 0.1% = 0.1 g 100 ml 1 g 1000 mg 0.1 g 100 ml = 0.08 g x ml = 0.08 g 0.1 g x ml = 100 ml 0.08 g x ml = 100 ml 0.08 g 0.1 g = 80 ml
Questions?