Transcript Dosage Calculations - Idaho Society of Health

Dosage Calculations
Mark Wagner, PharmD
Kirollos Hanna, PharmD, BCPS
PGY-2 Oncology Resident
St. Luke’s Mountain States Tumor Institute
March 5th, 2016
Disclosures
• We do not have any conflicts of interest or disclosures to make
• No medications will be discussed off-label
Learning Objectives
• Demonstrate the importance of calculations in
pharmacy practice.
• Calculate patient specific parameters effecting
drug therapy.
• Solve dosage calculations for common
pharmaceuticals.
Pharmaceutical Calculations
Includes areas in:
• Chemical and physical properties of drugs
• Drug absorption, metabolism, distribution, and
excretion (Pharmacokinetics)
• Statistical data for trials
• Drug development and formulations
• Drug dosage, dosage regimens, and patient
compliance
• Pharmacoeconomics
Pharmaceutical Calculations
Ensures:
• Patient safety
• Appropriate dosing
• Prevent toxicity
• Ensure therapeutic benefits
• Appropriate admixtures
• Concentrations
• Ratios
• Proportions
• Cost Benefit
Approaching Calculations
1. Take the time to read and consider
computations.
2. Estimate the quantity and units of measure to
satisfy the question.
3. Perform calculations using methods for
efficiency and understanding.
4. Read the problem again and recheck
calculations.
5. Consider the reasonableness of the answer.
Patient Specific
Calculations
Dosage Calculations
Patient: SM
• SM is a 64 year old female patient (height 5’5’’, weight 205
pounds) hospitalized for nosocomial pneumonia which is
responding to treatment. Her current antibiotic medications
include ceftazidime and imipenem. Her physician wishes to
initiate vancomycin at a loading dose of 15 mg/kg and
monitored by pharmacy. Her morning lab values include:
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Na 140 mEq/L
K 4.0 mEq/L
BUN 60 mg/dL
Scr 2.7 mg/dL
Glucose 222 mg/dL
Height & Weight Calculations
• Pounds (lb) to Kilograms (kg)
• Ideal body weight
• Inches (In) to Centimeters (cm)
• Body surface area
• Kidney Function
Pounds to Kilograms
÷ 2.2
Example Problems
• How much would a 100 lb male weight in kg?
• How much would a 500 lb female weight in kg?
• How much does 1000 grams weight in kg?
Example Problems Answers
• How much would a 100 lb male weight in kg?
• 45.4545454545 kg
• How much would a 500 lb female weight in kg?
• 227.2727272727 kg
• How much does 1000 grams weight in kg?
• 1 kg!!!
Ideal Body Weight (IBW)
• Healthy weight for a person
• Useful for medications that do not distribute to fat
• IBW (male) = 50kg + [(2.3kg) (each inch above 5’)]
• IBW (female) = 45.5kg + [(2.3kg) (each inch above 5’)]
Calculate SM’s IBW (height 5’5’’)
Calculate SM’s IBW (height 5’5’’)
45.5 + [(2.3) x 5]
45.5 + 11.5
57 kg
Is SM a healthy weight?
Inches to Centimeters
X 2.54
Example Problems
• Convert the height of a 6’ male to centimeters.
• How many centimeters would a 3’5’’ female be?
• If an infant is 2’ & 5 cm, how many cm is he?
Example Problems Answers
• Convert the height of a 6’ male to centimeters.
• 182.88 cm
• How many centimeters would a 3’5’’ female be?
• 104.14 cm
• If an infant is 2’ & 5 cm, how many cm is he?
• 65.96 cm
Body Surface Area (BSA)1
BSA (m2) = 0.007184 x Height(cm)0.725 x
Weight(kg)0.425
Renal Function
• Poor renal function leads to decreased drug
clearance.
• Aids in appropriate dosing for toxicity prevention.
• No benefit for patients with End Stage Renal
Dysfunction (ESRD).
• Must have: Height, weight, age, and serum
creatinine (SCr)
Renal Function
• Creatinine Clearance vs. Glomerular Filtration Rate
• Creatinine Clearance (CrCl)
• Exceeds GFR
• Creatinine is secreted by the proximal tubule as well as filtered by
the glomerulus.
• Measured by blood tests and urine tests
• Glomerular Filtration Rate (GFR)2
• Cannot be measured directly
• Filtration marker (inulin, iothalamate or iohexol) is the gold standard
for measurement – not typically used in practice
• Creatinine has traditionally been used to estimate GFR
• Adjusted to BSA.
• Kidney function is proportional to kidney size, which is proportional to
body surface area.
KDOQI Stages3,4
Stage
1
GFR*
90+
Description
Treatment stage
Normal kidney function but urine
Observation, control of blood
findings or structural abnormalities or
pressure.
genetic trait point to kidney disease
2
60-89
Mildly reduced kidney function, and
other findings (as for stage 1) point to
kidney disease
3A
3B
45-59
30-44
Moderately reduced kidney function
Observation, control of blood
pressure and risk factors.
4
15-29
Severely reduced kidney function
Planning for endstage renal
failure.
5
<15 or on Very severe, or endstage kidney failure
dialysis
(sometimes call established renal
failure)
Observation, control of blood
pressure and risk factors.
Treatment choices.
Renal Function5
Creatinine Clearance
(140 – patient age) x Weight (kg)
72 x SCr (mg/dL)
Female = CrCl x 0.85
Example Problems
Female, 64 years old, 5’5’’, 205 pounds, Scr 2.7 mg/dL
1. Calculate SM’s renal function using the Cockcroft and Gault
equation and her actual body weight.
2. Calculate SM’s renal function using the Cockcroft and Gault
equation and her ideal body weight.
Example Problems Answers
1. Calculate SM’s renal function using the Cockcroft and Gault
equation and her actual body weight.
CrCl = 30.96 ml/min
2. Calculate SM’s renal function using the Cockcroft and Gault
equation and her ideal body weight.
CrCl = 18.94 ml/min
Drug Specific
Calculations
Dosage Calculations
Drug Dosing Differences
• Pediatrics
• WEIGHT & AGE BASED DOSING!!!
• Adults
• Standard flat dosing
• Atorvastatin
• Height/weight based dosing
• Gemcitabine
• Multi-step dosing
• Vancomycin
Pediatric Dosing
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Not Small Adults!!!!
Specific dosing for almost all drugs
Age and weight are crucial
Determine appropriate dosage form (i.e. liquid, tablet, etc.)
Acetaminophen
• 10-15 mg/kg every 6 hours as needed
• Steps to solve
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Determine patient’s weight
Multiply by 10-15 mg/kg
Determine route of administration
Calculate volume/dose for product
http://sunvalleypediatricdentistry.com/tag/arizona-pediatrics/
Acetaminophen Problem
• TW’s 3 year old son AW is currently in your pediatric unit after
having developed a fever and possible infection. He is having
a fever and the nurses have asked the physician to order
acetaminophen to bring the fever down. The physician gives a
verbal order to the nurse for acetaminophen pharmacy to
dose. The patient weighs 26 lbs and the standard dosing for
acetaminophen is 10-15 mg/kg every 6 hours as needed for
fever or pain. The patient is unable to swallow tablets so you
will also need to use the liquid formulation which is 160 mg/5
mL. What is the appropriate dose and volume you would
order for this child, assuming a 15 mg/kg dose?
Acetaminophen Answer
• Weight = 26 lbs =
11.8 kg
• Dose = 11.8 kg x 15 mg/kg =
177 mg
• Volume = (177 mg/160 mg) x 5 mL =
5.53 mL
177 mg/5.53 mL PO every 6 hours as
needed for fever
Flat, Height/Weight based, and Multi-Step
ADULT DOSING
Flat Dosing
• Similar effect seen in majority of patients
• Atorvastatin
• 10 mg, 20 mg, 40 mg, 80 mg
• Based on risk factors
• No need for calculations
http://iverson.cm.utexas.edu/courses/310N/MOTD%20Fl05/MOTDsp04/Atorvastatin.html
Height/Weight Based Dosing
• Patient’s height and weight determine how much drug is
required to have similar effect
• BSA!
• Chemotherapy
• Gemcitabine 1000 mg/m2
• Steps to solve
• Determine patient’s height and weight
• Calculate BSA = X m2
• Multiply 1000 mg x X m2
BSA Problem
• JN is a 73 year old male who presents with metastatic
pancreatic cancer. The patient wishes to undergo palliative
treatment with gemcitabine 1000 mg/m2 weekly x7 weeks
then off 1 week, followed by weekly ¾ weeks every 28 days.
Patient is 5’ 8” tall and weighs 168 lbs. His renal and hepatic
function are within normal limits and his labs are all okay to
move forward with treatment. What dose of gemcitabine
should he receive?
http://www.realclearsports.com/lists/dominance_single_events/masters_jack_nicklaus.html
BSA Answer
• Height = 5’ 8” = 68” =
172.7 cm
• Weight = 168 lbs =
76.4 kg
• BSA = 0.007184 x 172.70.725 x 76.40.425 =
1.9 m2
• Dose = 1.9 m2 x 1000 mg/m2 =
1900 mg
Multi-Step Dosing
• Requires the use of patient’s weight, height, renal function,
and possibly many other factors
• Antibiotics!
• Vancomycin
• Renal function, height, and weight
• Steps to solve
• Obtain patient’s height, weight, serum creatinine, causative
infection, other patient factors affecting drug distribution
• Calculate creatinine clearance
• Select appropriate dosing
• Use the above to determine dose and frequency of dose for a
given patient
Vancomycin Problem
• JS is a 21 year old who presents with a bad case of cellulitis in
his left calf after a recent golf tournament down in Florida.
The physician decides to use vancomycin for empiric gram +
coverage and requests that pharmacy doses it. The patient is
5’ 7” and weighs 154 lbs, and his SCr is 0.9 mg/dL. All other
labs are within normal limits. How would you dose the
vancomycin given the following chart and dosing of 15-20
mg/kg (rounded to the nearest 250 mg increment)?
St. Luke’s Regional Medical Center. Vancomycin Dosing Guidelines. Appendix E MM026 TV
Vancomycin Answer
• Height = 5’ 7” = 67 inches =
170.2 cm
• Weight = 154 lbs =
70 kg
• CrCl = ((140 – 21) x 70 kg)/(72 x 0.9 mg/dL) =
128 mL/min
• Dose = 15 – 20 mg/kg x 70 kg =
1050 mg – 1400 mg- middle ground ~1250 mg
• Frequency of dosing
• CrCl >100 and pt. <40 years old = every 8 hour
• Dose =
1250 mg IV every 8 hours
ADDITIONAL PROBLEMS
IV Fluids
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Determining appropriate electrolyte concentrations
Custom IV bags may be necessary for certain patients
Fluid restrictions
154 mEq NaCl in 1L of NS, 4 mEq NaCl in 1mL of 23.4% NaCl
Steps to create custom bag
• Determine quantity of substance needed for final concentration
to be correct
• Determine volume of product you need to add to give you this
concentration
IV Fluid Problem
• A physician orders a 500 mL D5W ¼ NS IV infusion for a
patient. Your pharmacy does not stock this formulation in a
500 mL version and the compounder is down so you must
make this by hand. You have D5W 500 mL bags and 23.4%
NaCl solution. What volume of NaCl needs to be added to
give you the appropriate product?
http://www.shopmedvet.com/category/iv-fluids
IV Fluid Answer
• NS = 0.9% NaCl = 154 mEq/L so ¼ NS = 38.5 mEq/L =
19.25 mEq/500 mL
• 23.4% NaCl =
4 mEq/mL
• 19.25 mEq/4 mEq/mL =
4.81 mL
Add 4.81 mL of 23.4% NaCl to a 500 mL bag
of D5W to obtain a final product of 500 mL
of D5W ¼ NS
Conclusion
• Dosage calculations affect every patients life
• Small mistakes can lead to big problems
• Be vigilant!
Questions
https://www.copera.org/resources/newsletters/elevate/summer-2015/expired-drugs-continue-use-ordiscard
References
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Du Bois D, Du Bois EF: A formula to estimate the approximate
surface area if height and weight be known. 1916. Nutrition 5:303311; discussion 312-313, 1989.
Stevens LA, Levey AS. Measured GFR as a confirmatory test for
estimated GFR. J Am Soc Nephrol. 2009;20(11):2305-2313.
National Kidney Foundation: K/DOQI clinical practice guidelines
for chronic kidney disease: Evaluation, classification, and
stratification. Kidney Disease Outcomes Quality Initiative. Am J
Kidney Dis. 2002;39(suppl 1):S1-266.
Levey AS, Coresh J, Balk E, et al. National Kidney Foundation
practice guidelines for chronic kidney disease: evaluation,
classification, and stratification. Ann Intern Med. 2003;139(2):137147.
Cockcroft DW, Gault MH. Prediction of creatinine clearance from
serum creatinine. Nephron. 1976;16(1):31-41