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Pharmacy calculations are very important for prescribing, especially in regards to renal function and pediatric dosing. This assignment will focus on calculating renal function and medication doses for pediatric patients. Review the information below and then follow the subsequent instructions.

Upon successful completion of the course material, you will be able to:

  • Successfully calculate dosing for renal functions

Background Information

Renal function

Renal function can be calculated different ways but the most common ways are using the Cockcroft-Gault and CKD-EPI equations (see below). If adjustments to dosing are required due to decreased renal function, it is important to know which equation should be used. This can be found in a drug reference or the package insert for a particular medication. In general, if renal cutoffs are expressed as mL/min the Cockcroft-Gault equation should be used but if renal cutoffs are expressed in mL/min/1.73m2 then the CKD-EPI equation should be used. Different equations are necessary to calculate renal function in the pediatric population.

The Cockcroft-Gault equation is as follows:

  • CrCl = [(140-age) (ideal body weight)]/[(72) (SCr)] x 0.85 (if female)
    • age is expressed in years
    • weight is expressed in kg [ideal body weight (IBW) should be used except for obese patients, in which case adjusted body weight is typically used]
    • SCr is serum creatinine expressed in mg/dL
    • IBW (male) = 50kg + 2.3 kg for each inch of height over 5 feet
    • IBW (female) = 45.5kg + 2.3 kg for each inch of height over 5 feet
    • Adjusted body weight = IBW + [0.4 x (actual body weight – IBW)]

The CKD-EPI equation is as follows:

  • eGFR = 141 x min(SCr/κ, 1)α x max(SCr /κ, 1)-1.209 x 0.993Age x 1.018 [if female] x 1.159 [if Black]
    • eGFR (estimated glomerular filtration rate) = mL/min/1.73 m2
    • SCr is serum creatinine expressed in mg/dL
    • κ = 0.7 (females) or 0.9 (males)
    • α = -0.329 (females) or -0.411 (males)
    • min = indicates the minimum of SCr/κ or 1
    • max = indicates the maximum of SCr/κ or 1
    • age = years

Pediatric Dosing

In general, medication dosing for pediatric patients is based on weight. This makes it particularly important to be able to calculate doses appropriately. The most basic calculation is dose (ie. mg/kg) times weight (in kg).

Example

  • Cefdinir (Omnicef®) for acute otitis media is 14 mg/kg/day in 1-2 divided doses
    • What would the recommended dose be for a 22 lb patient?
    • First, lbs must be converted to kg.
      • 22 lb x (1 kg/2.2 lb) = 10 kg
    • Now, the dose can be calculated.
      • 14 mg/kg x 10 kg = 140 mg
    • Based on the dosing recommendation above, this could be given 2 different ways:
      • 140 mg once daily
      • 70 mg two times daily

For liquids or suspensions, medication doses are often given as the volume to be given to the patient. One must be aware of the available concentrations for a particular medication.

Example

  • Cefdinir (Omnicef®) is available in the following concentrations:
    • 125 mg/5 mL
    • 250 mg/5 mL
  • Using the example above, how many mL would be needed for 140 mg daily?
    • [140 mg / 125 mg] x 5 mL = 5.6 mL
    • [140 mg / 250 mg] x 5 mL = 2.8 mL
    • Often, we have to round to the nearest 0.5 mL to make it easier to administer using an oral syringe. Rounding the first option to 5.5 mL would give the patient 137.5 mg (5.5 mL x 125 mg/ 5 mL)

Instructions

  1. Print out the Calculations Quiz.
  2. Complete each question by hand and show your work.
  3. Scan the completed quiz and submit via the Assignment.

This assignment is worth 50 Points

Access the Assignment submission page

1. Using the Cockcroft-Gault equation, calculate creatinine clearance for the following patient: 70 year old female, 5’6” tall, 150 lb, SCr = 1.2 mg/dL. Round your answer to the nearest 1 mL/min.

2. A 1-year-old male weighing 28 lbs is diagnosed with acute otitis media and will be treated with amoxicillin/clavulanate (Augmentin®). The recommended dosing is 90 mg/kg/day, divided every 12 hours. How many mL of 600 mg/5 mL would be given with each dose? Round your answer to the nearest 0.5 mL.

3. A 7-year-old female weighing 55 lbs presents with a severe asthma exacerbation and will be treated with prednisolone (Orapred®). Recommended dosing is 1 mg/kg/day in 2 divided doses. How many milligrams would be prescribed per dose? Round your answer to the nearest mg.

4. A 10-year-old female weighing 80 lbs presents with a urinary tract infection and will be treated with sulfamethoxazole/trimethoprim (Bactrim®). Dosing for this medication is typically based on the trimethoprim component. Suspension is available as 200 mg of sulfamethoxazole and 40 mg of trimethoprim in every 5 mL. Dosing for this indication is trimethoprim 8 mg/kg/day divided every 12 hours. How many milliliters of sulfamethoxazole would be given with each dose? Round your answer to the nearest 0.5 mL.

5. A 6-year-old male weighing 50 lbs presents complaining of a sore throat and has a positive rapid strep test. He will be treated with amoxicillin (Amoxil®) with dosing of 50 mg/kg/day. How many milligrams/day would be prescribed? Round your answer to the nearest 5 mg.

6. From question #5 – how many mL would this be (using 400 mg/5 mL concentration)? Round your answer to the nearest 0.5 mL.

7. A 2 month old weighing 9 lbs presents with erosive esophagitis. This will be treated with ranitidine (Zantac®) 10 mg/kg/day in 2 divided doses. Ranitidine comes as a 15 mg/mL concentration. How many milliliters would be recommended for each dose? Round your answer to the nearest 0.1 mL.

8. From question #7 – how many mL would be needed for a 30 day supply? Round your answer to the nearest 5 mL but be sure to give adequate supply.

9. A mother of a 4 year old weighing 30 lbs calls your office asking for dosing recommendations for ibuprofen (Motrin®) for a fever. Recommended dosing is 5-10 mg/kg every 6-8 hours as needed. Ibuprofen suspension concentration is 100 mg/5 mL. What range, in milliliters, would be appropriate for this patient for each dose? Round your answer to the nearest 0.5 mL.

10. From question #9 – the mother decides that she would rather use acetaminophen (Tylenol®). Dosing recommendation is 10-15 mg/kg/dose and the suspension concentration is 160 mg/5 mL. What range, in milliliters, would be appropriate for this patient for each dose? Round your answer to the nearest 0.1 mL.

 
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