## Essay: Dosing for renal functions

*Essay: Dosing for renal functions*

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

Print out the Calculations Quiz.

Complete each question by hand and show your work.

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