Drug Dosage Based on Creatinine Clearance
Kidneys play an important role in clearing (excreting) drugs. If kidney function is impaired, this will slow down the clearance of drugs. In patients with kidney impairment, it is important to determine how well the kidneys are working and then calculate the dose based on the kidney function. The most frequently used measure of kidney function is creatinine clearance. Creatinine is a breakdown product of muscle metabolism and is eliminated from the body through filtration by the kidneys. A decreased renal (kidney) function would result in decreased creatinine clearance.
Creatinine clearance (CrCl) can be determined using the Cockcroft-Gault Equation:
$$ CrCl \ (males) = { (140 - age \ in \ years) \times Weight \ (kg) \over 72 \times Serum \ creatinine ({mg \over dl})}$$
(use IBW unless ABW is less than IBW)
The predicted creatinine production rate in females is 85% of the calculated value because they have a smaller muscle mass per kilogram, and it is muscle mass that determines creatinine production. The equation for females is:
$$ CrCl \ (females) = {CrCl \ for \ males \times 0.85} $$
(use IBW unless ABW is less than IBW)
Cockcroft-Gault equation takes into account the age of the patient as well, since muscle mass decreases with age. The weight used in the above equations is IBW. IBW is used based on the rationale that creatinine comes from lean muscle, so IBW would give a closer estimation of creatinine clearance. In addition, for this equation, cancelling the units does not work, so this equation is an exception to the rule that units should cancel out.
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Solved Problem: Calculate the creatinine clearance of a 60-year-old, 140 lb, 5’5” female? Her measured serum creatinine is 1.2 mg/dl. Approach: : Calculate her ABW and IBW, and use the Cockcroft-Gault equation to calculate her creatinine clearance $$Female∶45.5 kg+ (2.3 kg)(5)=57 Kg $$ Her ABW is 64 Kgs, Use the IBW in the equation to calculate the creatinine clearance, $$ CrCl \ (females) = { (140 - 60) \times 57 \over 72 \times 1.2} \times 0.85$$ Answer: 44.9 ml/min | ||||||||
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Solved Example: For piperacillin (an antibiotic) standard dosing for patients is:
What dose should be given to the above patient? Approach: Use the creatinine clearance value from the above problem, compare it to the dosing chart provided in this problem. Answer: 4 g IV q6h |