The National Kidney Foundation Kidney Disease Outcomes Quality Initiative defines chronic kidney disease (CKD) as the presence of kidney damage or a reduction in the glomerular filtration rate (GFR) for three months or longer.13 CKD can affect multiple organ systems and these pathophysiologic changes have been associated with profound alterations in the pharmacokinetics and pharmacodynamics of many drugs.
The gold standard to measure kidney function is the GFR. However, FDA-approved drug labels, which provide recommendations for adjustments of drug dosages for patients with impaired kidney function are, most commonly, based on the estimated creatinine clearance (eCrCl) determined by the Cockcroft-Gault equation.14 In the equation, eCrCl is expressed in mL/min, age in years, weight in kilograms, and serum creatinine (Scr) in mg/dL:
eCrCl = (140 - age x weight x 0.85 (for females) ÷ Scr x 72
The normal range for eCrCl for men and women 40 years of age or older is 107-139 mL/min and 87-107 mL/min, respectively. After 20 years of age, eCrCl is reduced by 6.5 mL/min every 10 years. In drug dosing, the eCrCl is used as a surrogate of GFR to determine renal function (Figure 2). The eCrCl is slightly higher than true GFR because, in addition to being filtered by the glomeruli, creatinine is also excreted by the proximal tubules.