The creatinine clearance (CrCl) is used to estimate the glomerular filtration rate (GFR). It’s hard to directly measure the GFR, so we use CrCl –something we can measure–to estimate the GFR. Creatinine is derived from the metabolism of creatine in skeletal muscle and from dietary meat. It is released into the blood at a fairly constant rate and has a stable plasma concentration. Just like inulin, creatinine is freely filtered across the glomerulus and is neither reabsorbed nor metabolized by the kidneys. However, tubular secretion in the proximal tubule accounts for about 10 to 20 % of urinary creatinine in people with a normal GFR and a progressively higher percentage as the GFR falls. The net result is a progressive overestimation of the GFR with more severe kidney disease.
If the percentage of creatinine that is secreted is ignored, then all of the filtered creatinine (equal to the product of the GFR and the serum creatinine concentration [SCr]) will be excreted (equal to product of the urine creatinine concentration [UCr] and the urine flow rate or volume [V]). Thus:
GFR x SCr = UCr x V
GFR = [UCr x V]/SCr
CrCl = [UCr x V]/SCr. This formula is called the creatinine clearance and tends to exceed the true GFR by 10 to 20 percent or more depending upon the proportion of urinary creatinine that is derived from tubular secretion. For more, see this article
Formulas like the Cockcroft-Gault Equation allow us to estimate GFR without using a 24-hour urine collection by just using their sex, weight, age, and serum creatinine.