Creatinine clearance

In physiology, the creatinine clearance is a measure of kidney function in patients with chronic kidney disease.

Creatinine clearance and glomerular filtration rate
The creatinine clearance approximates the glomerular filtration rate. However, the creatinine clearance systematically overestimates the GFR due to excretion creatinine by the renal tubules. The correction factor is below:
 * $$\mbox{Glomerlular filtration rate} = \mbox{0.81}\ \times \ \mbox{Creatinine clearance}$$

Direct measurement of the creatinine clearance
By measuring the amount of creatinine excreted in the urine over one day, the creatinine clearance may be calculated. Creatinine is an endogenous molecule, synthesized in the body, which is freely filtered by the glomerulus (but also secreted by the renal tubules in very small amounts). Creatinine clearance is therefore a close approximation of the GFR. The formula for the creatinine clearance is:


 * $$\mbox{Creatinine clearance }= \frac{\frac{\mbox{ Amount of creatinine in urine (mg)}}{\mbox{Duration of the urine collection (minutes)}}}{\mbox{Plasma creatinine concentration (mg}/\mbox{ml)}}$$

Example: A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour he excretes 75 mg of creatinine in the urine. The GFR is calculated as M/P (where M is the mass of creatinine excreted per unit time and P is the plasma concentration of creatinine).


 * $$\mbox{Creatinine clearance }= \frac{\frac{75\mbox{ mg}}{60\mbox{ mins}}}{0.01\mbox{ mg}/\mbox{ml}} = 125 \mbox{ ml}/\mbox{min}$$

The creatinine clearance systematically overestimates the GFR due to excretion creatinine by the renal tubules. The correction factor is below:
 * $$\mbox{Glomerlular filtration rate} = \mbox{0.81}\ \times \ \mbox{Creatinine clearance}$$

Cockcroft-Gault estimation of the creatinine clearance
The Cockcroft-Gault formula may be used to estimate the creatinine clearance without having to collect urine over a period of time. However, it does not correlate as strongly with the GFR as do the MRDR formula.


 * $$\mbox{Creatinine clearance} = \frac { \mbox{(140 - Age)} \times \mbox{Mass (in kilograms)}} {\mbox{72} \times \mbox{Plasma Creatinine (in mg/dl)}} \times \mbox{0.85 if female}$$

The Estimated Creatinine Clearance then estimates GFR:
 * $$\mbox{Glomerlular filtration rate} = \mbox{0.84}\ \times \ \mbox{Cockcroft-Gault formula}$$