How is creatinine clearance (CrCl) calculated in patients with azotemia?

Updated: Sep 19, 2018
  • Author: Moro O Salifu, MD, MPH, FACP; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Because creatinine normally is filtered as well as secreted into the renal tubules, CrCl may cause the GFR to be substantially overestimated, especially as kidney failure progresses because of maximal tubular excretion. More accurate determinations of GFR require the use of inulin clearance or a radiolabeled compound (eg, iothalamate). In practice, precise knowledge of the GFR is not required, and the disease process usually can be adequately monitored by using the estimated GFR (eGFR), which may be obtained with a number of different methods.

CrCl is best calculated by obtaining a 24-hour urine collection for creatinine and volume and then using the following formula:

CrCl (mL/min) = U/P × V

where U is the urine creatinine in mg/dL, P is the serum creatinine in mg/dL, and V is the 24-hour volume divided by 1440 (the number of minutes in 24 hours). An adequate 24-hour collection usually reflects a creatinine generation of 15-20 mg/kg in women and 20-25 mg/kg in men. When 24-hour creatinine is measured, the adequacy of the collection must be established prior to calculation of the creatinine clearance.

Alternatively, the GFR can be estimated by means of the Cockcroft and Gault formula, a bedside formula that uses the patient’s serum creatinine (mg/dL), age (y), and lean weight (kg), as follows:

CrCl (mL/min) = [(140 – age) × weight]/(72 × serum creatinine)

For women, the result of the equation is multiplied by 0.85.


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