The rate at which the kidneys filter fluid is an important measure of their functioning. This value is known as the glomerular filtration rate (GFR) and may be calculated directly by comparing the concentration of a freely filtered substance in the blood with its concentration in the urine. This is generally an impractical method of determining the GFR, so the GFR is generally an estimated value. The creatinine clearance rate (CCR) is a practical method for accurately estimating the GFR.

Examine the practice by clinicians of estimating the GFR with the CCR. Creatinine is produced naturally by the body and therefore does not need to be injected. It is freely filtered by the glomerulus, but it is also secreted by the renal tubules, so the CCR overestimates the GFR by about 15 percent.

Study the practicality of using the CCR to estimate the GFR. The direct calculation of the GFR would require a series of precisely timed injections of inulin into the patients. By comparison, creatinine is already in a steady state of concentration in the blood and may be easily measured.

Determine the urine flow rate. This quantity is the rate at which the patient excretes urine and may be calculated as R = V/T where R is the urine flow rate, V is the volume of urine and T is the time required to excrete the urine. The urine flow rate is typically measured in milliliters per minute (ml/min).

Calculate the CCR. This may be expressed mathematically as CCR = Uc/Pc x R where Uc is the concentration of creatinine in the urine, Pc is the concentration of creatinine in the blood plasma and R is the urine flow rate. The CCR is typically measured in ml/min.

Solve for the CCR with some typical values. A patient with a concentration of 1.25 mg/mL of creatinine in the urine, a concentration of 0.01 mg/mL of creatinine in the blood and a urine flow rate of 1 ml/min would have a CCR of 1.25/0.01 x 1 = 125 ml/min.