Clinical Information

Glomerular filtration rate (GFR) is the sum of filtration rates in all functioning nephrons and so an estimation of the GFR provides a measure of functioning nephrons of the kidney. A decrease in GFR implies either progressive renal disease, or a reversible process causing decreased nephron function (eg, severe dehydration). One of the most common methods used for estimating GFR is creatinine clearance.

Creatinine is derived from the metabolism of creatine from skeletal muscle and dietary meat intake, and is released into the circulation at a relatively constant rate. Thus, the serum creatinine concentration is usually stable. Creatinine is freely filtered by glomeruli and not reabsorbed or metabolized by renal tubules. However, approximately 15% of excreted urine creatinine is derived from proximal tubular secretion. Because of the tubular secretion of creatinine, the creatinine clearance typically overestimates the true GFR by 10% to 15%.

Creatinine clearance is usually determined from measurement of creatinine in a 24-hour urine specimen and from a serum specimen obtained during the same collection period. The creatinine clearance is then calculated by the equation:

2.54 cm=1 inch

1 kg=2.2 pounds (lbs)

Patient surface area (SA)=wt (kg)(.425) X ht (cm)(.725) X 0.007184

                                    Urine conc (mg/dL) x 24 hr Urine volume (mL)                                     

Uncorr creat clear=                                                1440 minutes_____    =mL/min

                                                          Plasma creat (mg/dL)       

                                 Urine conc (mg/dL) x 24 hr urine volume (mL)

Corr creat clear=                                        1440 minutes_______ x 1.73m(2)=

mL/min/1.73m(2)

                                                     Plasma creat (mg/dL)                   Patient SA