Creatinine Clearance Test

The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood. It is an imperfect measure of glomerular filtration rate. The use of creatinine to estimate glomerular filtration rate is based upon three assumptions: (1) creatinine is filtered through the glomerulus, (2) relatively low amounts of creatinine are reabsorbed through the nephron tubule, and (3) creatinine production is constant overtime. Although creatinine can provide a rough estimate of glomerular filtration rate, error may be encountered. Such factors as increased tubular reabsorption of creatinine, reduced creatinine generation from muscle tissue, and dietary changes in nitrogenous compounds may affect creatinine clearance.

Specimen Collection:

Properly collected specimens are essential for accuracy of the creatinine clearance test. Two specimens are collected, a 24-hour urine and a blood specimen that is drawn during the 24-hour urine collection or not more than 24 hours before or after the urine collection. The laboratory may add 6-mol/L HCl or boric acid to the container as preservative.

Proper instruction must be given to the patient prior to the collection of urine sample:

1. Void and discard this urine.
2. Start timing the 24-hour period immediately after voiding
3. Collect all the urine voided for the next 24 hours. Keep the urine in a cool place.
4. At 24 hours, void and add this urine to the collection container.
5. Bring the urine to the laboratory as soon as possible.

Calculation of Creatinine Clearance:

At the laboratory, the specimen is well mixed, the volume is measured, and then an aliquot is taken for testing. The urine specimen is diluted 1:10 with distilled water and measured for creatinine. The urine creatinine is multiplied by 10 for the dilution. Creatinine clearance is calculated as follows: 

Urine creatinine (mg/dL) x volume (mL) x average surface area (m2)

Plasma creatinine (mg/dL) x time (min) x patient surface area (m2)

Complete collection of the 24-hour urine is necessary in order to report the creatinine clearance accurately. Several formulas are available to estimate creatinine clearance. The Schwarz formula estimates creatinine clearance from serum creatinine.This formula is used for pediatric patients:

Creatinine clearance= (k Ht)Creat

where k=   0.45 if age < 1 year, k=   0.55 if age= 1 to 12 years, Ht= height in cm, and Creat= serum creatinine.

Reference Ranges:

Male=  95–130 mL/min

Female=  80–120 mL/min

Reference:
Clinical Chemistry: A Laboratory Perspective- Wendy L. Arneson MS MT(ASCP)