17.3 Standard Urea Clearance
If the volume of urine excreted/min is less than 2 ml/min, standard urea clearance is
calculated by comparing the observed clearance after adjustment for“U”with a
standard of 54 ml/min, the average normal urea clearance.
Standard urea clearanceðÞ¼%average normalobserved clearance 100
54
For any value of“U,”the urea clearance isfinally expressed as % of average
normal value.
20.5 Procedure
Urea clearance can be carried either from 24 h urine specimen or timed urine
specimen. 24 h urine specimen is collected in a bottle containing a preservative
(10 ml of 6 N HCl). In between the 24 h period, collect a blood sample also for
estimation of urea in plasma. After the collection of 24 h urine specimen, measure
the volume of urine to calculate volume of urine excreted/min.
If timed urine specimen is to be used, give a cupful (200–400 ml) water to drink,
and ask the patient to empty the bladder and note the time. Exactly after 1 h, collect
thefirst urine specimen, and mark it as urine sample“ 1 ”. After 5 min of collection of
urine sample, collect a blood sample for urea estimation. Exactly after 2 h of intake
of water, ask the patient to empty the bladder completely, and mark that specimen as
urine sample“ 2 ”. Measure the volume of both the urine samples, and estimate the
concentration of urea in plasma and both the urine samples. Then calculate standard
or maximum urea clearance from both the urine samples according to the volume of
urine excreted/min.
17.5 Precautions
- Collect exactly 24 h urine sample, and note the volume accurately.
- Use urine preservative for the collection of 24 h urine specimen.
- If timed urine specimen is collected, we can check the reliability of urine
collection. The differences between the 1st and 2nd hourly specimens should
not exceed 10%.
17.6 Clinical Significance...............................
There is wide range of urea clearance in normal persons. Normal maximum clear-
ance varies from 60 to 95 ml/min, with an average of 75 ml/min. The normal
standard urea clearance varies from 40 to 65 ml/min with an average of 54 ml/min.
72 17 To Determine Urea Clearance