Aliquot from timed urine (24 hour or 48 hour collection interval is preferred) collected without a preservative. The total volume collected must be measured, recorded, and included with the test request.
The blood specimen ideally should be drawn during the urine collection time interval, however blood specimens drawn no longer than 24 hours prior to the start of the collection or within 24 hours after the collection ends are usually acceptable.
Patient's height (within 1 inch) and weight (within 10 lbs) should be submitted with the request to allow for result correction to standardized 1.73 sq. meter body surface area.
Clearance is calculated using the usual method (U/P x V) and makes no correction for tubular reabsorption at slower urine flow rates, i.e. the "maximum urea clearance" is assumed.
The blood specimen must be ordered separately as either a BUN or as part of a Basic Metabolic Profile.